Winter storm warning

A winter storm warning (SAME code: WSW) is a hazardous weather statement issued by Weather Forecast Offices (WFO) of the National Weather Service (NWS) in the United States to alert the public that a winter storm is occurring or is about to occur in the area, usually within 36 hours of the storm's onset.

A similar warning is issued by Environment and Climate Change Canada (ECCC) through local forecast offices of the Meteorological Service of Canada.

Definition

Generally, a Winter Storm Warning is issued if the following criteria, at minimum, are forecast: snow accumulations usually between or greater than 4 to 7 inches (10 to 18 cm), or snow accumulations of usually three inches (7.6 cm) or more with significant accumulation of ice (sleet or freezing rain). In the Southern United States, where severe winter weather is much less common and any snow is a more significant event, warning criteria are lower, as low as one inch (2.5 cm) in the southernmost areas: as one goes from north to south, the accumulation estimates needed to meet warning criteria lessen. A warning can also be issued during high impact events of lesser amounts, usually early or very late in the season when trees have leaves and damage can result.

Winter Storm Warnings for snow events are issued when winds are less than 35 mph (56 km/h); if the storm is expected to produce winds above this speed for at least three hours accompanying moderate to heavy snowfall, a blizzard warning will be issued instead or as an upgrade to the winter storm warning. Usually, a large accumulation of ice alone with little to no snow will result in an ice storm warning, or in the case of light freezing rain, a winter weather advisory, a freezing rain advisory, or a freezing drizzle advisory. In lieu of issuing such a statement separately, Winter Storm Warnings can include verbiage indicative of a wind chill advisory or a wind chill warning, if potentially life-threatening wind chill values (determined by local criteria) are forecast to accompany winter precipitation during the warning's duration.

Beginning with the 2008–09 meteorological winter, the National Weather Service consolidated individual precipitation-specific bulletins for winter storms expected to create heavy snowfall (heavy snow warning), lake-effect snow (lake effect snow warning) or sleet accumulations (sleet warning or heavy sleet warning) within the warned area, replacing them with variants of the Winter Storm Warning product outlining the accordant hazards being forecast:

Winter storm warning for heavy snow

Replaced the heavy snow warning

Winter storm warning for heavy wet snow

Same as above, except when the snow will also be wet

Winter storm warning for heavy snow and blowing snow

When the criteria for both a WSW for heavy snow and a winter weather advisory for snow and blowing snow are met [near-blizzard conditions]^[1]

Winter storm warning for heavy sleet

Replaced the sleet warning^[2]

Winter storm warning for heavy snow and ice

When both the criteria for a WSW for heavy snow and an ice storm warning are met^[3]

Winter storm warning for snow and ice

When both the criteria for a winter weather advisory for snow and an ice storm warning are met

Winter storm warning for sleet and freezing rain

When both the criteria for a winter weather advisory for sleet and an ice storm warning are met

Winter storm warning for heavy lake-effect snow

Replaced the lake effect snow warning in some NWS county warning areas. (2017-2018 season)

Additionally, all of the above warning types may also include verbiage indicative of a wind advisory to indicate strong winds that are expected to accompany the precipitation (e.g., "winter storm warning for heavy wet snow and strong winds").

The generic "winter storm warning" terminology may be used on its own, typically to indicate that all types of winter precipitation (as a mixture or in periods before transitioning between types) are expected in high amounts; however, it may be defined generically at the forecaster's discretion regardless of whether or not this condition is met.

Example of a winter storm warning

URGENT - WINTER WEATHER MESSAGE
National Weather Service Grand Rapids MI
331 PM EDT Fri Apr 13 2018

MIZ037>040-043>046-050>052-140345-
/O.NEW.KGRR.WS.W.0003.180414T0900Z-180415T1600Z/
Mason-Lake-Osceola-Clare-Oceana-Newaygo-Mecosta-Isabella-Muskegon-
Montcalm-Gratiot-
Including the cities of Ludington, Baldwin, Reed City, Clare, 
Hart, Fremont, Big Rapids, Mount Pleasant, Muskegon, Greenville, 
and Alma
331 PM EDT Fri Apr 13 2018

...WINTER STORM WARNING IN EFFECT FROM 5 AM SATURDAY TO NOON EDT
SUNDAY...

* WHAT...Heavy mixed precipitation expected. Total snow and sleet
  accumulations of two to four inches and ice accumulations of a
  quarter to half an inch are expected.

* WHERE...Portions of central and west central Michigan.

* WHEN...From 5 AM Saturday to noon EDT Sunday.

* ADDITIONAL DETAILS...Expect power outages and tree damage due 
  to the ice. Travel will be impossible. Tree branches could 
  fall. Expect significant reductions in visibility at times.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

A Winter Storm Warning means significant amounts of snow, sleet
and ice will make travel very hazardous or impossible. Strong
winds are also expected.


&&

$$

Severe
storms

Thunderstorm	Advisory Watch PDS watch Warning
Tornado	Watch PDS watch Warning PDS warning Emergency
Marine thunderstorm	Special marine warning

Winter
weather

Advisories	Winter weather advisory
Winter storm	Watch Warning
Blizzard/blowing snow	Blowing snow advisory Blizzard watch Blizzard warning
Heavy snow	Warning
Lake-effect snow	Advisory Watch Warning
Snow squall	Warning
Ice	Freezing rain advisory Ice storm warning Ice storm watch
Avalanche	Warning Watch Advisory

Tropical
cyclones

Tropical storms	Watch Warning
Hurricanes	Watch Warning
Typhoons	Watch Warning
Cyclones	Watch Warning
Storm surge	Watch Warning

Flood

Flood	Watch Warning Advisory
Flash flood	Watch PDS watch Warning Emergency
Coastal flood	Advisory Watch Warning Statement
Lakeshore flood	Warning Advisory Watch Statement
River flood	Advisory Watch Warning
Urban and small stream flood	Advisory

Apparent
temperature

Heat index	Heat advisory Excessive heat watch Excessive heat warning
Wind chill	Advisory Watch Warning PDS warning
Extreme cold	Warning
Freeze	Watch Warning Hard freeze warning Frost advisory Hard freeze watch

Maritime
and
coastal
weather

Marine winds	Small craft advisory Brisk wind advisory Lake wind advisory
Gale	Watch Warning
Storm	Watch Warning
Hurricane-force wind	Warning Watch
Hazardous seas	Watch Warning
Freezing spray	Freezing spray advisory Heavy freezing spray watch Heavy freezing spray warning

High surfWind

Advisory-force winds	Wind advisory
Gale-force winds	High wind watch High wind warning
Major hurricane-force winds	Extreme wind warning

Airborne
particulates

Dust/sand	Blowing dust advisory Blowing dust warning Dust advisory Dust storm warning
Fog/Smoke	Dense fog advisory Dense smoke advisory
Ash	Ashfall advisory Ashfall warning
Air pollution	Air stagnation advisory Air quality alert

FireAviationWeather scales
used by NWSOtherTsunami