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REFERENCE SHEET FOR WARNING/ADVISORY THRESHOLDS
Last Updated: April 23, 2013

The following are National Weather Service criteria for issuing Advisories and Warnings for various weather events. Watches generally are issued with longer lead times in expectation of meeting Warning criteria.
TYPE OF ISSUANCE   WHEN ISSUED FOR SOUTHERN NEW ENGLAND
WINTER WEATHER ADVISORY

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 (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, Ocean Effect Snow, and/or Sleet

  • 3 inches averaged over a CT, MA, RI forecast zone in 12 hours
  • 4 inches averaged over a NH 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 < 1/4 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 ADVISORY

    Any accretion of freezing rain or freezing drizzle on road surfaces

    WIND CHILL ADVISORY

    Wind chill index between ‑15°F and ‑24°F for at least 3 hours using only the sustained wind (except -20°F to -29°F Southern NH)

    WINTER STORM WARNING

    When any of the following is expected within the next 12 to 36 hour:
    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
  • 8 inches averaged over a CT, MA, RI forecast zone in a 24 hour period
  • 9 inches averaged over a NH forecast zone in a 24 hour period
  • BLIZZARD WARNING 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 WARNING ½ inch or greater accretion of freezing rain in any zone
    WIND CHILL WARNING Wind chill index < -25°F for at least 3 hours using only sustained wind (except < -30°F Southern NH)
    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 mph (≥35 kts) for at least 1 hour; OR any gusts ≥ 58 mph (≥50 kts)
    SMALL CRAFT ADVISORY Sustained winds 25-33 kts AND/OR Seas ≥ 5 feet within 24 hours
    GALE WARNING Sustained winds 34-47 kts within 24 hrs from a non-tropical system
    STORM WARNING Sustained winds 48-63 kts within 24 hours from a non tropical system
    HURRICANE FORCE WIND WARNING Sustained winds or frequent gusts ≥ 64 kts (> 2 hrs) within 24 hours from a non-tropical system for coastal waters only
    TROPICAL STORM WARNING Sustained winds 39-73 mph (34-63 kts) (no gust criteria) associated with a tropical storm expected to affect a specified area within 24 hours
    HURRICANE WARNING

    Sustained winds ≥ 74 mph (64 kts) (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
    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 Merrimack, Nashua or Charles) is expected to reach or exceed flood stage
    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
    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
    HEAT WAVE Issued for non-criteria warning/advisory heat.  A heat wave is defined as 3 or more days of > 90ºF temperatures.
    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
    HIGH SURF ADVISORY When high surf poses a danger to life in the form RIP currents or breaking seas
    RED FLAG WARNING High degree of confidence that dry fuels and weather conditions support extremefire 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)


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