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AC - Convective outlook issued by the SPC.
Abbreviation for Anticipated Convection; the term originates from
the header coding [ACUS1] of the transmitted product. See SWODY1, SWODY2.
ACCAS (usually
pronounced ACK-kis) - AltoCumulus
CAStellanus; mid-level clouds (bases generally 8 to 15 thousand feet), of
which at least a fraction of their upper parts show cumulus-type development.
These clouds often are taller than they are wide, giving them a turret-shaped
appearance. ACCAS clouds are a sign of instability aloft, and may precede the
rapid development of thunderstorms.
Accessory Cloud - A
cloud which is dependent on a larger cloud system for development and
continuance. Roll clouds, shelf clouds, and wall clouds are examples of
accessory clouds.
Advection - Transport of an
atmospheric property by the wind. See cold advection, moisture advection, warm
advection.
Air-mass
Thunderstorm - Generally, a thunderstorm not associated with a front or
other type of synoptic-scale forcing mechanism. Air mass thunderstorms typically
are associated with warm, humid air in the summer months; they develop during
the afternoon in response to insolation, and dissipate rather quickly after
sunset. They generally are less likely to be severe than other types of
thunderstorms, but they still are capable of producing downbursts, brief heavy
rain, and (in extreme cases) hail over 3/4 inch in diameter. See popcorn
convection.
Since all thunderstorms are associated with some type of forcing mechanism,
synoptic-scale or otherwise, the existence of true air-mass thunderstorms is
debatable. Therefore the term is somewhat controversial and should be used with
discretion.
Algorithm - A computer program
(or set of programs) which is designed to systematically solve a certain kind of
problem. WSR-88D radars (NEXRAD) employ algorithms to analyze radar data and
automatically determine storm motion, probability of hail, VIL, accumulated
rainfall, and several other parameters.
Anticyclonic
Rotation - Rotation in the opposite sense as the Earth's rotation, i.e.,
clockwise in the Northern Hemisphere as would be seen from above. The opposite
of cyclonic rotation.
Anvil - The flat, spreading top of a Cb
(cumulonimbus), often shaped like an anvil. Thunderstorm anvils may spread
hundreds of miles downwind from the thunderstorm itself, and sometimes may
spread upwind (see back-sheared anvil).
Anvil Crawler -
[Slang], a lightning discharge occurring within the anvil of a thunderstorm,
characterized by one or more channels that appear to crawl along the underside
of the anvil. They typically appear during the weakening or dissipating stage of
the parent thunderstorm, or during an active MCS.
Anvil Dome - A large
overshooting top or penetrating top.
Anvil Rollover -
[Slang], a circular or semicircular lip of clouds along the underside of the
upwind part of a back-sheared anvil, indicating rapid expansion of the anvil.
See cumuliform anvil, knuckles, mushroom.
Anvil Zits - [Slang],
frequent (often continuous or nearly continuous), localized lightning discharges
occurring from within a thunderstorm anvil.
AP - Anomalous Propagation. Radar term for
false (non-precipitation) echoes resulting from nonstandard propagation of the
radar beam under certain atmospheric conditions.
Approaching (severe levels)
- A thunderstorm which contains winds of 35 to 49 knots (40 to 57 mph), or hail
1/2 inch or larger but less than 3/4 inch in diameter. See severe thunderstorm.
Arcus - A low, horizontal cloud
formation associated with the leading edge of thunderstorm outflow (i.e., the
gust front). Roll clouds and shelf clouds both are types of arcus clouds.
AVN - AViatioN model; one of the
operational forecast models run at NCEP. The AVN is run four times daily, at
0000, 0600, 1200, and 1800 GMT. As of fall 1996, forecast output was available
operationally out to 120 hours only from the 0000 and 1200 runs. At 0600 and
1800, the model is run only out to 72 hours.
Back-building
Thunderstorm - A thunderstorm in which new development takes place on
the upwind side (usually the west or southwest side), such that the storm seems
to remain stationary or propagate in a backward direction.
Backing Winds - Winds
which shift in a counterclockwise direction with time at a given location (e.g.
from southerly to southeasterly), or change direction in a counterclockwise
sense with height (e.g. westerly at the surface but becoming more southerly
aloft). The opposite of veering winds.
In storm spotting, a backing wind usually refers to the turning of a south or
southwest surface wind with time to a more east or southeasterly direction.
Backing of the surface wind can increase the potential for tornado development
by increasing the directional shear at low levels.
Back-sheared Anvil
- [Slang], a thunderstorm anvil which spreads upwind, against the flow aloft. A
back-sheared anvil often implies a very strong updraft and a high severe weather
potential.
Barber Pole - [Slang], a
thunderstorm updraft with a visual appearance including cloud striations that
are curved in a manner similar to the stripes of a barber pole. The structure
typically is most pronounced on the leading edge of the updraft, while drier air
from the rear flank downdraft often erodes the clouds on the trailing side of
the updraft.
Baroclinic Zone - A
region in which a temperature gradient exists on a constant pressure surface.
Baroclinic zones are favored areas for strengthening and weakening systems;
barotropic systems, on the other hand, do not exhibit significant changes in
intensity. Also, wind shear is characteristic of a baroclinic zone.
Barotropic System
- A weather system in which temperature and pressure surfaces are coincident,
i.e., temperature is uniform (no temperature gradient) on a constant pressure
surface. Barotropic systems are characterized by a lack of wind shear, and thus
are generally unfavorable areas for severe thunderstorm development. See
baroclinic zone.
Usually, in operational meteorology, references to barotropic systems refer
to equivalent barotropic systems - systems in which temperature gradients
exist, but are parallel to height gradients on a constant pressure surface. In
such systems, height contours and isotherms are parallel everywhere, and winds
do not change direction with height.
As a rule, a true equivalent barotropic system can never be achieved in the
real atmosphere. While some systems (such as closed lows or cutoff lows) may
reach a state that is close to equivalent barotropic, the term barotropic system
usually is used in a relative sense to describe systems that are really only
close to being equivalent barotropic, i.e., isotherms and height contours are
nearly parallel everywhere and directional wind shear is weak.
Bear's Cage - [Slang], a
region of storm-scale rotation, in a thunderstorm, which is wrapped in heavy
precipitation. This area often coincides with a radar hook echo and/or
mesocyclone, especially one associated with an HP storm.
The term reflects the danger involved in observing such an area visually,
which must be done at close range in low visibility.
Beaver('s) Tail - [Slang],
a particular type of inflow band with a relatively broad, flat appearance
suggestive of a beaver's tail. It is attached to a supercell's general updraft
and is oriented roughly parallel to the pseudo-warm front, i.e., usually east to
west or southeast to northwest. As with any inflow band, cloud elements move
toward the updraft, i.e., toward the west or northwest. Its size and shape
change as the strength of the inflow changes. See also inflow stinger.
Spotters should note the distinction between a beaver tail and a tail cloud.
A "true" tail cloud typically is attached to the wall cloud and has a
cloud base at about the same level as the wall cloud itself. A beaver tail, on
the other hand, is not attached to the wall cloud and has a cloud base at about
the same height as the updraft base (which by definition is higher than the wall
cloud). Unlike the beaver tail, the tail cloud forms from air that is flowing
from the storm's main precipitation cascade region (or outflow region). Thus, it
can be oriented at a large angle to the pseudo-warm front.
Blue Watch (or Blue Box) -
[Slang], a severe thunderstorm watch.
Boundary Layer - In
general, a layer of air adjacent to a bounding surface. Specifically, the term
most often refers to the planetary boundary layer, which is the layer
within which the effects of friction are significant. For the earth, this layer
is considered to be roughly the lowest one or two kilometers of the atmosphere.
It is within this layer that temperatures are most strongly affected by daytime
insolation and nighttime radiational cooling, and winds are affected by friction
with the earth's surface. The effects of friction die out gradually with height,
so the "top" of this layer cannot be defined exactly.
