Glossay, Terms and definitions for storm spotters.
Severe weather definitions here.
NOAA Technical Memorandum NWS SR-145 Michael Branick
NOAA/WFO Norman
The information below is from NOAA's NWS Glossary and is their expertise and work that has
made this section available. It is provided here to give you accurate and professional
definitions.
-S-
*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.
Video Clip of Shelf Cloud
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 - Storm Prediction Center. A national forecast center in Norman, Oklahoma, which is
part of NCEP. The SPC is responsible for providing short-term forecast guidance for severe
convection, excessive rainfall (flash flooding) and severe winter weather over the
contiguous United States.
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 and LP storms also are supercell varieties.
Surface-based Convection - Convection occurring within a surface-based layer, i.e., a
layer in which the lowest portion is based at or very near the earth's surface. Compare
with elevated convection.
SWEAT Index - Severe Weather ThrEAT index. A stability index developed by the Air Force
which incorporates instability, wind shear, and wind speeds as follows:
SWEAT=(12 Td 850 ) + (20 [TT-49]) +( 2 f 850) + f 500 + (125 [s+0.2]) where
Td 850 is the dew point temperature at 850 mb,
TT is the total-totals index,
f 850 is the 850-mb wind speed (in knots),
f 500 is the 500-mb wind speed (in knots), and
s is the sine of the angle between the wind directions at 500 mb and 850 mb (thus
representing the directional shear in this layer).
SWEAT values of about 250-300 or more indicate a greater potential for severe weather, but
as with all stability indices, there are no magic numbers.
The SWEAT index has the advantage (and disadvantage) of using only mandatory-level data
(i.e., 500 mb and 850 mb), but has fallen into relative disuse with the advent of more
detailed sounding analysis programs.
SWODY1, SWODY2 (sometimes pronounced swoe-dee) - The day-1 and day-2 convective outlooks
issued by SELS.
Synoptic Scale (or Large Scale) - Size scale referring generally to weather systems with
horizontal dimensions of several hundred miles or more. Most high and low pressure areas
seen on weather maps are synoptic-scale systems. Compare with mesoscale, storm-scale.
-T-
*Tail Cloud - A horizontal, tail-shaped cloud (not a funnel cloud) at low levels extending
from the precipitation cascade region of a supercell toward the wall
cloud (i.e., it usually is observed extending from the wall cloud
toward the north or northeast). The base of the tail cloud is about
the same as that of the wall cloud. Cloud motion in the tail cloud
is away from the precipitation and toward the wall cloud, with rapid
upward motion often observed near the junction of the tail and wall
clouds. See supercell.
Compare with beaver tail, which is a form of inflow band that normally attaches to the
storm's main updraft (not to the wall cloud) and has a base at about the same level as the
updraft base (not the wall cloud).
Tail-end Charlie - [Slang], the thunderstorm at the southernmost end of a squall line or
other line or band of thunderstorms. Since low-level southerly inflow of warm, moist air
into this storm is relatively unimpeded, such a storm often has a higher probability of
strengthening to severe levels than the other storms in the line.
Thermodynamic Chart (or Thermodynamic Diagram) - A chart containing
contours of pressure, temperature, moisture, and potential
temperature, all drawn relative to each other such that basic
thermodynamic laws are satisfied. Such a chart typically is used to
plot atmospheric soundings, and to estimate potential changes in
temperature, moisture, etc. if air were displaced vertically from a
given level. A thermodynamic chart thus is a useful tool in
diagnosing atmospheric instability.
Thermodynamics - In general, the relationships between heat and other properties (such as
temperature, pressure, density, etc.) In forecast discussions, thermodynamics usually
refers to the distribution of temperature and moisture (both vertical and horizontal) as
related to the diagnosis of atmospheric instability.
Theta-e (or Equivalent Potential Temperature) - The temperature a parcel of air would have
if a) it was lifted until it became saturated, b) all water vapor was condensed out, and
c) it was returned adiabatically (i.e., without transfer of heat or mass) to a pressure of
1000 millibars. Theta-e, which typically is expressed in degrees Kelvin, is directly
related to the amount of heat present in an air parcel. Thus, it is useful in diagnosing
atmospheric instability.
Theta-e Ridge - An axis of relatively high values of theta-e. Severe weather and excessive
rainfall often occur near or just upstream from a theta-e ridge.
Tilt Sequence - Radar term indicating that the radar antenna is scanning through a series
of antenna elevations in order to obtain a volume scan.
Tilted Storm or Tilted Updraft - A thunderstorm or cloud tower which is not purely
vertical but instead exhibits a slanted or tilted character. It is a sign of vertical wind
shear, a favorable condition for severe storm development.
*Tornado - A violently rotating column of air in contact with the ground and extending
from the base of a thunderstorm. A condensation funnel does not need to reach to the
ground for a tornado to be present; a debris cloud beneath a thunderstorm is all that is
needed to confirm the presence of a tornado, even in the total absence of a condensation
funnel.
