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.

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.

Winter Storm Warning for Northwest Minnesota

An example of a winter storm warning Emergency Alert System activation being issued in Norman County on 25 December 2023 at 7:18 PM CST.

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^[4]

  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 weather terminology (United States)

• National Weather Service
• Federal Emergency Management Agency
• Meteorological Service of Canada