Definition of NWS Chicago Warnings, Watches, and Advisories

Warnings     |     Watches     |      Advisories

 

Warnings
Conditions are occurring or imminent
Tornado A severe thunderstorm has developed and has either produced a tornado based on reports or radar has indicated intense low level rotation in the presence of atmospheric conditions conducive to tornado development
Severe Thunderstorm A severe thunderstorm has developed, capable of producing hail greater than one inch in diameter and/or 50 knot (58 mph) wind speeds
Flood Long duration areal or river flooding is occurring or is imminent, which may result from excessive rainfall, rapid snow melt, ice jams on rivers or other similar causes
Flash Flood Excessive rainfall producing thunderstorms have developed, leading to short duration flash flooding. A warning may also be issued if a dam break has occurred
Blizzard Wind speeds of 35 mph or greater, considerable falling or blowing snow, and visibilities frequently below a quarter mile are expected to prevail for three hours or more
Winter Storm Snow amounts of 6 inches or more in 12 hours, or 8 inches or more in 24 hours, or heavy sleet accumulations of one half inch or greater are expected. These may or may not be accompanied by wind or other phenomena. A warning may also be issued if conditions will be approaching blizzard criteria and/or have significant impacts, even if snowfall amounts are not expected to reach the criteria above
Ice Storm Ice accumulations of a quarter inch or more are expected
Lake Effect Snow Heavy lake effect snow is expected to produce amounts of 6 inches or more in 12 hours, or 8 inches or more in 24 hours
Wind Chill Widespread wind chill values around -30° or colder.  For NWS criteria across the area, click here.
Excessive Heat Chicago/Cook County: Three consecutive days with a peak heat index expected to reach 100-105° or two consecutive days with a forecast peak heat index 105-110° or one day with an expected peak heat index forecast at 110° or higher 
Elsewhere: Maximum heat index is expected to reach 110° or greater and a minimum heat index of 75° or greater for at least 48 hours
Freeze Widespread temperatures below freezing when during the growing season are expected
High Wind Synoptic scale gradient winds (those not associated with thunderstorms) or mesoscale winds associated with wake lows are expected to reach speeds of 40 mph or greater, sustained for at least one hour, or 58 mph or greater gusts
Red Flag This is intended primarily for land area managers and is issued when sustained 20 foot winds of 20 mph or higher, afternoon humidity values less than 25%, and ten hour fuel moisture of 8% or less are expected.

 

Watches
Conditions are favorable but not occurring or imminent at the time
Tornado Atmospheric conditions are favorable for the development of severe thunderstorms potentially capable of producing tornadoes
Severe Thunderstorm Atmospheric conditions are favorable for the development of severe thunderstorms, producing at least one inch diameter hail and/or 50 knot (58 mph) or greater wind speeds
Flood Atmospheric and hydrologic conditions are favorable for long duration areal or river flooding
Flash Flood Atmospheric and hydrologic conditions are favorable for short duration flash flooding and/or a dam break is possible
Blizzard Wind speeds of 35 mph or greater, considerable falling or blowing snow, and visibilities frequently below a quarter mile for three hours or more are possible
Winter Storm Severe winter storm conditions may occur, including heavy snow, significant ice or sleet accumulations, and any of those accompanied by strong winds that may lead to significant visibility reductions
Lake Effect Snow Potential heavy lake effect snow may produce snow amounts of 6 inches or more over 12 hours, or 8 inches or more over 24 hours
Wind Chill Widespread wind chill values around -30° or colder are possible
Excessive Heat Chicago/Cook County: There is upcoming potential for three consecutive days with a peak heat index expected to reach 100-105° or two consecutive days with a forecast peak heat index 105-110° or one day with an expected peak heat index forecast at 110° or higher 
Elsewhere: Maximum heat index may potentially reach 110° or greater and a minimum heat index of 75° or greater for at least 48 hours.
Freeze Widespread temperatures below freezing are possible during the growing season
High Wind The potential exists for synoptic scale gradient winds (those not associated with thunderstorms) or mesoscale winds associated with wake lows to reach speeds of 40 mph or greater, sustained for at least one hour, or 58 mph or greater gusts
Fire Weather This is mainly intended for area land managers and is issued when a high potential exists for upcoming sustained 20 foot winds of 20 mph or higher, afternoon humidity less than 25%, and ten hour fuel moisture of 8% or lower.

 

Advisories
Conditions are occurring or imminent
Special Weather Statement A thunderstorm or line of thunderstorms has developed and is capable of producing hail and gusty winds that approach but are below severe weather criteria, such as dime to nickel size hail and 40 to 55 mph winds.
Flood Thunderstorms have produced heavy rainfall that may result in ponding of water on roadways and in low-lying areas, as well as rises in small stream levels, none of which is expected to pose an immediate threat to life and property
Winter Weather Any one of a combination of the following winter weather elements is expected: 3 to 6 inches of snow, light sleet accumulation, light ice and snow or sleet accumulation, and localized significant visibility reductions due to snow and/or blowing snow.  This may be issued for 2 to 4 inches of snow as well
Freezing Rain Light ice accumulation is expected
Lake Effect Snow Lake effect snow is expected to produce amounts of 3 to 5 inches. This may be issued for 2 to 4 inches of snow as well
Wind Chill Widespread wind chill values around -20° or colder are expected.  For NWS critera across the area click here.
Heat Maximum heat index is expected to exceed 105°
Frost Widespread temperatures of 33 to 36 degrees are expected during the growing season
Wind Sustained winds of 30 to 39 mph and/or gusts of 45 mph or stronger are expected to last for at least one hour
Dense Fog Widespread visibility of a quarter mile or less is expected
Freezing Fog Widespread light ice accumulation is expected from freezing fog

 


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