There is a thin layer immediately above the earth's surface known as the surface
boundary layer (or simply the surface layer). This layer is only a part of
the planetary boundary layer, and represents the layer within which friction
effects are more or less constant throughout (as opposed to decreasing with
height, as they do above it). The surface boundary layer is roughly 10 meters
thick, but again the exact depth is indeterminate. Like friction, the effects of
insolation and radiational cooling are strongest within this layer.
Bow Echo - A radar echo which is
linear but bent outward in a bow shape. Damaging straight-line winds often occur
near the "crest" or center of a bow echo. Areas of circulation also
can develop at either end of a bow echo, which sometimes can lead to tornado
formation - especially in the left (usually northern) end, where the circulation
exhibits cyclonic rotation.
Box (or Watch Box) - [Slang], a severe
thunderstorm or tornado watch. See blue box, red box.
BRN - See Bulk Richardson Number.
Bubble High - A mesoscale
area of high pressure, typically associated with cooler air from the rainy
downdraft area of a thunderstorm or a complex of thunderstorms. A gust front or
outflow boundary separates a bubble high from the surrounding air.
Bulk
Bust - [Slang], an inaccurate forecast or
an unsuccessful storm chase; usually a situation in which thunderstorms or
severe weather are expected, but do not occur.
BWER - Bounded Weak Echo
Region. (Also known as a vault.) Radar signature within a thunderstorm
characterized by a local minimum in radar reflectivity at low levels which
extends upward into, and is surrounded by, higher reflectivities aloft . This
feature is associated with a strong updraft and is almost always found in the
inflow region of a thunderstorm. It cannot be seen visually. See WER.
CA - Cloud-to-Air lightning.
Cap
(or Capping Inversion) - A layer of relatively warm air aloft (usually several
thousand feet above the ground) which suppresses or delays the development of
thunderstorms. Air parcels rising into this layer become cooler than the
surrounding air, which inhibits their ability to rise further. As such, the cap
often prevents or delays thunderstorm development even in the presence of
extreme instability. However if the cap is removed or weakened, then explosive
thunderstorm development can occur. See CIN.
The
cap is an important ingredient in most severe thunderstorm episodes, as it
serves to separate warm, moist air below and cooler, drier air above. With the
cap in place, air below it can continue to warm and/or moisten, thus increasing
the amount of potential instability. Or, air above it can cool, which also
increases potential instability. But without a cap, either process
(warming/moistening at low levels or cooling aloft) results in a faster release
of available instability - often before instability levels become large enough
to support severe weather development.
CAPE
- Convective Available Potential
Energy. A measure of the amount of energy available for convection.
*Cb
- Cumulonimbus cloud, characterized by strong vertical development
in the form of mountains or huge towers topped at least partially by a smooth,
flat, often fibrous anvil. Also known colloquially as a "thunderhead."
CC - Cloud-to-Cloud lightning.
Cell
- Convection in the form of a single updraft, downdraft, or updraft/downdraft
couplet, typically seen as a vertical dome or tower as in a cumulus or towering
cumulus cloud. A typical thunderstorm consists of several cells (see
multi-cellular thunderstorm).
The
term "cell" also is used to describe the radar echo returned by an
individual shower or thunderstorm. Such usage, although common, is technically
incorrect.
*CG
- Cloud-to-Ground lightning flash.
Chaff
- Small strips of metal foil, usually dropped in large quantities from aircraft
or balloons. Chaff typically produces a radar echo which closely resembles
precipitation. Chaff drops once were conducted by the military in order to
confuse enemy radar, but now are conducted mainly for radar testing and
calibration purposes.
CIN
- Convective INhibition. A measure of the amount of energy needed
in order to initiate convection. Values of CIN typically reflect the strength of
the cap. They are obtained on a sounding by computing the area enclosed between
the environmental temperature profile and the path of a rising air parcel, over
the layer within which the latter is cooler than the former. (This area
sometimes is called negative area.)
Cirrus
- High-level clouds (16,000 feet or more), composed of ice crystals and
appearing in the form of white, delicate filaments or white or mostly white
patches or narrow bands. Cirrus clouds typically have a fibrous or hairlike
appearance, and often are semi-transparent. Thunderstorm anvils are a form of
cirrus cloud, but most cirrus clouds are not associated with thunderstorms.
Classic
Supercell - See supercell.
Clear
Slot - A local region of clearing
skies or reduced cloud cover, indicating an intrusion of drier air; often seen
as a bright area with higher cloud bases on the west or southwest side of a wall
cloud. A clear slot is believed to be a visual indication of a rear flank
downdraft.
Closed
Low - A low pressure area with a
distinct center of cyclonic circulation which can be completely encircled by one
or more isobars
or height contour lines. The term usually is used to distinguish a low pressure
area aloft from a low-pressure trough. Closed lows aloft typically are partially
or completely detached from the main westerly current, and thus move relatively
slowly (see cutoff low).
Cloud
Streets - Rows of cumulus or
cumulus-type clouds aligned parallel to the low-level flow. Cloud streets
sometimes can be seen from the ground, but are seen best on satellite
photographs.
Cloud
Tags - Ragged, detached cloud
fragments; fractus or scud.
Cold
Advection - Transport of cold air
into a region by horizontal winds.
Cold-air
Funnel - A funnel cloud or (rarely) a
small, relatively weak tornado that can develop from a small shower or
thunderstorm when the air aloft is unusually cold (hence the name). They are
much less violent than other types of tornadoes.
Cold
Pool - A region of relatively cold
air, represented on a weather map analysis as a relative minimum in temperature
surrounded by closed isotherms. Cold pools aloft represent regions of relatively
low stability, while surface-based cold pools are regions of relatively stable
air.
Collar
Cloud - A generally circular ring of
cloud that may be observed on rare occasions surrounding the upper part of a
wall cloud.
This
term sometimes is used (incorrectly) as a synonym for wall cloud.
Comma
Cloud - A synoptic scale cloud
pattern with a characteristic comma-like shape, often seen on satellite
photographs associated with large and intense low-pressure systems.
Comma
Echo - A thunderstorm radar echo
which has a comma-like shape. It often appears during latter stages in the life
cycle of a bow echo.
Condensation
Funnel - A funnel-shaped cloud
associated with rotation and consisting of condensed water droplets (as opposed
to smoke, dust, debris, etc.). Compare with debris cloud.
Confluence
- A pattern of wind flow in which air flows inward toward an axis oriented
parallel to the general direction of flow. It is the opposite of difluence.
Confluence is not the same as convergence. Winds often accelerate as they enter
a confluent zone, resulting in speed divergence which offsets the (apparent)
converging effect of the confluent flow.
Congestus
(or Cumulus Congestus) - same as towering cumulus.
Convection
- Generally, transport of heat and moisture by the movement of a fluid. In
meteorology, the term is used specifically to describe vertical transport of
heat and moisture, especially by updrafts and downdrafts in an unstable
atmosphere. The terms "convection" and "thunderstorms" often
are used interchangeably, although thunderstorms are only one form of
convection. Cbs, towering cumulus clouds, and ACCAS clouds all are visible forms
of convection. However, convection is not always made visible by clouds.
Convection which occurs without cloud formation is called dry convection, while
the visible convection processes referred to above are forms of moist
convection.
Convective
Outlook (sometimes called AC) - A
forecast containing the area(s) of expected thunderstorm occurrence and expected
severity over the contiguous United States, issued several times daily by the
SPC. The terms approaching, slight risk, moderate risk, and high risk are used
to describe severe thunderstorm potential. Local versions sometimes are prepared
by local NWS offices.
Convective
Temperature - The approximate
temperature that the air near the ground must warm to in order for surface-based
convection to develop, based on analysis of a sounding.
