National Weather Service United States Department of Commerce
The following are National Weather Service criteria for issuing Watches / Warnings / Advisories:
  • Watches: 50% confidence of meeting Warning criteria (generally within 36-48+ hours).
  • Advisories and Warnings: 80% confidence in the event occurrence (generally within 24-36 hours).

Severe Thunderstorm WarningExpandCollapse

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

Tornado WarningExpandCollapse

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

Winter Storm WarningExpandCollapse

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

More than one predominant hazard
  • 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.
Snow, Ocean Effect Snow, or Sleet
  • 6 inches averaged over a forecast zone in a 12 hour period
    (except 7 inches in Berkshire and Litchfield Counties).
  • 8 inches averaged over a CT, MA, RI forecast zone in a 24 hour period
    (except 9 inches in Berkshire and Litchfield Counties).

Blizzard WarningExpandCollapse

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.

Ice Storm WarningExpandCollapse

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

Winter Weather AdvisoryExpandCollapse

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

More than one predominant hazard

Winter weather event having more than one predominant hazard (i.e., 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, Ocean Effect Snow, and/or Sleet
  • 3 inches averaged over a CT, MA, RI forecast zone in 12 hours
Snow and Blowing Snow
  • Sustained or frequent gusts of 25 to 34 mph accompanied by falling and blowing snow occasionally reducing visibility to ≤ ¼ mi for < 3 hours
Blowing Snow
  • Widespread or localized blowing snow reducing visibility to ≤ ¼ mi with winds < 35 mph
Black Ice
  • A Special Weather Statement will usually be issued when sufficient moisture is expected to cause a thin layer of ice on road surfaces, typically on cloudless nights ("black ice"). At forecaster discretion a formal Winter Weather Advisory may be issued instead.

Freezing Rain AdvisoryExpandCollapse

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

Hurricane WarningExpandCollapse

Sustained winds ≥ 74 mph (≥ 64 kts) (no gust criteria) associated with a hurricane expected to affect a coastal or inland zone within 36 hours.

Tropical Storm WarningExpandCollapse

Sustained winds 39-73 mph (34-63 kts) (no gust criteria) associated with a tropical storm expected to affect a coastal or inland zone within 36 hours.

Extreme Wind WarningExpandCollapse

Short-fused warning for onset of extreme winds, defined as sustained at 111+ mph (equivalent to a Category 3+ hurricane).
This is typically used for the approach of the eyewall of a major landfalling hurricane.

Flash Flood WarningExpandCollapse

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 WarningExpandCollapse

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

River Flood WarningExpandCollapse

Water level at a River Forecast point along a main stem river (such as the Merrimack, Nashua or Charles) is expected to reach or exceed flood stage.

Flood AdvisoryExpandCollapse

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

Coastal Flood WarningExpandCollapse

Widespread serious (moderate or major) coastal flooding is expected to damage property AND/OR pose a threat to life.

Coastal Flood AdvisoryExpandCollapse

Minor coastal flooding expected. Examples include: splashover that causes a few roads to be briefly impassable, standing water in parking lots, etc.

High Wind WarningExpandCollapse

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

Wind AdvisoryExpandCollapse

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

Excessive Heat WarningExpandCollapse

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

Heat AdvisoryExpandCollapse

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

Heat WaveExpandCollapse

Issued for non-criteria warning/advisory heat. A heat wave is defined as 3 or more days of ≥ 90°F temperatures.

Freezing WarningExpandCollapse

When minimum shelter temperature drops to ≤ 32°F during growing season

Frost AdvisoryExpandCollapse

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

Wind Chill WarningExpandCollapse

Wind chill index ≤ -25°F for at least 3 hours using only sustained wind.

Wind Chill AdvisoryExpandCollapse

Wind chill index between -15°F and -24°F for at least 3 hours using only the sustained wind.

Special Marine WarningExpandCollapse

Brief / sudden occurrence of sustained wind or frequent gusts ≥ 34 kts, usually associated with thunderstorms, AND/OR hail ≥ ¾ inch in diameter; also issued for waterspouts.

Hurricane Force Wind WarningExpandCollapse

Sustained winds or frequent gusts ≥ 64 kts (> 2 hrs) within 24 hours from a non-tropical system (marine only).

Storm WarningExpandCollapse

Sustained winds or frequent gusts 48-63 kts (> 2 hrs) within 24 hours from a non-tropical system (marine only).

Gale WarningExpandCollapse

Sustained winds or frequent gusts 34-47 kts (> 2 hrs) within 24 hrs from a non-tropical system (marine only).

Small Craft AdvisoryExpandCollapse

Sustained winds or frequent gusts 25-33 kts (> 2 hrs) AND/OR Seas ≥ 5 feet within 24 hours (marine only).

High Surf AdvisoryExpandCollapse

When high surf poses a danger to life in the form of rip currents or breaking seas. Generally issued when 7+ foot incoming seas reported at the buoys from Memorial Day through September.

Red Flag WarningExpandCollapse

High degree of confidence that dry fuels and weather conditions support extreme fire danger within 24 hours using the following criteria as a guide:
  • Winds sustained or with frequent gusts ≥ 25 mph
  • Relative Humidity at or below 30% anytime during the day
  • Rainfall amounts for the previous 5 days less than 0.25 inches (except 3 days in pre-greenup)
  • Lightning after an extended dry period
  • Significant dry frontal passage
  • Dry thunderstorms
  • Keetch-Byram Drought Index values of 300 or greater(summer only)

Dense Fog AdvisoryExpandCollapse

Widespread visibility ≤ ¼ mile for at least 3 hours.

Freezing Fog AdvisoryExpandCollapse

Very light ice accumulation from predominantly fog at or below freezing temperatures.