National Weather Service United States Department of Commerce

Criteria may be adjusted in certain circumstances to take into account the expected impact of an event.  For example, a Winter Weather Advisory may be issued for less than an inch of snow if will occur at a peak travel time, such as around the normal morning commute time, and thus potentially have a significant impact.

Severe Weather Events...

Severe Thunderstorm Warning

  • Thunderstorm wind gusts of 58 mph or greater,
  • AND/OR hail measuring 1" in diameter or larger.

Tornado Warning

  • Tornado observed or indicated on radar.

 

Winter Weather Events...

Blizzard Warning

  • Sustained wind or frequent gusts of 35 mph or higher,
  • AND visibility less than 1/4 mile in snow and/or blowing snow for 3 hours or longer.

Ice Storm Warning

  • Damaging ice accumulations of 1/4" or more.

Winter Storm Warning

  • Snowfall of 4" or more in 12 hours or 6" or more in 24 hours,
  • OR sleet accumulation of 1/2" or more in 24 hours.

Winter Weather Advisory

  • Snowfall of at least 1" but less than 4" in a 12 hour period,
  • AND/OR visibility reduced to 1/4 mile or less in blowing snow,
  • AND/OR sleet accumulation of less than 1/2".

Freezing Rain Advisory

  • Ice accumulations of less than 1/4".

Wind Chill Warning

  • Wind chills of 25 below or colder are expected. In most cases a minimum wind speed of 10 mph should be occurring or forecast before a warning is issued.

Wind Chill Advisory

  • Wind chills of 10 below to 24 below are expected. In most cases a minimum wind speed of 10 mph or more should be occurring or forecast before an advisory is issued.

 

Non-Precipitation Events...

High Wind Warning

  • Sustained non-thunderstorm wind speeds of at least 40 mph for an hour or more,
  • OR non-thunderstorm wind gusts of at least 58 mph regardless of duration.

Wind Advisory

  • Sustained non-thunderstorm wind speeds between 30 and 39 mph for at least one hour,
  • OR non-thunderstorm wind gusts of 40 to 57 mph for any duration.

Lake Wind Advisory

  • Issued when sustained non-thunderstorm wind speeds reach 20 to 29 mph for at least one hour,
  • OR non-thunderstorm wind gusts reach between 35 to 39 mph.

Freeze Warning

  • Widespread temperatures are forecast to be 30ºF or below for 2 hours or more during the growing season.  The beginning of the spring frost/freeze season and ending of the fall frost/freeze season is determined in conjunction with the University of Kentucky Agricultural Weather Center.

Frost Advisory

  • Widespread frost during the growing season.  The beginning of the spring frost/freeze season and ending of the fall frost/freeze season is determined in conjunction with the University of Kentucky Agricultural Weather Center.

Excessive Heat Warning

  • Issued when the heat index is expected to reach around 110ºF or higher,
  • AND the minimum heat index will remain around 75ºF or higher during a 48 hour period.
  • A warning would also be appropriate if heat advisory criteria are expected to be reached for 4 consecutive days.

Heat Advisory

  • Issued for heat index of around 105ºF

Dense Fog Advisory

  • Widespread visibility 1/4 mile or less due to dense fog.

Dense Smoke Advisory

  • Widespread visibility 1/4 mile or less due to dense smoke.

Dust Storm Warning

  • Widespread visibility 1/4 mile or less due to blowing dust.

Blowing Dust Advisory

  • Widespread visibility of between 1/4 mile and 1 mile.

 

Fire Weather Events...

Red Flag Warning

  • Sustained 20-foot wind speeds of 15 mph or greater,
  • AND minimum relative humidity of 25% or less,
  • AND 10-hour fuel moisture values of 8% or less.

 

National guidance regarding NWS Watch/Warning/Advisory products can be found in the following NWS Directives:
NWSI 10-511 WFO Severe Weather Products Specification,
NWSI 10-513 WFO Winter Weather Products Specifications,
NWSI 10-515 WFO Non-Precipitation Weather Products Specification,
NWSI 10-401 Fire Weather Services Product Specification