Watch, Warning, and Advisory Criteria at NWS Chicago

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Definition of NWS Chicago Warnings, Watches, and Advisories

Warnings | Watches | Advisories

For more on NWS Hazard Simplification, see here.

Warnings Conditions are occurring or imminent
Tornado	A severe thunderstorm has radar indications of intense low level rotation in the presence of atmospheric conditions conducive to tornado development, and/or is producing a tornado based on reports. These warnings are impact-based meaning they include a tornado tag and a tornado damage threat tag at the bottom of the warning. The tornado tag is either "Radar Indicated" or "Observed", with the latter giving confirmation a tornado has been reported with the storm. The tornado damage threat tag is for progressively worse expected damage and are "Considerable" and "Catastrophic".
Severe Thunderstorm	A severe thunderstorm or a line of severe thunderstorms capable of producing hail greater than one inch in diameter and/or 50 knot (58 mph) wind speeds. These warnings are impact-based meaning they include wind and/or hail tags and at times a damage threat tag at the bottom. The wind and/or hail tag is either "Radar Indicated" or "Observed", with the latter giving confirmation that severe weather has been reported with the storm. The damage threat tag is for progressively worse expected damage and are "Considerable" and "Destructive". An additional tag of "Tornado Possible" will be included when warning forecasters suspect a tornado is possible in addition to the damaging winds and/or hail.
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. These warnings are impact-based meaning they include a flash flood tag and at times a damage threat tag at the bottom. The flash flood tag will be either "Radar Indicated", "Gauge Indicated", or "Observed", with the last of those three giving confirmation that flash flooding has been reported with the storm. The damage threat tag is for progressively worse expected damage and are "Considerable" and "Catastrophic". An additional tag of "Expected Rainfall Rate" will be included when warning forecasters have enough confidence to include expected rainfall rates with the storm.
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.
Snow Squall	These are short-fused, smaller warnings (i.e. considerably smaller in space and time than Winter Storm Warnings) and issued for localized, intense areas of snow/wind expected to cause sharply reduced visibility of 1/4 mile or less often with sub-freezing road temperatures. These are not issued when Winter Storm or Blizzard Warnings are in effect.
Ice Storm	Ice accumulations of a quarter inch or more are expected.
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.
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.
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 accumulation, and localized significant visibility reductions due to snow and/or blowing snow. This may be issued for less than 3 inches of snow if significant impacts are expected.
Wind Chill	Widespread wind chill values around -20° or colder are expected. For NWS criteria 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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