Severe Weather Glossary

AC (Anticipated Convection)- The convective outlook issued by the Norman, Oklahoma Storm Prediction Center for three consecutive days.

ACCAS- The acronym for Altocumulus Castellanus clouds. ACCAS are mid-level convective clouds with widespread vertical development that may precede rapidly developing thunderstorms. Almost entirely composed of water vapor, these clouds exist at around 6,500 to 23,000 feet and have billowy tops and relatively high bases that indicate instability in the atmosphere.

Accessory Cloud- These clouds depend on larger cloud systems for sustainment. A wall cloud is an example of an accessory cloud.

Air-Mass Thunderstorm- Your garden-variety late summer day thunderstorm that develops due to the warm, humid summer air late in the afternoon. They typically dissipate after sunset and are not severe. They are not associated, as a rule, with fronts or other type of initiating mechanism.

Anvil Crawler- A slang term for the discharge of lightning within one or more channels crawling underside a thunderstorm's anvil.

AP (Anomalous Propogation)- Radio wave transmission forward in space occurring in non-standard conditions in the atmosphere.

 Arcus- A shelf cloud that is arched, dense, and occurs near the leading edge of a thunderstorm gust front due to warm, stable airflow..

AVN (Aviation model)- An obsolete forecast model, replaced by GFS or Global Forecast System, running at the National Centers for Environmental Prediction.

Back-building Thunderstorm- West or Southwest upwind development is the hallmark of this seemingly stationary or backward direction thunderstorm.

Backing Winds- Counterclockwise-shifting winds coming from a certain direction, like North to Northeast, or winds being shifted to a certain direction in a heightened counterclockwise fashion, East surface wind becoming westerly aloft. Backing winds are the opposite of veering winds and increase tornado potential as low level directional shear is increased.

Barber Pole- A slang term for pronounced cloud striations during a thunderstorm updraft that is similar to the appearance of a barber pole.

Baroclinic Zone- Regions where temperature grades are present on a pressure surface that is constant. Systems that are strengthening or weakening prefer this wind shear characteristic zone.

Barotropic System- Temperature and pressure surfaces are uniform in this weather system known for its lack of wind shear unfavorable for thunderstorm development.

Beavers Tail- A slang term for a type of inflow band that has the appearance of a beaver's tail. Beavers tail bands are not attached to wall clouds and run roughly parallel to a pseudo-warm front. Its shape changes as the inflow strength changes. Its cloud base is around the height of the updraft base.

Boundary Layer- The lowest one to two kilometers of atmosphere, this layer of air is adjacent to the planet or boundary surface. Daytime insulation and the radial cooling of the night are the temperatures most affected within this layer. Winds are also affected here by the surface of the earth as the friction is significant.

Bulk Richardson Number- The CAPE ratio of a parcel lifted to the vertical wind shear of its environment that it was lifted. For CAPEs 1500 to 3000 J/kg, it the number sizes up well with the type of thunderstorm. BRNs <45 support supercells, the others support BRNs >45. BRNs are bulk measures, so storm rotation is poorly supported.

BWER- Stands for Bounded Weak Echo Region. The acronym is a radar feature identifying a supercell's strongest updraft where particles cannot form in the lower and middle thunderstorm levels .

Cap- The cap measures the ability of a layer of warm air to restrain low-level parcel ascent. It separates the lower warm, moist air from the higher, colder, drier air, making it a very important measure of instability and the potential for severe thunderstorms. Thunderstorms growing quickly near strongly capped regions become severe thunderstorms.

Cell- Single or couplet up or downdraft convection seen in the cumulus cloud's vertical tower. Typically, thunderstorms are multi-cellular. Cells can also be indicated by radar echo.

Chaff- Small metal foil strips dropped in great quantity from balloons or aircraft that produce radar echos that are similar to precipitation. Such drops are used for testing and calibrating radars, although the military formerly use them to confuse the enemy's radar equipment.

Cirrus- Feather-like, thin ice-crystal clouds that form at around 16,500 to 45,000 feet. Although most cirrus clouds are fair weather clouds, thunderstorm anvils are a type of cirrus cloud.

Clear Slot- Higher-based, bright, dry area, a West or Southwest region of clearing skies and less cloud cover that may be a visual clue to a rear flank downdraft.

Cold Advection- Horizontal winds transporting colder air into a region.

