Cold air funnel

A funnel cloud that can develop from a small shower or thunderstorm when the air aloft is usually cold. Cold air funnels rarely become tornadoes, and when they do, they are almost always weak tornadoes.

Doppler radar

Radar that can detect location and intensity of precipitation, and measure radial velocity. Radial velocity is the instantaneous component of motion of precipitation particles parallel to the radar beam (i.e., toward or away from the radar).

Downburst

A strong downdraft from a thunderstorm resulting in an outward burst of damaging winds on or near the ground. Downburst winds are often 50 to 100 mph and in rare cases can be 100 to 150 mph. The winds usually blow in one direction and may also be known as straight-line winds.

Flash flood

Rapid rises in water in streams, creeks and storm drains that pose a threat to life and property. Flash floods are usually caused by excessive rainfall but ice jams and dam or levee failures can also cause flash flooding.

Flash Flood Warning

A flash flood is occurring or imminent. Go to higher ground! Stay away from small streams, creeks, and storm drains!

Flash Flood Watch

Conditions are favorable for flash flooding. Remain alert and be ready to move to higher ground.

Funnel cloud

A rotating column of air, extending from a towering cumulus or cumulonimbus cloud, that is not in contact with the ground.

Hail

Precipitation in the forms of lumps of ice that occur with some thunderstorms.

Severe thunderstorm

A thunderstorm that produces wind gusts of 58 mph or greater and/or hail 3/4 inch diameter or larger.

Severe Thunderstorm Warning

A thunderstorm with strong damaging winds and/or large damaging hail has been indicated by Doppler radar or sighted by Skywarn spotters. Go indoors and stay away from windows!

Severe Thunderstorm Watch

Conditions are favorable for the development of severe thunderstorms in and close to the watch area. Remain alert and be ready to move to a place of safety.

Squall line

A line of thunderstorms or squalls which may extend over a hundred miles.

Tornado

A violently rotating column of air, extending from a towering cumulus or cumulonimbus cloud, in contact with the ground. see TornadoProject.com

Tornado Watch

Conditions are favorable for the development of severe thunderstorms and tornadoes. Remain alert and be ready to move to a place of safety.

Tornado Warning

A tornado has been indicated by Doppler radar or sighted by Skywarn spotters. Seek shelter in a basement or in a small interior room in a reinforced building!

Urban/Small Stream Flood Advisory

The flooding of streets, low-lying areas, urban storm drains and creeks is either occurring or imminent. Flooding will cause significant inconvenience, but is generally not an immediate threat to life or property.

Waterspout

A small, relatively weak rotating column of air over water beneath a towering cumulus or cumulonimbus cloud. Waterspouts can occur over southern Lake Michigan, usually in late summer or early fall.

Severe thunderstorm watches and tornado watches are issued by the Storm Prediction Center in Kansas City, Missouri. A watch may cover a large portion of Illinois and will usually be in effect for a time period of about 6 hours.

Flash flood watches are issued by the Chicago Weather Service Forecast Office. They may be valid for a large portion of Illinois and they are usually in effect for 6 to 12 hours.

Tornado warnings, severe thunderstorm warnings, flash glood warnings, and urban/small stream flood advisories are issued by the Chicago Weather Service Forecast Office on a county-by-county basis. Tornado and severe thunderstorm warnings are usually in effect for 30 to 60 minutes. Flood warnings and advisories can be in effect for 1 to 6 hours.

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