Calculation
of the convective temperature involves many assumptions, such that thunderstorms
sometimes develop well before or well after the convective temperature is
reached (or may not develop at all). However, in some cases the convective
temperature is a useful parameter for forecasting the onset of convection.
Convergence
- A contraction of a vector field; the opposite of divergence. Convergence in a
horizontal wind field indicates that more air is entering a given area than is
leaving at that level. To compensate for the resulting "excess,"
vertical motion may result: upward forcing if convergence is at low levels, or
downward forcing (subsidence) if convergence is at high levels. Upward forcing
from low-level convergence increases the potential for thunderstorm development
(when other factors, such as instability, are favorable). Compare with
confluence.
Core
Punch - [Slang], a penetration by a
vehicle into the heavy precipitation core of a thunderstorm.
Core
punching is not a recommended procedure for storm spotting.
Cumuliform
Anvil - A thunderstorm anvil with
visual characteristics resembling cumulus-type clouds (rather than the more
typical fibrous appearance associated with cirrus). A cumuliform anvil arises
from rapid spreading of a thunderstorm updraft, and thus implies a very strong
updraft. See anvil rollover, knuckles, mushroom.
Cumulus
- Detached clouds, generally dense and with sharp outlines, showing vertical
development in the form of domes, mounds, or towers. Tops normally are rounded
while bases are more horizontal. See Cb,
towering cumulus.
Cumulus
Congestus (or simply Congestus)
- Same as towering cumulus.
Cutoff
Low - A closed low which has become
completely displaced (cut off) from basic westerly current, and moves
independently of that current. Cutoff lows may remain nearly stationary for
days, or on occasion may move westward opposite to the prevailing flow aloft
(i.e., retrogression).
"Cutoff
low" and "closed low" often are used interchangeably to describe
low pressure centers aloft. However, not all closed lows are completely removed
from the influence of the basic westerlies. Therefore, the recommended usage of
the terms is to reserve the use of "cutoff low" only to those closed
lows which clearly are detached completely from the westerlies.
Cyclic
Storm - A thunderstorm that undergoes
cycles of intensification and weakening (pulses) while maintaining its
individuality. Cyclic supercells are capable of producing multiple tornadoes
(i.e., a tornado family) and/or several bursts of severe weather.
A
storm which undergoes only one cycle (pulse), and then dissipates, is known as a
pulse storm.
Cyclogenesis
- Development or intensification of a low-pressure center (cyclone).
*Cyclonic
Circulation (or Cyclonic Rotation) -
Circulation (or rotation) which is in the same sense as the Earth's rotation,
i.e., counterclockwise (in the Northern Hemisphere) as would be seen from above.
Nearly all mesocyclones and strong or violent tornadoes exhibit cyclonic
rotation, but some smaller vortices, such as gustnadoes, occasionally rotate
anticyclonically (clockwise). Compare with anticyclonic rotation.
dBZ
- Nondimensional "unit" of radar reflectivity which represents a
logarithmic power ratio (in decibels, or dB) with respect to radar reflectivity
factor, Z.
The
value of Z is a function of the amount of radar beam energy that is
backscattered by a target and detected as a signal (or echo). Higher values of Z
(and dBZ) thus indicate more energy being backscattered by a target. The amount
of backscattered energy generally is related to precipitation intensity,
such that higher values of dBZ that are detected from precipitation areas
generally indicate higher precipitation rates. However, other factors can affect
reflectivity, such as width of the radar beam, precipitation type, drop size, or
the presence of ground clutter or AP. WSR-88D radars can detect reflectivities
as low as -32 dBZ near the radar site, but significant (measurable)
precipitation generally is indicated by reflectivities of around 15 dBZ or more.
Values of 50 dBZ or more normally are associated with heavy thunderstorms,
perhaps with hail, but as with most other quantities, there are no reliable
threshold values to confirm the presence of hail or severe weather in a given
situation. See VIP for threshold dBZ values associated with each VIP level.
*Debris
Cloud - A rotating "cloud"
of dust or debris, near or on the ground, often appearing beneath a condensation
funnel and surrounding the base of a tornado.
This
term is similar to dust whirl, although the latter typically refers to a
circulation which contains dust but not necessarily any debris. A dust plume, on
the other hand, does not rotate. Note that a debris cloud appearing beneath a
thunderstorm will confirm the presence of a tornado, even in the absence of a
condensation funnel.
Delta
T - A simple representation of the
mean lapse rate within a layer of the atmosphere, obtained by calculating the
difference between observed temperatures at the bottom and top of the layer.
Delta Ts often are computed operationally over the layer between pressure levels
of 700 mb and 500 mb, in order to evaluate the amount of instability in
mid-levels of the atmosphere. Generally, values greater than about 18 indicate
sufficient instability for severe thunderstorm development.
Derecho
- (Pronounced deh-REY-cho), a widespread and usually fast-moving windstorm
associated with convection. Derechos include any family of downburst clusters
produced by an extratropical MCS, and can produce damaging straight-line winds
over areas hundreds of miles long and more than 100 miles across.
Dew
Point (or Dew-point Temperature) - A
measure of atmospheric moisture. It is the temperature to which air must be
cooled in order to reach saturation (assuming air pressure and moisture content
are constant).
Differential
Motion - Cloud motion that appears to
differ relative to other nearby cloud elements, e.g. clouds moving from left to
right relative to other clouds in the foreground or background. Cloud rotation
is one example of differential motion, but not all differential motion indicates
rotation. For example, horizontal wind shear along a gust front may result in
differential cloud motion without the presence of rotation.
Difluence
(or Diffluence) - A pattern of wind flow in which air moves outward (in a
"fan-out" pattern) away from a central axis that is oriented parallel
to the general direction of the flow. It is the opposite of confluence.
Difluence
in an upper level wind field is considered a favorable condition for severe
thunderstorm development (if other parameters are also favorable). But difluence
is not the same as divergence. In a difluent flow, winds normally decelerate as
they move through the region of difluence, resulting in speed convergence which
offsets the apparent diverging effect of the difluent flow.
Directional
Shear - The component of wind shear
which is due to a change in wind direction with height, e.g., southeasterly
winds at the surface and southwesterly winds aloft. A veering wind with height
in the lower part of the atmosphere is a type of directional shear often
considered important for tornado development.
Diurnal
- Daily; related to actions which are completed in the course of a calendar day,
and which typically recur every calendar day (e.g., diurnal temperature rises
during the day, and diurnal falls at night).
Divergence
- The expansion or spreading out of a vector field; usually said of horizontal
winds. It is the opposite of convergence. Divergence at upper levels of the
atmosphere enhances upward motion, and hence the potential for thunderstorm
development (if other factors also are favorable).
Doppler
Radar - Radar that can measure radial
velocity, the instantaneous component of motion parallel to the radar beam
(i.e., toward or away from the radar antenna).
*Downburst
- A strong downdraft resulting in an outward burst of damaging winds on or near
the ground. Downburst winds can produce damage similar to a strong tornado.
Although usually associated with thunderstorms, downbursts can occur with
showers too weak to produce thunder. See dry and wet microburst.
Downdraft
- A small-scale column of air that rapidly sinks toward the ground, usually
accompanied by precipitation as in a shower or thunderstorm. A downburst is the
result of a strong downdraft.
Downstream
- In the same direction as a stream or other flow, or toward the direction in
which the flow is moving.
Dry
Adiabat - A line of constant
potential temperature on a thermodynamic chart.
Dry
Line - A boundary separating moist
and dry air masses, and an important factor in severe weather frequency in the
Dry-line
Bulge - A bulge in the dry line,
representing the area where dry air is advancing most strongly at lower levels
(i.e., a surface dry punch). Severe weather potential is increased near and
ahead of a dry line bulge.
Dry-line
Storm - Generally, any thunderstorm
that develops on or near a dry line. The term often is used synonymously with LP
storm, since the latter almost always occurs near the dry line.