Cold-air Funnel- A type of funnel cloud or small, weaker tornado developing from unusually cold air aloft in the atmosphere.

Collar Cloud- Although sometimes used incorrectly as a synonym for a wall cloud, this circular cloud ring rarely surrounds the upper regions of a wall cloud.

Comma Cloud- A large scale, comma-shaped cloud pattern associated with low-pressure systems that are large and intense.

Confluence- An inward wind flow pattern toward an axis parallel to the flow direction. Confluence is the antonym of difluence and not the same as convergence.

Convection- Used interchangeably with the term thunderstorm, convection is the vertical heat and moisture transported by liquid movement in up and downdrafts when the atmosphere is unstable. Thunderstorms are actually only one type of convection. Visible forms of convention also include cumulonimbus, towering cumulus and ACCAS clouds. Without the cloudy visual, convection is called dry convection. When clouds are present, convection is moist convection.

Cumulus- Characterized by mostly flat bases and domed tops, these clouds are formed in convective currents and do not form precipitation.

Cyclic Storm- These storms undergo cycles of intensity and weakening, called pulses, while sustaining itself as a storm. Cyclic supercells can produce several severe weather bursts as well as multiple tornados, while single cycle storms are called pulse storms.

dBZ- The reflectivity factor, known by this logarithmic equation- dBZ = 10 log ( z / 1 mm⁶ m³).

Delta T- The Delta T represents the mean lapse rate within an atmospheric layer. This is calculated by the difference between observed temperatures at the bottom and top layer of the atmospheric layer between the pressures of 700 and 500 megabars. Values >18 indicate instability sufficient for developing thunderstorms.

Derecho- A derecho is a windstorm associated with convection that is widespread and a fast-mover. It can produce straight line winds that can damage property over regions that are hundreds of miles long and over 100 miles across. The two types of derecho are progressive and serial. Derecho is pronounced "day-RAY-cho."

Dew Point- Along with humidity, the dew point is a measure of moisture in the atmosphere. The dew point is a temperature reading of the air which must be cooled at constant pressure and moisture content so that saturation can happen. The higher the dew point, the higher the amount of water vapor in the air mass. When the dew point and the temperature are the same, fog will occur.

Doppler Radar- The powerfully sensitive Doppler radar, also known as WSR-88D, was created in 1988 to obtain such meteorological readings as- radial velocity, standard reflectivity echoes, wind speed and atmospheric pictures as well as timed storm total precipitation images.

Dry Line- Located typically north to south across the central and southern Plains states in the spring and early summer months as the boundary separates warm, moist Gulf Air from the colder, drier desert air. The dry line moves east during the afternoon and west during the night. The dry line often triggers severe weather such as supercells.

Elevated Convection- This is a thunderstorm occurring on the cold surface of a cold front. They form aloft a very stable atmospheric layer. Severe weather is possible with an elevated convection.

Energy Helicity Index- Used to forecast supercells, this index utilizes both vertical shear and instability. Its equation is- EHI = (CAPE x SRH)/160,000. The higher the value, the more unstable the condition and strong vertical shear. Values over one show a higher threat of tornadic activity, values equal or greater than five are rare, and indicate the potential for violent tornadoes. The index just measures the potential, not the certainty.

Equilibrium Level- In the upper troposphere, the temperature of a parcel of saturated air becomes equal to the environment.

Feeder Bands- Feeder bands are the same as inflow bands, lines of low clouds that feed into an updraft of a convection from east to south.

Front- The transition area between two different density air masses. Those two air masses will have different temperatures. Types of fronts are dependent on the advancing air- cold, occluded, stationary and warm fronts.

Fujita Scale- The University of Chicago's Dr. Theodore Fujita devised this older method of classifying tornadoes by strength from F0, the weakest, to F5, the most violent. This scale has been replaced by the Enhanced Fujita Scale.

Gust Front- A gust front forms when a thunderstorm's rain-cooled air and the down draft reaches the ground and spreads out. Also associated with a shelf cloud, gustnado, and downburst.

Gustnado or Gustinado- (slang) A gust front tornado, usually very weak and quickly dissipated. More associated with shelf than wall clouds. No storm-scale rotation. 

Helicity- Corkscrew patterned property of a moving fluid proportional to flow strength, vertical wind shear, and vorticity, or turn in the air flow. Measured relative to a storm's motion, helicity is computed usually from the ground to three kilometers up into the atmosphere.

