Shelf cloud picture taken at Riverton NWS office on May 18, 2018.

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Severe Weather Terminology

Funnel Cloud: A funnel-shaped cloud, extended outward or downward from a thunderstorm, that corresponds to a rotating column of air. If the rotation is violent and reaches the ground, the funnel cloud is associated with a tornado.

Tornado: A violently rotating column of air, in contact with the ground, that extends from the base of a thunderstorm to the ground. This is often visible as a funnel cloud with swirling dust or debris near the surface.

Severe Thunderstorm: A thunderstorm that produces hail one inch or larger in diameter (quarter size) and/or a wind gust to 58 mph or higher.

Microburst: A convective downdraft with an affected outflow area of less than 2½ miles wide and peak winds lasting less than 5 minutes. Microbursts may induce dangerous horizontal/vertical wind shears, which can adversely affect aircraft performance and cause property damage.

Straight-line Winds: Generally, any wind that is not associated with rotation, used mainly to differentiate them from tornadic winds.

Flash Flood: A sudden inundation of water in low-lying areas, usually brought on by heavy rain, dam break, rapid snowmelt or ice jams.

Watch: The potential exists for severe weather to occur within the next 8 hours but the exact location and timing is not known. Action can be taken to protect property such as putting your vehicle in the garage, putting away patio furniture, etc.

Warning: Severe weather either is occurring or will be shortly. Immediate action should be taken to protect yourself by going to the lowest portion of a sturdy building, or into a closet, hallway or room without windows.

High Wind Safety Information

High winds can be produced by severe thunderstorm(s) or larger scale non thunderstorm winds i.e. downslope Foehn Winds, accelerated gap flow, mountain waves etc.  High Winds are sustained winds over 58 mph.

Severe Thunderstorm(s) Winds

High winds produced by thunderstorm(s) are usually referred to as straight line winds and can exceed 100 mph.  A single strong thunderstorm, clusters of storms, or a line of storms can all produce severe winds.  Often, high winds can be felt during an ongoing severe thunderstorm, but sometimes severe winds can be felt ahead of a storm, cluster of storms, or squall line.  Severe winds ahead of a storm are ususally associated with the storm's gustfront or outflow boundary.  However, severe winds can be caused by:

Downbursts are usually caused by air being dragged down by precipitation -sometimes called a rain bomb.  Downbursts can happen very quickly, cause visibility to briefly drop to near zero, and have damaging winds over 100 mph.  Dry microburst are possible as well; these begin the same way as a wet microburst, but the precipitation evaporates aloft leaving just the downdraft of air.

A Rear Flank Downdraft is a region of dry air wrapping around the back of a mesocyclone in a supercell thunderstorm. These areas of descending air are thought to be essential in the production of many supercell tornadoes.   

A Derecho is a line of intense, widespread, and fast-moving windstorms and a line of severe thunderstorms that move across a great distance and is characterized by potentially widespread damaging winds.

Wyoming experiences a full range of weather related hazards. The following graphics show the number of public reported severe weather related events across Wyoming. This data is from 1950 through 2015. The severe weather season in Wyoming runs roughly from mid-April through September with the majority of the severe weather occurring between 2 p.m. and 7 p.m.