National Weather Service United States Department of Commerce

TYPE OF ISSUANCE 

What is Expected To Occur

WINTER WEATHER ADVISORY

When any of the following is expected within the next 12 to 24 hours:
 

Blowing Snow…

  • Visibility substantially reduced over a wide area due to wind-driven snow with winds less than 35 mph. Usually NOT issued for localized occurrences.


Freezing Rain…

  • Any accretion of freezing rain or freezing drizzle on road surfaces


Mixed Precipitation…

  • Winter weather event having more than one predominant hazard (ie., snow and ice, snow and sleet, or snow, ice & sleet) meeting or exceeding advisory criteria for at least one of the precipitation elements, but remaining below warning criteria.


Snow…

  • 4 inches averaged over a forecast zone in 12 hours


Snow and Blowing Snow…

  • 4 inches snowfall averaged over a forecast zone in 12 hours with visibility substantially reduced over a wide area due to wind-driven snow with winds less than 35 mph.


Wind Chill…

  • Wind chill index between -20°F and -34°F for at least 3 hours with a sustained wind speed of at least 10 mph.

WINTER STORM WARNING

When any of the following is expected within the next 12 to 36 hours:
 

Blizzard…

  • Sustained winds or frequent gusts > 35 mph AND considerable falling and/or blowing snow frequently reducing visibility < ¼ for > 3 hours (Blizzard conditions need to be the predominant condition over a 3 hour period)


Heavy Snow…

  • 7 inches averaged over a forecast zone in a 12 hour period
  • 10 inches averaged over a forecast zone in a 24 hour period


Ice Storm…

  • ½ inch or greater accretion of freezing rain in any zone

 

Sleet…

  • Pure sleet >= 1”


Wind Chill…

  • Wind chill index less than -35°F for at least 3 hours with a sustained wind of at least 10 mph.


Winter Storm…

  • Winter weather event having more than one predominant hazard {ie. heavy snow and blowing snow (below blizzard conditions), snow and ice, snow and sleet, sleet and ice, or snow, sleet and ice} meeting or exceeding warning criteria for at least one of the precipitation elements.

WIND ADVISORY

Sustained winds 31-39 mph (27-34 kts) for at least 1 hour; OR any gusts to 46-57 mph (40-49 kts)

HIGH WIND WARNING

Sustained winds 40-73 mph (≥35 kts) for at least 1 hour; OR any gusts ≥ 58 mph (≥50 kts)

SEVERE THUNDERSTORM WARNING

Thunderstorms with wind gusts ≥ 58 mph (50 kts) AND/OR hail ≥1" in diameter

TORNADO WARNING

Likelihood of a tornado within the given area based on radar or actual sighting; usually accompanied by conditions indicated above for "Severe Thunderstorm Warning"

FLOOD ADVISORY

Expected inundation of some low lying and poor drainage areas, resulting in a nuisance to the public but not a threat to life and property.

FLASH FLOOD WARNING

Rapid and extreme flow of high water into a normally dry area, or a rapid water level rise in a stream or creek above a predetermined flood level, beginning within a short timeframe from the onset of heavy rain.  A dam or levee failure, or water released from an ice jam is also considered

FLOOD WARNING

Expected overflow or inundation by water which causes or will cause damage and/or a threat to life

RIVER FLOOD WARNING

Water level at a River Forecast point along a main stem river (such as the Penobscot, Aroostook, Saint John) is expected to reach or exceed flood stage

EXCESSIVE HEAT WARNING

Daytime heat indices of  105°F for 2 or more hours

HEAT ADVISORY

Daytime heat indices of 100ºF-104ºF for 2 or more hours

DENSE FOG ADVISORY

Widespread visibility ≤1/4 mile for at least 3 hours

FREEZING FOG ADVISORY

Very light ice accumulation from predominantly FZFG

FROST ADVISORY

Issued under clear, light wind conditions with forecast minimum shelter temperature 33-36ºF during growing season

FREEZE WARNING

When minimum shelter temperature drops to < 32ºF during growing season

RED FLAG WARNING

  • The mean of 1 hour and 10 hour fuel moistures are less than 10% in combination with the following weather conditions
  • Relative Humidity <=20%
  • Winds 10 – 14 mph
  • Wind speed and relative humidity ratios have different criteria

SMALL CRAFT ADVISORY

Sustained winds or frequent gusts of 29-38 mph (25-33kts) AND/OR Seas ≥ 5 feet within 24 hours

GALE WARNING

Sustained winds or frequent gusts of 39-55 mph (34-47kts) within 24hrs from a non-tropical system

STORM WARNING

Sustained winds or frequent gusts 55-73 mph (48-63kts) within 24 hours from a non-tropical system

HURRICANE FORCE WIND WARNING

Sustained winds or frequent gusts ≥ 74 mph (≥ 64kts) (> 2hrs) within 24 hours from a non-tropical system for coastal waters only

TROPICAL STORM WARNING

Sustained winds 39-73 mph (34-63kts) (no gust criteria) associated with a tropical storm expected to affect a specified area within 24 hours

HURRICANE WARNING

Sustained winds ≥ 74 mph (64kts) (no gust criteria) associated with a hurricane expected to affect a specified area within 24 hours

SPECIAL MARINE WARNING

Brief/sudden occurrence of sustained wind or frequent gusts ≥ 34 knots, usually associated with thunderstorms; AND/OR hail ≥3/4" in diameter; also issued for waterspouts

COASTAL FLOOD ADVISORY

Minor coastal flooding expected within 12 hours.  Examples include: splash over causing a few roads briefly impassable, standing water in parking lots, etc.

COASTAL FLOOD WARNING

Coastal flooding expected within 12 hours; widespread serious coastal flooding which damages property AND/OR is a threat to life

FREEZING SPRAY ADVISORY

Light to moderate accumulation of ice is expected on vessels.

HEAVY FREEZING SPRAY WARNING

An heavy accumulation of freezing water droplets on a vessel at a rate of 2 cm per hour or greater caused by some appropriate combination of cold water, wind, cold air temperature, and vessel movement