Dry
Microburst - A microburst with little
or no precipitation reaching the ground; most common in semi-arid regions. They
may or may not produce lightning. Dry microbursts may develop in an otherwise
fair-weather pattern; visible signs may include a cumulus cloud or small Cb with
a high base and high-level virga, or perhaps only an orphan anvil from a dying
rain shower. At the ground, the only visible sign might be a dust plume or a
ring of blowing dust beneath a local area of virga. Compare with wet microburst.
Dry
Punch - [Slang], a surge of drier
air; normally a synoptic-scale or mesoscale process. A dry punch at the surface
results in a dry line bulge. A dry punch aloft above an area of moist air at low
levels often increases the potential for severe weather.
Dry
Slot - A zone of dry (and relatively
cloud-free) air which wraps east- or northeastward into the southern and eastern
parts of a synoptic scale or mesoscale low pressure system. A dry slot generally
is seen best on satellite photographs.
Dry
slot should not be confused with clear slot, which is a storm-scale phenomenon.
Dust
Devil - A small atmospheric vortex
not associated with a thunderstorm, which is made visible by a rotating cloud of
dust or debris (dust whirl). Dust devils form in response to surface heating
during fair, hot weather; they are most frequent in arid or semi-arid regions.
*Dust
Plume - A non-rotating
"cloud" of dust raised by straight-line winds. Often seen in a
microburst or behind a gust front.
If
rotation is observed, then the term dust whirl or debris cloud should be used.
*Dust
Whirl - A rotating column of air
rendered visible by dust. Similar to debris cloud; see also dust devil,
gustnado, tornado.
Dynamics
- Generally, any forces that produce motion or affect change. In operational
meteorology, dynamics usually refer specifically to those forces that produce
vertical motion in the atmosphere.
ECMWF
-
Elevated
Convection - Convection occurring
within an elevated layer, i.e., a layer in which the lowest portion is based
above the earth's surface. Elevated convection often occurs when air near the
ground is relatively cool and stable, e.g., during periods of isentropic lift,
when an unstable layer of air is present aloft. In cases of elevated convection,
stability indices based on near-surface measurements (such as the lifted index)
typically will underestimate the amount of instability present. Severe weather
is possible from elevated convection, but is less likely than it is with
surface-based convection.
Energy
Helicity Index (or EHI) - An index
that incorporates vertical shear and instability, designed for the purpose of
forecasting supercell thunderstorms. It is related directly to storm-relative
helicity in the lowest 2 km (SRH, in m2/s2) and CAPE (in
j/kg) as follows:
EHI=(
Thus,
higher values indicate unstable conditions and/or strong vertical shear. Since
both parameters are important for severe weather development, higher values
generally indicate a greater potential for severe weather. Values of 1 or more
are said to indicate a heightened threat of tornadoes; values of 5 or more are
rarely observed, and are said to indicate potential for violent tornadoes.
However, there are no magic numbers or critical threshold values to confirm or
predict the occurrence of tornadoes of a particular intensity.
Enhanced
V - A pattern seen on satellite
infrared photographs of thunderstorms, in which a thunderstorm anvil exhibits a
V-shaped region of colder cloud tops extending downwind from the thunderstorm
core. The enhanced V indicates a very strong updraft, and therefore a higher
potential for severe weather.
Enhanced
V should not be confused with V notch, which is a radar signature.
Enhanced
Wording - An option used by the SPC
in tornado and severe thunderstorm watches when the potential for strong/violent
tornadoes, or unusually widespread damaging straight-line winds, is high. The
statement "THIS IS A PARTICULARLY DANGEROUS SITUATION WITH THE POSSIBILITY
OF VERY DAMAGING TORNADOES" appears in tornado watches with enhanced
wording. Severe thunderstorm watches may include the statement "THIS IS A
PARTICULARLY DANGEROUS SITUATION WITH THE POSSIBILITY OF EXTREMELY DAMAGING
WINDS," usually when a derecho event is occurring or forecast to occur. See
PDS watch.
Entrance
Region - The region upstream from a
wind speed maximum in a jet stream (jet max), in which air is approaching
(entering) the region of maximum winds, and therefore is accelerating. This
acceleration results in a vertical circulation that creates divergence in the
upper-level winds in the right half of the entrance region (as would be viewed
looking along the direction of flow). This divergence results in upward motion
of air in the right rear quadrant (or right entrance region) of the jet max.
Severe weather potential sometimes increases in this area as a result. See also
exit region, left exit region.
Equilibrium
Level (or EL) - On a sounding, the
level above the level of free convection (LFC) at which the temperature of a
rising air parcel again equals the temperature of the environment.
The
height of the EL is the height at which thunderstorm updrafts no longer
accelerate upward. Thus, to a close approximation, it represents the height of
expected (or ongoing) thunderstorm tops. However, strong updrafts will continue
to rise past the EL before stopping, resulting in storm tops that are higher
than the EL. This process sometimes can be seen visually as an overshooting top
or anvil dome.
The
EL typically is higher than the tropopause, and is a more accurate reference for
storm tops.
Eta
Model - One of the operational
numerical forecast models run at NCEP. The Eta is run twice daily, with forecast
output out to 48 hours.
Exit
Region - The region downstream from a
wind speed maximum in a jet stream (jet max), in which air is moving away from
the region of maximum winds, and therefore is decelerating. This deceleration
results in divergence in the upper-level winds in the left half of the exit
region (as would be viewed looking along the direction of flow). This divergence
results in upward motion of air in the left front quadrant (or left exit region)
of the jet max. Severe weather potential sometimes increases in this area as a
result. See also entrance region, right entrance region.
F
scale - See Fujita Scale.
Feeder
Bands - Lines or bands of low-level
clouds that move (feed) into the updraft region of a thunderstorm, usually from
the east through south (i.e., parallel to the inflow). Same as inflow bands.
This
term also is used in tropical meteorology to describe spiral-shaped bands of
convection surrounding, and moving toward, the center of a tropical cyclone.
*Flanking
Line - A line of cumulus or towering
cumulus clouds connected to and extending outward from the most active part of a
supercell, normally on the southwest side. The line normally has a stair-step
appearance, with the tallest clouds closest to the main storm, and generally
coincides with the pseudo-cold front.
Forward
Flank Downdraft - The main region of
downdraft in the forward, or leading, part of a supercell, where most of the
heavy precipitation is. Compare with rear flank downdraft. See pseudo-warm front.
Front
- A boundary or transition zone between two air masses of different density, and
thus (usually) of different temperature. A moving front is named according to
the advancing air mass, e.g., cold front if colder air is advancing.
Fractus
- Ragged, detached cloud fragments; same as scud.
Fujita
Scale (or F Scale) - A scale of wind damage
intensity in which wind speeds are inferred from an analysis of wind damage:
F0 (weak): 40-
72 mph, light damage.
F1 (weak): 73-112
mph, moderate damage.
F2 (strong): 113-157 mph,
considerable damage.
F3 (strong): 158-206 mph,
severe damage.
F4 (violent): 207-260 mph, devastating damage.
F5 (violent): 261-318 mph, (rare) incredible damage.
All
tornadoes, and most other severe local windstorms, are assigned a single number
from this scale according to the most intense damage caused by the storm.
*Funnel
Cloud - A condensation funnel
extending from the base of a towering cumulus or Cb,
associated with a rotating column of air that is not in contact with the
ground (and hence different from a tornado). A condensation funnel is a tornado,
not a funnel cloud, if either a) it is in contact with the ground or b) a
debris cloud or dust whirl is visible beneath it.
Ground
Clutter - A pattern of radar echoes
from fixed ground targets (buildings, hills, etc.) near the radar. Ground
clutter may hide or confuse precipitation echoes near the radar antenna.
