REFERENCE SHEET FOR WARNING/ADVISORY THRESHOLDS
Last Updated: June 4th, 2010
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 VERMONT AND NORTHERN NEW YORK |
| 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 Advisory…
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 -29°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…
6 inches averaged over a forecast zone in a 12 hour period
9 inches averaged over a forecast zone in a 24 hour period
Ice Storm…
½ inch or greater accretion of freezing rain in any zone
Wind Chill…
Wind chill index less than -30°F for at least 3 hours with a sustained wind of at least 5 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 Merrimack, Nashua or Charles) 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 |
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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