Hodograph- Vertical wind tip vectors in the lowest 7000 meters of the atmosphere is graphed in this polar coordinate plane. As the space between dots increase, wind increases.

HP Storm- Also known as a high-precipitation supercell in which the heavy precipitation falls on the trailing mesocyclone side making tornado identification very difficult and dangerous. Storm chasers are always advised never to monitor an HP storm too closely for that reason.

Humidity- Known as relative humidity, it is a measure of water vapor in the air, much like the dew point.

Inflow Bands- Moving toward or into a convection, these low bands of clouds are parallel to the low-level winds. A bowed inflow band may indicate tornadic rotation.

Insolation- Sunshine and its ensuing heat.

Instability- A prerequisite for severe weather, instability in the atmosphere is when the vertical temperature distribution allows warm air to rise and to accelerate.

Inversion- Temperature inversion, or an increase in temperature with height. Present in the lower cap.

Isobar- Barometric lines of pressure that are equal and depicted on a weather map.

Jet Stream- Strong winds that control the movement of high and low pressure systems and their front along a narrow band that can reach wind speeds of over 200 mph. The jet stream is usually found around 30 to 40,000 feet into the atmosphere.

Knuckles- A slang tern describing lumpy cloud formations on the underside of a thunderstorm anvil; not Mammatus clouds. They indicate a very strong updraft.

Land Spout- (slang) A tornado that does not emanate from organized storm rotation.

Lapse Rate- Atmospheric variable change, usually height and temperature. Steep lapse rates imply instability. Some rates- global-6.5°C/km; adiabatic-the normal rate of change (9.8°C/km); moist adiabatic lapse rate-(4.9°C/km).

LEWP- Line Echo Wave Pattern.    

Lifted Index- Determines thunderstorm potential. A lifted index < -5 means that air is very unstable and that there is a potential for heavy or strong thunderstorms. Thunderstorms are probable when the index reads -3 to -5, while 0 to -2 means that thunderstorms are possible as the air is only marginally unstable.

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).

LSR- The acronym for a Local Storm Report that the National Weather Service produces to inform of significant and severe weather events.

Mammatus Clouds- These smooth, round, sack-like clouds protrude from the underside of a cloud or thunderstorm anvil. They accompany, but do not produce severe weather.

MCC- Mesoscale Convective Complex.

MCS- Mesoscale Convective System.

Meridonial Flow- Large north to south, longitudinal atmospheric flow in which the zonal flow which runs east to west is weaker than normal.

Mesocyclone- A radar-indicated rotation in which there are three types of azimuthal shear patterns including the uncorrelated and 3D Correlated Shears. The 3D shear is indicated on the radar as a thin, yellow circle.

Moisture Convergence- Measure of the moisture converging into a region that takes converging winds and moisture advection into account. If persistent, the region is considered favorable for thunderstorms if instability is present.

Morning Glory- A long cloud band appearing during the early morning, atmospherically stable hours.

NCEP- Stands for the NOAA's National Centers for Environmental Prediction. Its mission is to mitigate economic loss and to protect life and property through providing accurate national weather forecasts for up to seven days. Climate predictions are created for two-week time frames up to one year. There are nine national centers that make up the NCEP, which was established in 1958.

NGM- Pressure/precipitation forecast model that plots in 12-hour intervals up to 48 hours at a time.

NOAA- The acronym for the National Oceanographic and Atmospheric Administration that is the umbrella for the NCEP.

Orographic- Physical geography-related information.

Outflow Boundary- Similar to a cold front, the outflow boundary separates thunderstorm cooled-air from the surrounding air. Outflows may exist for up to a day after a thunderstorm and may travel hundreds of miles.

Overhang- High reflectivity region of a radar at the mid and upper levels of a weak-echo region. Found on the inflow side of a convection.

Positive Area- The layer that a lifted parcel is warmer than its surrounding environment. The measure of convection energy that is available. Also see CAPE.

Potential Temperature- The temperature a dry air parcel would need if brought to a standard pressure of 1000 mb.

Pseudo-Cold Front- A form of gust front that is characterized by the advancement of the downdraft toward the inflow.

Pseudo-Warm Front- Boundary between supercell inflow and forward-flank downdraft region extending outward from a mesocyclonic center.

Pulse Storm- A convection where a brief updraft occurs during and immediately after a short bout of severe weather within a thunderstorm. Usually produces large hail and damaging winds, but not tornadoes.