Gunge
- [Slang], anything in the atmosphere that restricts visibility for storm
spotting, such as fog, haze, precipitation (steady rain or drizzle), widespread
low clouds (stratus), etc.
Gust
Front - The leading edge of gusty
surface winds from thunderstorm downdrafts; sometimes associated with a shelf
cloud or roll cloud. See also downburst, gustnado, outflow boundary.
Gustnado
(or Gustinado) - [Slang], gust front tornado. A small tornado, usually weak and
short-lived, that occurs along the gust front of a thunderstorm. Often it is
visible only as a debris cloud or dust whirl near the ground. Gustnadoes are not
associated with storm-scale rotation (i.e. mesocyclones); they are more likely
to be associated visually with a shelf cloud than with a wall cloud.
Helicity
- A property of a moving fluid which represents the potential for helical flow
(i.e. flow which follows the pattern of a corkscrew) to evolve. Helicity is
proportional to the strength of the flow, the amount of vertical wind shear, and
the amount of turning in the flow (i.e. vorticity). Atmospheric helicity is
computed from the vertical wind profile in the lower part of the atmosphere
(usually from the surface up to 3 km), and is measured relative to storm motion.
Higher values of helicity (generally, around 150 m2/s2 or
more) favor the development of mid-level rotation (i.e. mesocyclones). Extreme
values can exceed 600 m2/s2.
High
Risk (of severe thunderstorms) -
Severe weather is expected to affect more than 10 percent of the area. A high
risk is rare, and implies an unusually dangerous situation and usually the
possibility of a major severe weather outbreak. (See slight risk, moderate risk,
convective outlook.)
Hodograph
- A plot representing the vertical distribution of horizontal winds, using polar
coordinates. A hodograph is obtained by plotting the end points of the wind
vectors at various altitudes, and connecting these points in order of increasing
height. Interpretation of a hodograph can help in forecasting the subsequent
evolution of thunderstorms (e.g., squall line vs. supercells, splitting vs.
non-splitting storms, tornadic vs. nontornadic storms, etc.).
Hook
(or Hook Echo) - A radar reflectivity pattern characterized by a hook-shaped
extension of a thunderstorm echo, usually in the right-rear part of the storm
(relative to its direction of motion). A hook often is associated with a
mesocyclone, and indicates favorable conditions for tornado development.
HP
Storm or HP Supercell -
High-Precipitation storm (or High-Precipitation supercell). A supercell
thunderstorm in which heavy precipitation (often including hail) falls on the
trailing side of the mesocyclone. Precipitation often totally envelops the
region of rotation, making visual identification of any embedded tornadoes
difficult and very dangerous. Unlike most classic supercells, the region of
rotation in many HP storms develops in the front-flank region of the storm
(i.e., usually in the eastern portion). HP storms often produce extreme and
prolonged downburst events, serious flash flooding, and very large damaging hail
events.
Mobile
storm spotters are strongly advised to maintain a safe distance from any storm
that has been identified as an HP storm; close observations (e.g., core
punching) can be extremely dangerous. See bear's cage.
Humidity
- Generally, a measure of the water vapor content of the air. Popularly, it is
used synonymously with relative humidity.
Impulse
- See upper level system.
Inflow
Bands (or Feeder Bands) - Bands of
low clouds, arranged parallel to the low-level winds and moving into or toward a
thunderstorm. They may indicate the strength of the inflow of moist air into the
storm, and, hence, its potential severity. Spotters should be especially wary of
inflow bands that are curved in a manner suggesting cyclonic rotation; this
pattern may indicate the presence of a mesocyclone.
Inflow
Jets - Local jets of air near the
ground flowing inward toward the base of a tornado.
Inflow
Notch - A radar signature
characterized by an indentation in the reflectivity pattern on the inflow side
of the storm. The indentation often is V-shaped, but this term should not be
confused with V-notch. Supercell thunderstorms often exhibit inflow notches,
usually in the right quadrant of a classic supercell, but sometimes in the
eastern part of an HP storm or in the rear part of a storm (rear inflow notch).
Inflow
Stinger - A beaver tail cloud with a
stinger-like shape.
Insolation
- Incoming solar radiation. Solar heating; sunshine.
Instability
- The tendency for air parcels to accelerate when they are displaced from their
original position; especially, the tendency to accelerate upward after being
lifted. Instability is a prerequisite for severe weather - the greater the
instability, the greater the potential for severe thunderstorms. See lifted
index.
Inversion
- Generally, a departure from the usual increase or decrease in an atmospheric
property with altitude. Specifically it almost always refers to a temperature
inversion, i.e., an increase in temperature with height, or to the layer within
which such an increase occurs. An inversion is present in the lower part of a
cap.
Isentropic
Lift - Lifting of air that is
traveling along an upward-sloping isentropic surface.
Isentropic
lift often is referred to erroneously as overrunning, but more accurately
describes the physical process by which the lifting occurs. Situations involving
isentropic lift often are characterized by widespread stratiform clouds and
precipitation, but may include elevated convection in the form of embedded
thunderstorms.
Isentropic
Surface - A two-dimensional surface
containing points of equal potential temperature.
Isobar
- A line connecting points of equal pressure.
Isodrosotherm
- A line connecting points of equal dew point temperature.
Isohyet
- A line connecting points of equal precipitation amounts.
Isopleth
- General term for a line connecting points of equal value of some quantity.
Isobars, isotherms, etc. all are examples of isopleths.
Isotach
- A line connecting points of equal wind speed.
Isotherm
- A line connecting points of equal temperature.
Jet
Max (or Speed Max, Jet Streak) - a
point or area of relative maximum wind speeds within a jet stream.
Jet
Streak - A local wind speed maximum
within a jet stream.
Jet
Stream - Relatively strong winds
concentrated in a narrow stream in the atmosphere, normally referring to
horizontal, high-altitude winds. The position and orientation of jet streams
vary from day to day. General weather patterns (hot/cold, wet/dry) are related
closely to the position, strength and orientation of the jet stream (or jet
streams). A jet stream at low levels is known as a low-level jet.
Knuckles
- [Slang], lumpy protrusions on the edges, and sometimes the underside, of a
thunderstorm anvil. They usually appear on the upwind side of a back-sheared
anvil, and indicate rapid expansion of the anvil due to the presence of a very
strong updraft. They are not mammatus clouds. See also cumuliform anvil, anvil
rollover.
Laminar
- Smooth, non-turbulent. Often used to describe cloud formations which appear to
be shaped by a smooth flow of air traveling in parallel layers or sheets.
Landspout
- [Slang], a tornado that does not arise from organized storm-scale rotation and
therefore is not associated with a wall cloud (visually) or a mesocyclone (on
radar). Landspouts typically are observed beneath Cbs or towering cumulus clouds
(often as no more than a dust whirl), and essentially are the land-based
equivalents of waterspouts.
Lapse
Rate - The rate of change of an
atmospheric variable, usually temperature, with height. A steep lapse rate
implies a rapid decrease in temperature with height (a sign of instability) and
a steepening lapse rate implies that destabilization is occurring.
Large-scale
- See synoptic-scale.
Left
Front Quadrant (or Left Exit Region)
- The area downstream from and to the left of an upper-level jet max (as would
be viewed looking along the direction of flow). Upward motion and severe
thunderstorm potential sometimes are increased in this area relative to the wind
speed maximum. See also entrance region, right rear quadrant.
Left
Mover - A thunderstorm which moves to
the left relative to the steering winds, and to other nearby thunderstorms;
often the northern part of a splitting storm. See also right mover.
LEWP
- Line Echo Wave Pattern. A bulge in a thunderstorm line producing a wave-shaped
"kink" in the line. The potential for strong outflow and damaging
straight-line winds increases near the bulge, which often resembles a bow echo.
Severe weather potential also is increased with storms near the crest of a LEWP.