Tornado Family - A series of tornadoes produced by a single supercell, resulting in damage
path segments along the same general line.
Total-Totals Index - A stability index and severe weather forecast tool, equal to the
temperature at 850 mb plus the dew point at 850 mb, minus twice the temperature at 500 mb.
The total-totals index is the arithmetic sum of two other indices: the Vertical Totals
Index (temperature at 850 mb minus temperature at 500 mb) and the Cross Totals Index (dew
point at 850 mb minus temperature at 500 mb). As with all stability indices there are no
magic threshold values, but in general, values of less than 50 or greater than 55 are
considered weak and strong indicators, respectively, of potential severe storm
development.
Tower - (Short for towering cumulus), a cloud element showing appreciable upward vertical
development.
Towering Cumulus - (Same as congestus.) A large cumulus cloud with great vertical
development, usually with a cauliflower-like appearance, but lacking the characteristic
anvil of a Cb. (Often shortened to "towering cu," and abbreviated TCU.)
Transverse Bands - Bands of clouds oriented perpendicular to the flow in which they are
embedded. They often are seen best on satellite photographs. When observed at high levels
(i.e., in cirrus formations), they may indicate severe or extreme turbulence. Transverse
bands observed at low levels (called transverse rolls or T rolls) often indicate the
presence of a temperature inversion (or cap) as well as directional shear in the low- to
mid-level winds. These conditions often favor the development of strong to severe
thunderstorms.
Transverse Rolls - Elongated low-level clouds, arranged in parallel bands and aligned
parallel to the low-level winds but perpendicular to the mid-level flow. Transverse rolls
are one type of transverse band, and often indicate an environment favorable for the
subsequent development of supercells. Since they are aligned parallel to the low-level
inflow, they may point toward the region most likely for later storm development.
T Rolls - [Slang], same as transverse rolls.
Triple Point - The intersection point between two boundaries (dry line, outflow boundary,
cold front, etc.), often a focus for thunderstorm development.
Triple point also may refer to a point on the gust front of a supercell, where the warm
moist inflow, the rain-cooled outflow from the forward flank downdraft, and the rear flank
downdraft all intersect; this point is a favored location for tornado development (or
redevelopment).
Tropopause - The upper boundary of the troposphere, usually characterized by an abrupt
change in lapse rate from positive (decreasing temperature with height) to neutral or
negative (temperature constant or increasing with height). See Fig. 6, sounding.
Troposphere - The layer of the atmosphere from the earth's surface up to the tropopause,
characterized by decreasing temperature with height (except, perhaps, in thin layers - see
inversion, cap), vertical wind motion, appreciable water vapor content, and sensible
weather (clouds, rain, etc.).
Trough - An elongated area of relatively low atmospheric pressure, usually not associated
with a closed circulation, and thus used to distinguish from a closed low. The opposite of
ridge.
Turkey Tower - [Slang], a narrow, individual cloud tower that develops and falls apart
rapidly. The sudden development of turkey towers from small cumulus clouds may signify the
breaking of a cap.
TVS - Tornadic Vortex Signature. Doppler radar signature in the radial velocity field
indicating intense, concentrated rotation - more so than a mesocyclone. Like the
mesocyclone, specific criteria involving strength, vertical depth, and time continuity
must be met in order for a signature to become a TVS. Existence of a TVS strongly
increases the probability of tornado occurrence, but does not guarantee it. A TVS is not a
visually observable feature.
-U-
UKMET - A medium-range numerical weather prediction model operated by the United Kingdom
METeorological Agency.
Updraft - A small-scale current of rising air. If the air is sufficiently moist, then the
moisture condenses to become a cumulus cloud or an individual tower of a towering cumulus
or Cb.
Updraft Base - Alternate term for a rain-free base.
Upper Level System - A general term for any large-scale or mesoscale disturbance capable
of producing upward motion (lift) in the middle or upper parts of the atmosphere. This
term sometimes is used interchangeably with impulse or shortwave.
Upslope Flow - Air that flows toward higher terrain, and hence is forced to rise. The
added lift often results in widespread low cloudiness and stratiform precipitation if the
air is stable, or an increased chance of thunderstorm development if the air is unstable.
Upstream - Toward the source of the flow, or located in the area from which the flow is
coming.
UVM (or UVV) - Upward Vertical Motion (or Velocity).
-V-
VAD - Velocity Azimuth Display. A radar display on which mean radial velocity is plotted
as a function of azimuth. See VWP.
Vault - Same as BWER.
Veering Winds - Winds which shift in a clockwise direction with time at a given location
(e.g., from southerly to westerly), or which change direction in a clockwise sense with
height (e.g., southeasterly at the surface turning to southwesterly aloft). The latter
example is a form of directional shear which is important for tornado formation. Compare
with backing winds.
