Winter Storm Alerts: What Watch, Warning and Advisory Mean

Direct answer: A winter storm watch means get ready, a warning means take action, and an advisory means use caution. The National Weather Service uses four tiers — outlook, watch, warning and advisory — and the difference between them is how certain forecasters are, and how serious the expected weather is. A watch means dangerous winter weather is possible. A warning means it is happening, about to happen, or almost certain — and it threatens life or property. An advisory means wintry weather is expected but below warning strength: still a real hazard, and still worth changing your plans for.

The four tiers, in order

Outlook. The earliest heads-up, covering the day 3 to 7 forecast period. The National Weather Service puts outlooks in the Hazardous Weather Outlook when winter weather may cause a significant impact and eventually lead to a watch or warning. At this range, forecast uncertainty is high: roughly a 30 to 50 percent chance that the event will occur and reach warning criteria. An outlook is for people who need considerable lead time to prepare.

Watch. A watch is generally issued in the 24 to 72 hour forecast time frame, when the risk of a hazardous winter weather event has increased to a 50 to 80 percent certainty that warning thresholds will be met. It is intended to give enough lead time for people to set their plans in motion. A Winter Storm Watch means conditions are favorable for a winter storm event — heavy sleet, heavy snow, ice storm, heavy snow and blowing snow, or a combination — to meet or exceed local warning criteria in the next 24 to 72 hours. The Burlington, Vermont office's snow criterion is 7 or more inches during an event covering at least half of the forecast zone or most of its population, and the ice criterion is half an inch or more over at least half of the zone. An Extreme Cold Watch means apparent temperatures may reach or exceed minus 30 degrees Fahrenheit.

Warning. Warnings are issued when a hazardous winter weather event is occurring, is imminent, or has a very high probability of occurrence — generally greater than 80 percent — and the conditions pose a threat to life or property. A Winter Storm Warning means such an event is expected to meet or exceed local warning criteria in the next 12 to 36 hours, using the same snow and ice criteria as a watch. A Blizzard Warning is more specific: a blizzard event is imminent or expected in the next 12 to 36 hours, with sustained winds or frequent gusts of 35 miles per hour or greater accompanying falling or blowing snow that frequently reduces visibility to less than a quarter of a mile for three or more hours. An Ice Storm Warning means ice of half an inch or more over at least half of the zone in the next 12 to 36 hours, and an Extreme Cold Warning means wind chill temperatures may reach or exceed minus 30 degrees Fahrenheit.

Advisory. An advisory is for the same kind of event, with the same high probability, but below warning criteria: less serious conditions that cause significant inconvenience and, if caution is not exercised, could lead to situations that may threaten life or property. A Winter Weather Advisory means a winter storm event is expected to meet advisory criteria in the next 12 to 36 hours but stay below warning criteria — for snow, 4 or more inches in 12 hours or less covering at least half of the zone, and for ice, any accumulation less than half an inch. A Winter Weather Advisory can also be issued for black ice. A Cold Weather Advisory means wind chill temperatures may reach or exceed minus 20 degrees Fahrenheit.

Why the numbers change by location

There is no single national snowfall threshold, because the criteria are set locally. The Burlington criteria above are one office's numbers; your local office may use different ones. In North Dakota, for example, a Winter Storm Warning is issued for 6 inches of snow in 12 hours or 8 inches in 24 hours, with half an inch of sleet, an Ice Storm Warning for a quarter inch or more of ice, and an Extreme Cold Warning for wind chill of minus 40 degrees Fahrenheit or colder. In Colorado, a Winter Storm Warning is issued when heavy snow is occurring or will develop in the next 36 hours, possibly with wind greater than 15 miles per hour and blowing snow. Always read the alert for your own forecast area, not a number you remember from somewhere else.

What to do at each level

At a watch, get ready: set your plans in motion, check supplies, and be ready to change travel plans. That is what the lead time is for. At a warning, take action: the threat to life or property is real, so avoid travel, stay sheltered, and check warnings on our weather alerts page before you set out. At an advisory, use caution: slow down, allow extra time, and do not assume a smaller snowfall means a safe road — black ice and blowing snow can be dangerous at any accumulation. For the wider winter plan, read our severe cold and winter storm safety guide.

Frequently asked questions

What is the difference between a watch and a warning?

A watch means dangerous winter weather is possible — generally a 50 to 80 percent certainty that warning thresholds will be met, 24 to 72 hours out. A warning means the event is occurring, imminent, or has a very high probability (generally over 80 percent) and threatens life or property.

What exactly is a blizzard warning?

A blizzard event imminent or expected in the next 12 to 36 hours, with sustained winds or frequent gusts of 35 miles per hour or greater and falling or blowing snow frequently reducing visibility to less than a quarter of a mile for three or more hours.

Does an advisory mean it is safe?

No. An advisory means conditions stay below warning criteria but cause significant inconvenience — and, in the National Weather Service's words, if caution is not exercised they could lead to situations that may threaten life or property.

Where do I check current winter alerts?

Check our weather alerts page and your local National Weather Service forecast office. Remember that thresholds are set locally, so read the alert for your own forecast area.

Sources

• National Weather Service Burlington - Expanded Winter Weather Terminology
• National Weather Service Boulder - Watch Warning Advisory Products Explained
• National Weather Service Bismarck - North Dakota Winter Weather Awareness Week

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