Lifted
Index (or LI) - A common measure of
atmospheric instability. Its value is obtained by computing the temperature that
air near the ground would have if it were lifted to some higher level (around
18,000 feet, usually) and comparing that temperature to the actual temperature
at that level. Negative values indicate instability - the more negative, the
more unstable the air is, and the stronger the updrafts are likely to be with
any developing thunderstorms. However there are no "magic numbers" or
threshold LI values below which severe weather becomes imminent.
Loaded
Gun (Sounding) - [Slang], a sounding
characterized by extreme instability but containing a cap, such that explosive
thunderstorm development can be expected if the cap can be weakened or the air
below it heated sufficiently to overcome it.
Longwave
Trough - A trough in the prevailing
westerly flow aloft which is characterized by large length and (usually) long
duration. Generally, there are no more than about five longwave troughs around
the Northern Hemisphere at any given time. Their position and intensity govern
general weather patterns (e.g., hot/cold, wet/dry) over periods of days, weeks,
or months. Smaller disturbances (e.g., shortwave troughs) typically move more
rapidly through the broader flow of a longwave trough, producing weather changes
over shorter time periods (a day or less).
Low-level
Jet (abbrev. LLJ) - A region of
relatively strong winds in the lower part of the atmosphere. Specifically, it
often refers to a southerly wind maximum in the boundary layer, common over the
Plains states at night during the warm season (spring and summer).
The
term also may be used to describe a narrow zone of strong winds above the
boundary layer, but in this sense the more proper term would be low-level jet stream.
LP
Storm (or LP Supercell) - Low-Precipitation
storm (or Low-Precipitation supercell). A supercell thunderstorm
characterized by a relative lack of visible precipitation. Visually similar to a
classic supercell, except without the heavy precipitation core. LP storms often
exhibit a striking visual appearance; the main tower often is bell-shaped, with
a corkscrew appearance suggesting rotation. They are capable of producing
tornadoes and very large hail. Radar identification often is difficult relative
to other types of supercells, so visual reports are very important. LP storms
almost always occur on or near the dry line, and thus are sometimes referred to
as dry line storms.
LSR
- Local Storm Report. A product issued by local NWS offices
to inform users of reports of severe and/or significant weather-related events.
Mammatus
Clouds - Rounded, smooth, sack-like
protrusions hanging from the underside of a cloud (usually a thunderstorm
anvil). Mammatus clouds often accompany severe thunderstorms, but do not produce
severe weather; they may accompany non-severe storms as well.
MCC
- Mesoscale Convective Complex. A large MCS, generally
round or oval-shaped, which normally reaches peak intensity at night. The formal
definition includes specific minimum criteria for size, duration, and
eccentricity (i.e., "roundness"), based on the cloud shield as seen on
infrared satellite photographs:
MCCs
typically form during the afternoon and evening in the form of several isolated
thunderstorms, during which time the potential for severe weather is greatest.
During peak intensity, the primary threat shifts toward heavy rain and flooding.
MCS
- Mesoscale Convective System. A complex of thunderstorms
which becomes organized on a scale larger than the individual thunderstorms, and
normally persists for several hours or more. MCSs may be round or linear in
shape, and include systems such as tropical cyclones, squall lines, and MCCs
(among others). MCS often is used to describe a cluster of thunderstorms that
does not satisfy the size, shape, or duration criteria of an MCC.
Medium
Meridional
Flow - Large-scale atmospheric flow
in which the north-south component (i.e., longitudinal, or along a meridian) is
pronounced. The accompanying zonal (east-west) component often is weaker than
normal. Compare with zonal flow.
*Mesocyclone
- A storm-scale region of rotation, typically around 2-6 miles in diameter and
often found in the right rear flank of a supercell (or often on the eastern, or
front, flank of an HP storm). The circulation of a mesocyclone covers an area
much larger than the tornado that may develop within it.
Properly
used, mesocyclone is a radar term; it is defined as a rotation signature
appearing on Doppler radar that meets specific criteria for magnitude, vertical
depth, and duration. Therefore, a mesocyclone should not be considered a
visually-observable phenomenon (although visual evidence of rotation, such as
curved inflow bands, may imply the presence of a mesocyclone).
Mesohigh
- A mesoscale high pressure area, usually associated with MCSs or their
remnants.
Mesolow
(or Sub-synoptic Low) - A mesoscale low-pressure center. Severe weather
potential often increases in the area near and just ahead of a mesolow.
Mesolow
should not be confused with mesocyclone, which is a storm-scale phenomenon.
Mesonet
- A regional network of observing stations (usually surface stations) designed
to diagnose mesoscale weather features and their associated processes.
Mesoscale
- Size scale referring to weather systems smaller than synoptic-scale systems
but larger than storm-scale systems. Horizontal dimensions generally range from
around 50 miles to several hundred miles. Squall lines, MCCs, and MCSs are
examples of mesoscale weather systems.
*Microburst
- A small, concentrated downburst affecting an area less than 4 kilometers
(about 2.5 miles) across. Most microbursts are rather short-lived (5 minutes or
so), but on rare occasions they have been known to last up to 6 times that long.
Mid-level
Cooling - Local cooling of the air in
middle levels of the atmosphere (roughly 8 to 25 thousand feet), which can lead
to destabilization of the entire atmosphere if all other factors are equal.
Mid-level cooling can occur, for example, with the approach of a mid-level cold
pool.
Moderate
Risk (of severe thunderstorms) -
Severe thunderstorms are expected to affect between 5 and 10 percent of the
area. A moderate risk indicates the possibility of a significant severe weather
episode. See high risk, slight risk, convective outlook.
Moisture
Advection - Transport of moisture by
horizontal winds.
Moisture
Convergence - A measure of the degree
to which moist air is converging into a given area, taking into account the
effect of converging winds and moisture advection. Areas of persistent moisture
convergence are favored regions for thunderstorm development, if other factors
(e.g., instability) are favorable.
Morning
Glory - An elongated cloud band,
visually similar to a roll cloud, usually appearing in the morning hours, when
the atmosphere is relatively stable. Morning glories result from perturbations
related to gravitational waves in a stable boundary layer. They are similar to
ripples on a water surface; several parallel morning glories often can be seen
propagating in the same direction.
MRF
- Medium-Range Forecast model; one of the operational forecast models run at
NCEP. The MRF is run once daily, with forecast output out to 240 hours (10
days).
Multi-cell(ular)
Thunderstorm - A thunderstorm
consisting of two or more cells, of which most or all are often visible at a
given time as distinct domes or towers in various stages of development.
Nearly
all thunderstorms (including supercells) are multi-cellular, but the term often
is used to describe a storm which does not fit the definition of a supercell.
*Multiple-vortex
(or Multi-vortex) Tornado - a tornado in
which two or more condensation funnels or debris clouds are present at the same
time, often rotating about a common center or about each other. Multiple-vortex
tornadoes can be especially damaging. See suction vortex.
Mushroom
- [Slang], a thunderstorm with a well-defined anvil rollover, and thus having a
visual appearance resembling a mushroom.
NCEP
-
Negative-tilt
Trough - An upper level system which is
tilted to the west with increasing latitude (i.e., with an axis from southeast
to northwest). A negative-tilt trough often is a sign of a developing or
intensifying system.
NEXRAD
- NEXt-Generation Weather RADar. Technologically-advanced weather
radar being deployed to replace WSR-57 and WSR-74 units. NEXRAD is a
high-resolution Doppler radar with increased emphasis on automation, including
use of algorithms and automated volume scans. NEXRAD units are known as WSR-88D.
NGM
- Nested Grid Model; one of the operational forecast models
run at NCEP. The NGM is run twice daily, with forecast output out to 48 hours.
NMC
-
NOAA
- National Oceanographic and Atmospheric Administration.
Nocturnal
- Related to nighttime, or occurring at night.
Nowcast
- A short-term weather forecast, generally out to six hours or less.