Vertically-stacked System - A low-pressure system, usually a closed low or cutoff low,
which is not tilted with height, i.e., located similarly at all levels of the atmosphere.
Such systems typically are weakening and are slow-moving, and are less likely to produce
severe weather than tilted systems. However, cold pools aloft associated with
vertically-stacked systems may enhance instability enough to produce severe weather.
VIL - Vertically-Integrated Liquid water. A property computed by RADAP II and WSR-88D
units that takes into account the three-dimensional reflectivity of an echo. The maximum
VIL of a storm is useful in determining its potential severity, especially in terms of
maximum hail size.
VIP - Video Integrator and Processor, which contours radar reflectivity (in dBZ) into six
VIP levels:
VIP 1 (Level 1, 18-30 dBZ) - Light precipitation
VIP 2 (Level 2, 30-38 dBZ) - Light to moderate rain.
VIP 3 (Level 3, 38-44 dBZ) - Moderate to heavy rain.
VIP 4 (Level 4, 44-50 dBZ) - Heavy rain
VIP 5 (Level 5, 50-57 dBZ) - Very heavy rain; hail possible.
VIP 6 (Level 6, >57 dBZ) - Very heavy rain and hail; large hail possible.
*Virga - Streaks or wisps of precipitation falling from a cloud but evaporating before
reaching the ground. In certain cases, shafts of virga may precede a microburst; see dry
microburst.
V Notch - A radar reflectivity signature seen as a V-shaped notch in the downwind part of
a thunderstorm echo. The V-notch often is seen on supercells, and is thought to be a sign
of diverging flow around the main storm updraft (and hence a very strong updraft). This
term should not be confused with inflow notch or with enhanced V, although the latter is
believed to form by a similar process. See supercell.
Volume Scan - A radar scanning strategy in which sweeps are made at successive antenna
elevations (i.e., a tilt sequence), and then combined to obtain the three-dimensional
structure of the echoes. Volume scans are necessary to determine thunderstorm type, and to
detect features such as WERs, BWERs, and overhang.
Vorticity - A measure of the local rotation in a fluid flow. In weather analysis and
forecasting, it usually refers to the vertical component of rotation (i.e., rotation about
a vertical axis) and is used most often in reference to synoptic scale or mesoscale
weather systems. By convention, positive values indicate cyclonic rotation.
Vort Max - (Slang; short for vorticity maximum), a center, or maximum, in the vorticity
field of a fluid.
VWP - VAD Wind Profile. A radar plot of horizontal winds, derived from VAD data, as a
function of height above a Doppler Radar. The display is plotted with height as the
vertical axis and time as the horizontal axis (a so-called time-height display), which
then depicts the change in wind with time at various heights. This display is useful for
observing local changes in vertical wind shear, such as backing of low-level winds,
increases in speed shear, and development or evolution of nearby jet streams (including
low-level jets).
This product often is referred to erroneously as a VAD.
-W-
*Wall Cloud - A localized, persistent, often abrupt lowering from a rain-free base. Wall
clouds can range from a fraction of a mile up to nearly five miles in diameter, and
normally are found on the south or southwest (inflow) side of the thunderstorm. When seen
from within several miles, many wall clouds exhibit rapid upward motion and cyclonic
rotation. However, not all wall clouds rotate. Rotating wall clouds usually develop before
strong or violent tornadoes, by anywhere from a few minutes up to nearly an hour. Wall
clouds should be monitored visually for signs of persistent, sustained rotation and/or
rapid vertical motion. See supercell.
"Wall cloud" also is used occasionally in tropical meteorology to describe the
inner cloud wall surrounding the eye of a tropical cyclone, but the proper term for this
feature is eyewall.
Warm Advection - Transport of warm air into an area by horizontal winds.
Low-level warm advection sometimes is referred to (erroneously) as overrunning. Although
the two terms are not properly interchangeable, both imply the presence of lifting in low
levels.
Warning - A product issued by NWS local offices indicating that a particular weather
hazard is either imminent or has been reported. A warning indicates the need to take
action to protect life and property. The type of hazard is reflected in the type of
warning (e.g., tornado warning, blizzard warning). See short-fuse warning.
Watch - An NWS product indicating that a particular hazard is possible, i.e., that
conditions are more favorable than usual for its occurrence. A watch is a recommendation
for planning, preparation, and increased awareness (i.e., to be alert for changing
weather, listen for further information, and think about what to do if the danger
materializes).
Watch Box (or Box) - [Slang], a severe thunderstorm or tornado watch.
Waterspout - In general, a tornado occurring over water. Specifically, it normally refers
to a small, relatively weak rotating column of air over water beneath a Cb or towering
cumulus cloud. Waterspouts are most common over tropical or subtropical waters.
