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NOAA Technical Memorandum NWS SR-145









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).



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 (Fig. 6). 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.


Spin-up - [Slang], a small-scale vortex initiation, such as what may be seen when a gustnado, landspout, or suction vortex forms.


*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.




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 (Fig. 7). Storms exhibiting these characteristics often are called classic supercells; however HP storms (Fig. 3) and LP storms (Fig. 5) also are supercell varieties.



  • Td 850 is the dew point temperature at 850 mb,
  • TT is the total-totals index,


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.


*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 Fig. 7, 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. (See Fig. 6, sounding.)




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.



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.



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.



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.


UKMET - A medium-range numerical weather prediction model operated by the United Kingdom METeorological Agency.


Updraft Base - Alternate term for a rain-free base.



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).


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.




VIP - Video Integrator and Processor, which contours radar reflectivity (in dBZ) into six VIP levels:






Vort Max - (Slang; short for vorticity maximum), a center, or maximum, in the vorticity field of a fluid.


This product often is referred to erroneously as a VAD.
















Wind Shear - See shear.







The author expresses his most sincere thanks to the many individuals who provided input to the glossary. 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.


  • 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.

