4 Ingredients Needed For Severe Weather

Moisture, Instability, Lift, and Shear

 

Anvil

Flat spreading top of a cumulonibus, often shaped like an anvil.

 

Back-Building Thunderstorm

A thunderstorm in which new development takes place on the upwind side of a thunderstorm (usually the west or southwest side) so the storm seems to remain stationary or seems to build backwards.

 

Bear's Cage (slang)

An rotating area inside a thunderstorm, usually wrapped in heavy precipitation (and large hail).  This area is often associated with the hook echo or mesocyclone on radar.

 

Beaver Tail (slang)

An inflow band with a long flat appearance, similar to the tail of a beaver, attached to a storm's updraft.

 

Bow Echo

A radar echo, showing linear storms, which looks similar to a bow (bow & arrow).  The most damaging winds often occur near the center or leading part of the bow echo.

 

Cap (Capping Inversion)

A layer of warm air aloft that suppress the development of thunderstorms.  Usually the higher up in the atmosphere you go, the colder the air gets.  This allows warm moist air to rise and thunderstorms to develop.  However, if warm moist air starts to rise, and runs into another layer of warm air, it cannot continue to lift.  This will prevent thunderstorm initiation.

 

CAPE (Convective Available Potential Energy)

A measure of the amount of energy available for convection.  Measured in joules per kilogram (j/kg), the higher the CAPE, the more unstable the atmosphere.

 

Clear Slot

A region of clearing skies, indicating an intrusion of drier air, often seen on the backside of a wall cloud.  It is a visual indication of a rear-flank downdraft.

 

Convection

Vertical transport of heat and moisture (by updrafts and downdrafts) in an unstable atmosphere.  Thunderstorms are one form of convection.

 

Convective Outlook

A forecast of expected thunderstorm development (slight risk, moderate risk, high risk).  Updated and issued by the Storm Prediction Center several times per day.

 

Core Punch (slang)

Penetration, by vehicle, into the heavy precipitation core of a thunderstorm (Bear's Cage).

 

CWA

County Warning Area.  A specific geographic area served by a particular National Weather Service office.

 

Debris Cloud

A rotating cloud of dust or debris, on or near the ground, beneath a funnel cloud or around the base of a tornado.

 

Derecho

A widespread, fast-moving, long-lived windstorm associated with convection.  Derechos can produce damaging straight-line winds and can be hundreds of miles long.

 

Dew Point

A measure of moisture in the atmosphere.  The dew point is the temperature at which air will condense.

 

Directional Shear

The change in wind direction with height.

 

Downburst

A strong downdraft resulting in an outward burst of damaging winds on or near the ground. 

 

Downdraft

A small-scale column of air that rapidly sinks towards the ground.  A downburst is the result of a strong downdraft.

 

Enhanced Fujita Scale

A scale for estimating the wind speed of a tornado by the damage it caused.

 

Front

A boundary between two different air masses of different densities.

 

Funnel Cloud

A rotating column of air extending from the base of a thunderstorm, NOT in contact with the ground.

 

Gust Front

The leading edge of gusty surface winds from a thunderstorm, sometimes associated with a shelf cloud or downburst.

 

Hook Echo

A radar echo characterized by a hook-shape and usually found on the right, rear part of the storm.  It is an indication of a mesocylone and indicates that conditions are favorable for the development of a tornado.

 

Humidity

A measure of the water vapor content in the air.

 

HWO

Hazardous Weather Outlook.  A text product issued by the National Weather Service.

 

Inflow Bands

Bands of low clouds, moving towards or into a thunderstorm.

 

Inflow Notch

A radar signature characterized by an indentation in the reflectivity patter on the inflow side of a thunderstorm.  It is often V-shaped.

 

Instability

The tendency for a parcel of air to accelerate when displaced from its original position, especially the tendency to accelerate upward after being lifted.

 

Jet Stream

Strong, high-altitude winds concentrated in a narrow stream.  General weather patterns are closely associated with the position, strength, and orientation of the jet stream.  A jet stream at low levels of the atmosphere is known as a low-level jet.

 

Lifted Index

A common measure of atmospheric instability.  When computed, it is the temperature the air near the ground would be if lifted to a higher level (usually around 18,000 feet) and compared to the current temperature at that level.  Negative amounts indicate instability and the more negative the number is, the more unstable the air is, and the stronger the updrafts are likely to be within a developing thunderstorm.

 

Mammatus

Rounded, smooth, sack-like protrusions hanging from the underside of a cloud (usually a thunderstorm anvil).  Mammatus often accompany severe weather, but are not producers of it.

 

Mesocyclone

A storm-scale region of rotation, typically 2-6 miles in diameter, and often found in the right rear flank of a supercell.  The circulation of a mesocyclone is much larger than a tornado that may develop within it.

 

Microburst

A small, concentrated downburst affecting an area less than 2 1/2 miles wide.