The exact definition of waterspout is debatable. In most cases the term is reserved for
small vortices over water that are not associated with storm-scale rotation (i.e., they
are the water-based equivalent of landspouts). But there is sufficient justification for
calling virtually any rotating column of air a waterspout if it is in contact with a water
surface.
Wedge (or Wedge Tornado) - [Slang], a large tornado with a condensation funnel that is at
least as wide (horizontally) at the ground as it is tall (vertically) from the ground to
cloud base.
The term "wedge" often is used somewhat loosely to describe any large tornado.
However, not every large tornado is a wedge. A true wedge tornado, with a funnel at least
as wide at the ground as it is tall, is very rare.
Wedges often appear with violent tornadoes (F4 or F5 on the Fujita Scale), but many
documented wedges have been rated lower. And some violent tornadoes may not appear as
wedges (e.g., Xenia, OH on 3 April 1974, which was rated F5 but appeared only as a series
of suction vortices without a central condensation funnel). Whether or not a tornado
achieves "wedge" status depends on several factors other than intensity - in
particular, the height of the environmental cloud base and the availability of moisture
below cloud base. Therefore, spotters should not estimate wind speeds or F-scale ratings
based on visual appearance alone. However, it generally is safe to assume that most (if
not all) wedges have the potential to produce strong (F2/F3) or violent (F4/F5) damage.
WER - Weak Echo Region. Radar term for a region of relatively weak (reflectivity at low
levels on the inflow side of a thunderstorm echo, topped by stronger reflectivity in the
form of an echo overhang directly above it. The WER is a sign of a strong
updraft on the inflow side of a storm, within which precipitation is held aloft. When the
area of low reflectivity extends upward into, and is surrounded by, the higher
reflectivity aloft, it becomes a BWER.
Wet Microburst - A microburst accompanied by heavy precipitation at the surface. A rain
foot may be a visible sign of a wet microburst. See dry microburst.
Wind Shear - See shear.
Wrapping Gust Front - A gust front which wraps around a mesocyclone, cutting off the
inflow of warm moist air to the mesocyclone circulation and resulting in an occluded
mesocyclone.
WSR-57, WSR-74 - NWS Weather Surveillance Radar units, replaced by WSR-88D units.
WSR-88D - Weather Surveillance Radar - 1988 Doppler; NEXRAD unit.
-Z-
Zonal Flow - Large-scale atmospheric flow in which the east-west component (i.e.,
latitudinal) is dominant. The accompanying meridional (north-south) component often is
weaker than normal. Compare with meridional flow.
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Glossary (A-B)
Glossary (C-H)
Glossary (I-R)
Glossary (S-Z)
Acknowledgements
The following individuals were instrumental in contributing, through helpful
comments and suggestions: David Andra, Dave Beusterien, Dr. Harold Brooks, Bill Bunting,
Don Burgess, E. Brian Curran, Dr. Charles A. Doswell III, Mike Emlaw, Mike Foster, Dave
Gold, Paul Janish, Tim Marshall, Alan Moller, Mike Morgan, Steve Parker, Steve Piltz,
Robert Prentice, Jim Purpura, Gene Rhoden, Lans Rothfusz, Dan Smith, Greg Stumpf, Steve
Vasiloff. Steve Nelson and Doug Speheger both were instrumental in setting up the glossary
on the WSFO Norman home page.
Bibliography
- American Meteorology Society, 1990: Glossary of Meteorology.
American Meteorological Society Press, Boston.
- Caracena, Fernando, Ronald L. Holle, and Charles A. Doswell III,
1989: Microbursts - A Handbook for Visual Identification. NOAA, Environmental
Research Laboratories, National Severe Storms Laboratory.
- Doswell, Charles A. III, 1982: The Operational Meteorology of
Convective Weather. Volume I: Operational Mesoanalysis. NOAA Technical Memorandum NWS
NSSFC-5.
- Doswell, Charles A. III, 1985: The Operational Meteorology of
Convective Weather. Volume II: Storm Scale Analysis. NOAA Technical Memorandum ERL
ESG-15.
- Fujita, T. T., 1985: The Downburst - Microburst and Macroburst.
SMRP Research Paper No. 210, University of Chicago, 122 pp.
- Marshall, Tim, 19--: Storm Chase Manual. Published
annually in association with Storm Track. Contact: 1336 Brazos Blvd, Lewisville TX
75067.
- Marshall, Tim (Editor): Storm Track. Published
bi-monthly by Master Graphics, Lewisville TX. Contact the editor, 1336 Brazos Blvd,
Lewisville TX 75067, for subscription information.
- National Weather Service, 1982: Spotter's Guide for
Identifying and Reporting Severe Local Storms. Available from most National Weather
Service offices, or from the U. S. Dept. of Commerce, NOAA, Rockville MD 20852.
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