NSSFC
-
NSSL
- National Severe Storms Laboratory, in
NWP
- Numerical Weather Prediction.
NWS
- National Weather Service.
Occluded
Mesocyclone - A mesocyclone in which
air from the rear-flank downdraft has completely enveloped the circulation at
low levels, cutting off the inflow of warm unstable low-level air.
Orographic
- Related to, or caused by, physical geography (such as mountains or sloping
terrain).
Orographic
Lift - Lifting of air caused by its
passage up and over mountains or other sloping terrain.
Orphan
Anvil - [Slang], an anvil from a
dissipated thunderstorm, below which no other clouds remain.
Outflow
Boundary - A storm-scale or mesoscale
boundary separating thunderstorm-cooled air (outflow) from the surrounding air;
similar in effect to a cold front, with passage marked by a wind shift and
usually a drop in temperature. Outflow boundaries may persist for 24 hours or
more after the thunderstorms that generated them dissipate, and may travel
hundreds of miles from their area of origin. New thunderstorms often develop
along outflow boundaries, especially near the point of intersection with another
boundary (cold front, dry line, another outflow boundary, etc.; see triple
point).
Overhang
- Radar term indicating a region of high reflectivity at middle and upper levels
above an area of weak reflectivity at low levels. (The latter area is known as a
weak-echo region, or WER.) The overhang is found on the inflow side of a
thunderstorm (normally the south or southeast side).
Overrunning
- A weather pattern in which a relatively warm air mass is in motion above
another air mass of greater density at the surface. Embedded thunderstorms
sometimes develop in such a pattern; severe thunderstorms (mainly with large
hail) can occur, but tornadoes are unlikely.
Overrunning
often is applied to the case of warm air riding up over a retreating layer of
colder air, as along the sloping surface of a warm front. Such use of the term
technically is incorrect, but in general it refers to a pattern characterized by
widespread clouds and steady precipitation on the cool side of a front or other
boundary.
*Overshooting
Top (or Penetrating Top) - A
dome-like protrusion above a thunderstorm anvil, representing a very strong
updraft and hence a higher potential for severe weather with that storm. A
persistent and/or large overshooting top (anvil dome) often is present on a
supercell. A short-lived overshooting top, or one that forms and dissipates in
cycles, may indicate the presence of a pulse storm or a cyclic storm..
PDS
Watch - [Slang], a tornado watch with
enhanced wording (Particularly Dangerous Situation).
Pendant
Echo - Radar signature generally
similar to a hook echo, except that the hook shape is not as well defined.
Penetrating
Top - Same as overshooting top.
Popcorn
Convection - [Slang], Showers and
thunderstorms that form on a scattered basis with little or no apparent
organization, usually during the afternoon in response to diurnal heating.
Individual thunderstorms typically are of the type sometimes referred to as
air-mass thunderstorms: they are small, short-lived, very rarely severe, and
they almost always dissipate near or just after sunset.
Positive
Area - The area on a sounding
representing the layer in which a lifted parcel would be warmer than the
environment; thus, the area between the environmental temperature profile and
the path of the lifted parcel. Positive area is a measure of the energy
available for convection; see CAPE.
Positive
CG - A CG flash that delivers
positive charge to the ground, as opposed to the more common negative charge.
Positive CGs have been found to occur more frequently in some severe
thunderstorms. Their occurrence is detectable by most lightning detection
networks, but visually it is not considered possible to distinguish between a
positive CG and a negative CG. (Some claim to have observed a relationship
between staccato lightning and positive CGs, but this relationship is as yet
unproven.)
Positive-tilt
Trough - An upper level system which
is tilted to the east with increasing latitude (i.e., from southwest to
northeast). A positive-tilt trough often is a sign of a weakening weather
system, and generally is less likely to result in severe weather than a
negative-tilt trough if all other factors are equal.
Potential
Temperature - The temperature a
parcel of dry air would have if brought adiabatically (i.e., without transfer of
heat or mass) to a standard pressure level of 1000 mb.
PPINE
- Plan Position Indicates No Echoes, referring to the fact that a radar detects
no precipitation within its range.
Profiler
- An instrument designed to measure horizontal winds directly above its
location, and thus measure the vertical wind profile. Profilers operate on the
same principles as Doppler radar.
Pseudo-Cold
Front - A boundary between a
supercell's inflow region and the rear-flank downdraft (or RFD). It extends
outward from the mesocyclone center, usually toward the south or southwest (but
occasionally bows outward to the east or southeast in the case of an occluded
mesocyclone), and is characterized by advancing of the downdraft air toward the
inflow region. It is a particular form of gust front. See also pseudo-warm
front.
Pseudo-Warm
Front - A boundary between a
supercell's inflow region and the forward-flank downdraft (or FFD). It extends
outward from at or near the mesocyclone center, usually toward the east or
southeast, and normally is either nearly stationary or moves northward or
northeastward ahead of the mesocyclone. See pseudo-cold front and beaver tail.
Pulse
Storm - A thunderstorm within which a
brief period (pulse) of strong updraft occurs, during and immediately after
which the storm produces a short episode of severe weather. These storms
generally are not tornado producers, but often produce large hail and/or
damaging winds. See overshooting top, cyclic storm.
PVA
- Positive Vorticity Advection. Advection of higher values
of vorticity into an area, which often is associated with upward motion
(lifting) of the air. PVA typically is found in advance of disturbances aloft
(i.e., shortwaves), and is a property which often enhances the potential for
thunderstorm development.
RADAP
II - RAdar DAta Processor
II, attached to some WSR-57 and WSR-74 radar units. It automatically controls
the tilt sequence and computes several radar-derived quantities at regular
intervals, including VIL, storm tops, accumulated rainfall, etc.
Radial
Velocity - Component of motion toward
or away from a given location. As "seen" by Doppler radar, it is the
component of motion parallel to the radar beam. (The component of motion perpendicular
to the beam cannot be seen by the radar. Therefore, strong winds blowing
strictly from left to right or from right to left, relative to the radar, can not
be detected.)
Rain
Foot - [Slang], a horizontal bulging
near the surface in a precipitation shaft, forming a foot-shaped prominence. It
is a visual indication of a wet microburst.
*Rain-free
Base - A dark, horizontal cloud base
with no visible precipitation beneath it. It typically marks the location of the
thunderstorm updraft. Tornadoes may develop from wall clouds attached to the
rain-free base, or from the rain-free base itself - especially when the
rain-free base is on the south or southwest side of the main precipitation area.
Note
that the rain-free base may not actually be rain free; hail or large rain drops
may be falling. For this reason, updraft base is more accurate.
Rear
Flank Downdraft (or RFD) - A region
of dry air subsiding on the back side of, and wrapping around, a mesocyclone. It
often is visible as a clear slot wrapping around the wall cloud. Scattered large
precipitation particles (rain and hail) at the interface between the clear slot
and wall cloud may show up on radar as a hook or pendant; thus the presence of a
hook or pendant may indicate the presence of an RFD.
Red
Watch or Red Box - [Slang], a tornado
watch.
Reflectivity
- Radar term referring to the ability of a radar target to return energy; used
to derive echo intensity, and to estimate precipitation intensity and rainfall
rates. See dBZ, VIP.
Relative
Humidity - A dimensionless ratio,
expressed in percent, of the amount of atmospheric moisture present relative to
the amount that would be present if the air were saturated. Since the latter
amount is dependent on temperature, relative humidity is a function of both
moisture content and temperature. As such, relative humidity by itself does not
directly indicate the actual amount of atmospheric moisture present. See dew
point.
Retrogression
(or Retrograde Motion) - Movement of a weather system in a direction opposite to
that of the basic flow in which it is embedded, usually referring to a closed
low or a longwave trough which moves westward.