 

Moisture Advection

That transport of moisture by horizontal winds.

 

Multi-cellular Thunderstorm

A thunderstorm consisting of two or more cells, of which most are often visible as domes or towers in various stages of development.

 

Multiple Vortex Tornado

A tornado in which two or more condensation funnels or debris clouds are visible at the same time, often rotating around a common center.  It looks like a large (somewhat clear) tornado, with several smaller tornadoes inside it.

 

NCEP

National Centers for Environmental Prediction

 

NOAA

National Oceanic and Atmospheric Administration

 

NSSL

National Severe Storms Laboratory

 

NWS

National Weather Service

 

Outflow Boundary

A storm-scale boundary separating thunderstorm cooled air (outflow) from surrounding air.  Similar to a cold front.

 

Overshooting Top

A dome-like protrusion above a thunderstorm anvil, representing a very strong updraft and likely a higher potential for severe weather.

 

PDS (Potentially Dangerous Situation)

It is enhanced wording, usually associated with a tornado watch, indicating a potential for strong or violent tornadoes, or widespread wind damage.

 

Rain Foot (slang)

A horizontal bulge near the surface in a precipitation shaft, forming the shape of a foot.  Usually an indication of a wet microburst.

 

Rain Free Base

The base of a thunderstorm updraft area with no visible precipitation.  This is the area that wall clouds and tornadoes usually form.  It is now referred to as the Updraft Base because although no rain falls in this area, the rotation of a storm can sometimes wrap the area in rain.  This will obscure the view of a wall cloud or tornado.

 

RFD (Rear Flank Downdraft)

A region of dry air subsiding on the back side of, and wrapping around a mesocyclone.  It is often visible as a clear slot wrapping around the wall cloud.  This area can contain strong winds.

 

Roll Cloud

A low, horizontal tube-shaped cloud associated with a thunderstorm gust front.  Roll clouds are completely detached from the thunderstorm base and other clouds, making them different from shelf clouds.

 

Scud

Small, ragged, low cloud fragments that are not attached to a cloud base.

 

Shear

A variation in wind speed and direction with height over a short distance.

 

Shelf Cloud

A low, horizontal wedge shaped cloud, associated with a thunderstorm gust front.  A shelf cloud differs from a roll cloud in that it is attached to the cloud above it (usually a thunderstorm).  Rising cloud motion can often be seen on the leading (outer) part of the cloud (and it can look smooth), while the underside of the cloud often appears turbulent and wind-torn.

 

SLC's (Scary Looking Clouds) (slang)

Clouds that can often look like storm clouds (such as funnel clouds or wall clouds) but are not.  They are often mistakenly called in by spotters when the spotter fails to identify them properly.  Knowing where you are at in relation to the storm, and know what storm structure you are observing can help to eliminate this issue.  As an example, a shelf cloud often has what is referred to as "shark's teeth" on the underside of it and these "teeth" can look a lot like a funnel cloud.  However, a shelf cloud is outflow dominant and usually on the leading edge of the storm, and usually won't be accompanied by a tornado.  Tornadoes usually develop in the updraft (inflow) area of a storm and are usually located on the tailing end of the storm.

 

SPC

Storm Prediction Center

 

Squall Line

A solid or nearly solid band of active thunderstorms.

 

Straight-Line Winds

Any wind that is not associated with rotation.  Straight-line winds can be very powerful and cause severe damage.

 

Supercell

A thunderstorm with a persistent, rotating updraft.

 

Tornado

A violently rotating column of air in contact with the ground and extending from the base of a thunderstorm.  The condensation of the funnel does not need to reach all of the way to the ground for a tornado to be present.  A debris cloud is all that is needed to confirm the presence of a tornado.

 

Triple Point

The intersection point between two boundaries, and often the focus for severe 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).

 

TVS (Tornado Vortex Signature)

Doppler radar signature indicating intense, concentrated rotation.  Existence of a TVS strongly increases the probability of a tornado occurring, however is does not guarantee it.  A TVS is a radar feature and is not a visually observable feature of a storm.

 

Updraft

A small-scale current of rising air.

 

Updraft Base

The base of a thunderstorm updraft area with no visible precipitation.  This is the area that wall clouds and tornadoes usually form.  While rain doesn't fall in this area of the storm, it can become rain wrapped from the storm's rotation.

 

Virga

Streaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground.

 

Wall Cloud

A localized, persistent, abrupt lowering from the Updraft Base of a thunderstorm, where funnel clouds and tornadoes usually develop.  Wall clouds exhibit rapid upward motion and may or may not rotate.

 

Warning

A NWS product indicating that a particular weather hazard is imminent or has been reported.  A warning indicates the need to take action to protect life and property.

 

Watch

A NWS product indicating that a particular weather hazard is possible.  A watch is a recommendation for planning and increased awareness.