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NWS Alerts Criteria
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).
- For the full listing of criteria, please consult this summary.
Thunderstorms with wind gusts ≥ 58 mph (50 kts) and/or hail ≥ 1-inch in diameter and/or a tornado.
Likelihood of a tornado within the given area based on radar or actual sighting; usually accompanied by conditions indicated above for "Severe Thunderstorm Warning".
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
- 8 inches averaged over a CT, MA, RI forecast zone in a 24 hour period
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.
½ inch or greater accretion of freezing rain in any zone.
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
Freezing Rain
- Any accretion of freezing rain or freezing drizzle on road surfaces.
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.
Sustained winds ≥ 74 mph (≥ 64 kts) (no gust criteria) associated with a hurricane expected to affect a coastal or inland zone within 36 hours.
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.
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.
When the danger of life-threatening inundation from rising water moving inland from the shoreline somewhere within the specified area, generally within 36 hours,
is in association with a tropical, subtropical, or post-tropical cyclone.
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.
Expected overflow or inundation by water which causes or will cause damage and/or a threat to life.
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.
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.
Widespread serious (moderate or major) coastal flooding is expected to damage property AND/OR pose a threat to life.
Moderate Coastal Flooding
Widespread flooding of vulnerable shore roads and/or basements due to height of storm tide and/or wave action. Numerous roads become impassable for a time, and closures are needed. Lives may be at risk for people who put themselves in harm’s way such as trying to drive through flooded roadways. A few neighborhoods may become isolated around the time of high tide. Isolated damage of very vulnerable structures such as docks or house decks/porches near the high tide line may be observed. Erosion of dunes and bluffs is likely due to waves on top of an elevated water level. Any stairs on the slopes of bluffs/dunes may be damaged.
Major Coastal Flooding
Coastal flooding severe enough to be life threatening along the immediate shore and to cause damage to at least some coastal homes/businesses along with widespread flooding of vulnerable shore roads and/or basements due to storm tide and/or wave action. Vulnerable homes or businesses may be severely damaged or destroyed. Numerous roads are impassable, some with washouts, and unsafe to cross on foot or by vehicle. A number of neighborhoods become isolated, possibly for an extended time. Response requires some evacuations and movement of vehicles and other property to beyond the reach of the storm tide and waves. Severe erosion of dunes and bluffs due to waves on top of abnormally high water levels should be expected. Erosion may be severe enough for some bluffs to slump or dunes to breach. Erosion of bluffs/dunes may threaten nearby structures or infrastructure. Major coastal flooding is covered by a Coastal Flood Warning with additional language as appropriate to indicate that the flooding will be major, severe, destructive, damaging, etc.
Minor coastal flooding expected. Examples include: splashover that causes a few roads to be briefly impassable, standing water in parking lots, etc.
Minor Coastal Flooding
Flooding of the most vulnerable shore roads and/or basements due to height of storm tide or wave splashover. Majority of roads remain passable with only isolated closures due to nuisance flooding or rip rap. Spotty erosion of dunes or along the base of bluffs could occur due to waves on top of an elevated water level. There is no significant threat to life, and any impact on property is minimal.
Sustained winds ≥ 40 mph (≥ 35 kts) for at least 1 hour; OR any gusts ≥ 58 mph (≥ 50 kts)
Sustained winds 31-39 mph (27-34 kts) for at least 1 hour; OR any gusts to 46-57 mph (40-49 kts)
Daytime heat indices of ≥ 105°F for 2 or more hours.
Daytime heat indices of 95°F-99°F for 2 or more hours over 2 consecutive days, or 100°F-104°F for 2 or more hours over 1 day.
Issued for non-criteria warning/advisory heat. A heat wave is defined as 3 or more days of ≥ 90°F temperatures.
When minimum shelter temperature drops to ≤ 32°F during growing season
Issued under clear, light wind conditions with forecast minimum shelter temperature 33-36°F during growing season
Wind chill index ≤ -25°F for at least 3 hours using only sustained wind.
Wind chill index between -15°F and -24°F for at least 3 hours using only the sustained wind.
Wind chill index between -15°F and -24°F for at least 3 hours using only the sustained wind.
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.
Sustained winds or frequent gusts ≥ 64 kts (> 2 hrs) within 24 hours from a non-tropical system (marine only).
Sustained winds or frequent gusts 48-63 kts (> 2 hrs) within 24 hours from a non-tropical system (marine only).
Sustained winds or frequent gusts 34-47 kts (> 2 hrs) within 24 hrs from a non-tropical system (marine only).
Sustained winds or frequent gusts 25-33 kts (> 2 hrs) AND/OR Seas ≥ 5 feet within 24 hours (marine only).
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.
When conditions yield ice accumulation rates < 2 cm / hr ( 0.8 inch / hour). Favorable conditions for ice accumulation occur when air temperatures over the waters are ≤ 23F and sustained winds ≥ 25 kts and sea surface temperatures ≤ 43F and seas ≥ 4 feet.
When conditions yield ice accumulation rates ≥ 2 cm / hr ( 0.8 inch / hr ). Favorable conditions for ice accumulation occur when air temperatures over the waters are ≤ 23F and sustained winds ≥ 25 kts and sea surface temperatures ≤ 43F and seas ≥ 4 feet.
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)
Widespread visibility ≤ ¼ mile for at least 3 hours.
Very light ice accumulation from predominantly fog at or below freezing temperatures.
Widespread or localized smoke reducing visibilities to ¼ mile or less.
Atmospheric conditions stable enough to cause air pollutants to accumulate in a given area. Criteria developed in conjunction with the local or state EPA and the product issued at their request.
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