Return
Flow - South winds on the back (west)
side of an eastward-moving surface high pressure system. Return flow over the
central and eastern
Right
Entrance Region (or Right Rear
Quadrant) - The area upstream from and to the right of an upper-level jet max
(as would be viewed looking along the direction of flow). Upward motion and
severe thunderstorm potential sometimes are increased in this area relative to
the wind speed maximum. See also exit region, left front quadrant.
Ridge
- An elongated area of relatively high atmospheric pressure; the opposite of
trough.
*Right
Mover - A thunderstorm that moves
appreciably to the right relative to the main steering winds and to other nearby
thunderstorms. Right movers typically are associated with a high potential for
severe weather. (Supercells often are right movers.) See left mover, splitting
storm.
Right
Rear Quadrant - see Right Entrance
Region.
*Roll
Cloud - A low, horizontal tube-shaped
arcus cloud associated with a thunderstorm gust front (or sometimes with a cold
front). Roll clouds are relatively rare; they are completely detached from the
thunderstorm base or other cloud features, thus differentiating them from the
more familiar shelf clouds. Roll clouds usually appear to be "rolling"
about a horizontal axis, but should not be confused with funnel clouds.
*Rope
(or Rope Funnel) - A narrow, often contorted condensation funnel usually
associated with the decaying stage of a tornado. See rope stage.
Rope
Cloud - In satellite meteorology, a
narrow, rope-like band of clouds sometimes seen on satellite images along a
front or other boundary.
The
term sometimes is used synonymously with rope or rope funnel.
Rope
Stage - The dissipating stage of a
tornado, characterized by thinning and shrinking of the condensation funnel into
a rope (or rope funnel). Damage still is possible during this stage.
RUC
- Rapid Update Cycle, a numerical model run at NCEP that
focuses on short-term (up to 12 h) forecasts and small-scale (mesoscale) weather
features. Forecasts are prepared every 3 hours for the contiguous
*Scud
(or Fractus) - Small, ragged, low cloud fragments that are unattached to a
larger cloud base and often seen with and behind cold fronts and thunderstorm
gust fronts. Such clouds generally are associated with cool moist air, such as
thunderstorm outflow.
SELS
- SEvere Local Storms Unit, former name of the Operations
Branch of the Storm Prediction Center (SPC) in Norman, OK (formerly in Kansas
City, MO).
*Severe
Thunderstorm - A thunderstorm which
produces tornadoes, hail 0.75 inches or more in diameter, or winds of 50 knots
(58 mph) or more. Structural wind damage may imply the occurrence of a severe
thunderstorm. See approaching (severe).
Shear
- Variation in wind speed (speed shear) and/or direction (directional shear)
over a short distance. Shear usually refers to vertical wind shear, i.e., the
change in wind with height, but the term also is used in Doppler radar to
describe changes in radial velocity over short horizontal distances.
*Shelf
Cloud - A low, horizontal
wedge-shaped arcus cloud, associated with a thunderstorm gust front (or
occasionally with a cold front, even in the absence of thunderstorms). Unlike
the roll cloud, the shelf cloud is attached to the base of the parent cloud
above it (usually a thunderstorm). Rising cloud motion often can be seen in the
leading (outer) part of the shelf cloud, while the underside often appears
turbulent, boiling, and wind-torn.
Short-Fuse
Warning - A warning issued by the NWS
for a local weather hazard of relatively short duration. Short-fuse warnings
include tornado warnings, severe thunderstorm warnings, and flash flood
warnings. Tornado and severe thunderstorm warnings typically are issued for
periods of an hour or less, flash flood warnings typically for three hours or
less.
Shortwave
(or Shortwave Trough) - A disturbance in the mid or upper part of the atmosphere
which induces upward motion ahead of it. If other conditions are favorable, the
upward motion can contribute to thunderstorm development ahead of a shortwave.
Slight
Risk (of severe thunderstorms) -
Severe thunderstorms are expected to affect between 2 and 5 percent of the area.
A slight risk generally implies that severe weather events are expected to be
isolated. See high risk, moderate risk, convective outlook.
Sounding
- A plot of the vertical profile of temperature and dew point (and often winds)
above a fixed location . Soundings are used extensively in severe weather
forecasting, e.g., to determine instability, locate temperature inversions,
measure the strength of the cap, obtain the convective temperature, etc.
SPC
-
Speed
Shear - The component of wind shear
which is due to a change in wind speed with height, e.g., southwesterly winds of
20 mph at 10,000 feet increasing to 50 mph at 20,000 feet. Speed shear is an
important factor in severe weather development, especially in the middle and
upper levels of the atmosphere.
Spin-up
- [Slang], a small-scale vortex initiation, such as what may be seen when a
gustnado, landspout, or suction vortex forms.
Splitting
Storm - A thunderstorm which splits
into two storms which follow diverging paths (a left mover and a right mover).
The left mover typically moves faster than the original storm, the right mover,
slower. Of the two, the left mover is most likely to weaken and dissipate (but
on rare occasions can become a very severe anticyclonic-rotating storm), while
the right mover is the one most likely to reach supercell status.
*Squall
Line - A solid or nearly solid line
or band of active thunderstorms.
Staccato
Lightning - A CG lightning discharge
which appears as a single very bright, short-duration stroke, often with
considerable branching.
Steering
Winds (or Steering Currents) - A
prevailing synoptic scale flow which governs the movement of smaller features
embedded within it.
Storm-relative
- Measured relative to a moving thunderstorm, usually referring to winds, wind
shear, or helicity.
Storm-scale
- Referring to weather systems with sizes on the order of individual
thunderstorms. See synoptic scale, mesoscale.
*Straight-line
Winds - Generally, any wind that is
not associated with rotation, used mainly to differentiate them from tornadic
winds.
Stratiform
- Having extensive horizontal development, as opposed to the more vertical
development characteristic of convection. Stratiform clouds cover large areas
but show relatively little vertical development. Stratiform precipitation, in
general, is relatively continuous and uniform in intensity (i.e., steady rain
versus rain showers).
Stratocumulus
- Low-level clouds, existing in a relatively flat layer but having individual
elements. Elements often are arranged in rows, bands, or waves. Stratocumulus
often reveals the depth of the moist air at low levels, while the speed of the
cloud elements can reveal the strength of the low-level jet.
Stratus
- A low, generally gray cloud layer with a fairly uniform base. Stratus may
appear in the form of ragged patches, but otherwise does not exhibit individual
cloud elements as do cumulus and stratocumulus clouds. Fog usually is a
surface-based form of stratus.
Striations
- Grooves or channels in cloud formations, arranged parallel to the flow of air
and therefore depicting the airflow relative to the parent cloud. Striations
often reveal the presence of rotation, as in the barber pole or
"corkscrew" effect often observed with the rotating updraft of an LP
storm.
Subsidence
- Sinking (downward) motion in the atmosphere, usually over a broad area.
Sub-synoptic
Low - Essentially the same as mesolow.
Suction
Vortex (sometimes Suction Spot) - A
small but very intense vortex within a tornado circulation. Several suction
vortices typically are present in a multiple-vortex tornado. Much of the extreme
damage associated with violent tornadoes (F4 and F5 on the Fujita scale) is
attributed to suction vortices.
*Supercell - A thunderstorm with a persistent rotating updraft. Supercells are rare, but are responsible for a remarkably high percentage of severe weather events - especially tornadoes, extremely large hail and damaging straight-line winds. They frequently travel to the right of the main environmental winds (i.e., they are right movers). Radar characteristics often (but not always) include a hook or pendant, bounded weak echo region (BWER), V-notch, mesocyclone, and sometimes a TVS. Visual characteristics often include a rain-free base (with or without a wall cloud), tail cloud, flanking line, overshooting top, and back-sheared anvil, all of which normally are observed in or near the right rear or southwest part of the storm . Storms exhibiting these characteristics often are called classic supercells; however HP storms&nbs