Boston / Norton, MA

Weather Forecast Office

Watch / Warning / Advisory 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).

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For the full listing of criteria, please consult this summaryresource.

Severe
Winter
Tropical
Flood
Wind
Heat
Cold
Marine
Fire
Fog / Smoke / Pollution

Severe Thunderstorm WarningExpandCollapse

Thunderstorms with wind gusts ≥ 58 mph (50 kts) and/or hail ≥ 1-inch in diameter and/or a tornado.

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 for all but the Berkshires.
7 inches averaged over a forecast zone in the Berkshires.

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

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.

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.

Storm Surge WarningExpandCollapse

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.

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.

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.

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.

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.

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 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.

Heat WaveExpandCollapse

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

Freeze 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.

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.

Freezing Spray AdvisoryExpandCollapse

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.

Freezing Spray WarningExpandCollapse

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.

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)