Radial Velocity- Also known as Doppler velocity, this motion is what the Doppler radar detects parallel to the radar beam.

Rain Foot- (slang) A horizontal bulge near the precipitation shaft, indicating a wet microburst.

Rear Flank Downdraft- Dry air region on the back of and wrapping around of a mesocyclone. May be indicated on radar as a hook.

Relative Humidity- Same as humidity, expressed as a percent of the amount of moisture in the air relative to the moisture that would be in the air if it were saturated.

Retrogression- A weather system moving in the opposite direction of the basic embedded flow. Usually a westward movement of air.

Rope Cloud- Also called a rope funnel, a narrow, string-like band of clouds often appearing along a front or boundary.

Scud- Associated with convection outflow and cool moist air, these small, ragged, clouds are fragmented and low, while unattached to a larger base cloud. Usually noted behind cold and convection gust fronts.

SELS- The acronym for the Severe Local Storms Unit which is the previous name of the Norman, Oklahoma's Storm Prediction Center Operations Branch.

Severe Thunderstorm- A convection is considered severe is winds reach 58 mph or more which can result in property damage, and at least one inch hail, or a tornado.

Shelf Cloud- Attached to a parent cloud base, shelf clouds are low, horizontal, and wedge-shaped arcus clouds that exist with a convection gust front, cold front, or no thunderstorm at all. Is accompanied by straight-line winds and precipitation.

Shortwave- Also called a shortwave trough, this is a mid or upper atmospheric disturbance inducing upward motion in advance of the disturbance. Along with instability, this upward motion can precipitate the development of a thunderstorm.

Speed Shear- The change in wind speed and height. A speed shear is an important part of the wind shear that factors into severe weather development in the mid to upper atmospheric levels.

Splitting Storm- A convection splitting into two storms with diverging paths. The right sided storm is more likely to become a supercell as it moves slower than the left sided storm.

Stratiform- Clouds with much horizontal development that cover great areas but have little vertical development.

Stratocumulus- Globulous masses of clouds full of water vapor that form from stratus clouds as they are breaking with cumulus clouds. They exist up to 6,500 feet from the ground.

Stratus- A uniform, low, sheet cloud made up of mostly water vapor that is located up to 6,500 feet. Fog is a stratus cloud that has its foundation on the ground.

Supercell- A convection with an updraft that is rotating and persistent. They generally move right and are rare, but are responsible for most severe weather events like tornadoes, large hail and other damaging winds.

Tail Cloud- Not a funnel cloud, a tail cloud extends from the precipitation cascade supercell region toward the wall cloud at low levels. Motion is toward the wall cloud and away from the precipitation.

Thermodynamics- Temperature and moisture distribution as related to instability in the atmosphere.

Tornado- Violent, rotating air column that connects with the ground and extends from a convection base.

Transverse Bands- Clouds perpendicular to the air flow where they are embedded.

Triple Point- The intersection between two boundaries or fronts. This is a focus for thunderstorm development.

Troposphere- Characterized by decreasing temperature, vertical wind motion and water vapor, the troposphere extended from the ground to the tropopause.

Trough- The opposite of a ridge, the trough is a long area of low atmospheric pressure.

Updraft- Vertical air currents.

Upper Level System- Umbrella term for any large mesoscale disturbance in the mid to upper atmosphere. Used interchangeably with shortwave.

Veering Winds- Indicative of warm air advection, veering winds change in a clockwise direction with time at a certain location or due to height.

Vertically-stacked System- A closed or cutoff low pressure system not tilted with height and that are less likely to produce severe weather.

Volume Scan- Successive sweeps at radar antenna elevations that are combined with three-dimensional echo structures.

Vorticity- Local rotation vector measure in a fluid flow, referring to rotation on a vertical axis that if are measured positively may indicate cyclonic rotation.

Wall Cloud- Formed during supercells, a wall cloud is localized, persistent, and abrupt. They range from hundreds of feet to almost five miles in diameter and are found on the inflow side of a convection.

Warning- Issued by the National Weather Service to indicate that severe weather is present in the local area and that action must be taken to protect life and property.

Waterspout- A rotating column of air over a body of water that is violent. Once on shore, they usually dissipate.

Wedge- (slang) A rare, large tornado that is as wide as it is tall and has a condensation funnel.

Zonal Flow- Latitudinal atmospheric or east-west flow of air. The accompanying meridian flow is usually weaker.

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