Being deep into the winter season, we've heard a few people ask "why was there a winter storm watch when we knew the storm was coming?". The answer is all in the "when". To start, a warning issued by the National Weather Service (NWS) means hazardous winter conditions are either occurring or are imminent. Usually, this is issued within 24 hours of an event. In short, when a warning is released, now is the time to take action! A watch, on the other hand, means that there is the potential for hazardous wintry conditions to occur. This is usually released 24-72 hours before an event and means this is a good time to prepare and have a plan in case conditions warrant an upgrade to a warning. As a side note, the NWS can also issue an advisory at any time. When an advisory is issued, you should just be aware of your surrounding weather conditions. So, now that we know what actions we should take when a watch, warning or advisory is issued, what ARE the different types of winter watches and warnings? Take a deep breath as we break down the major ones, since there are plenty.
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(Ice Storm Warning and Wind Chill Warning criteria. Images are courtesy of the NWS.)
Ice Storm
There is no ice storm watch, only a warning (but there is a freezing rain advisory). An ice storm warning is issued when freezing rain is forecast to produce a significant and perhaps damaging amount of ice. Much of New England needs at least a half inch of freezing rain to prompt a warning, while the Mid-Atlantic needs just a quarter of an inch.
Wind Chill
Using the wind chill chart by the NWS , when a combination of cold temperatures and strong winds has the potential to cause harm to exposed human skin, a wind chill watch or wind chill warning is issued. The threshold for what determines a wind chill warning is based partially on the climate of a location. Looking at the image above, even though a -20ºF wind chill is frigid, those in New Hampshire see this often enough where it is not enough to warrant a wind chill warning (NH needs a -30°F wind chill). However, for those in Virginia, -20°F wind chills are rare enough that a wind chill warning is issued.
Winter Storm
This is probably the one you're most accustomed to hearing, with our classic Nor'easters. Whenever large amounts of snow, sleet or freezing rain are anticipated, a winter storm watch is issued and can be upgraded to a winter storm warning when the storm is at or less than 24 hours from beginning. Specifically for snow, the criteria used for issuing a winter storm watch or warning is based on how much snow falls in a 12 or 24 hour period. There is plenty of variation in snow amounts across the East Coast to issue a watch or warning. For example, it takes five inches of snow in 24 hours over much of Virginia for a winter storm warning to be issued, while in northern New England a location needs nine or more inches over the same time period!
Blizzard
While many like to throw the word "blizzard" around, there is actually strict criteria for meeting a blizzard watch or blizzard warning, they are:
- Sustained winds or frequent wind gusts of 35 mph or greater (near gale force).
AND
- Visibility reduced to a quarter mile or less thanks to snow OR blowing snow.
ALL FOR
- Three hours or longer
Based on the official criteria, we do not actually need snow to fall for a blizzard warning...blowing snow can do the trick. Contrary to what may be out there, there is no temperature requirement for a blizzard warning.
(Winter Storm Warning criteria based on 12 hour and 24 hour snow forecasts. Images are courtesy of the NWS.)
Watches and warnings are issued by the NWS not only during the winter. Severe weather season brings its own set of watches and warnings, which you can check out in a previous blog. During the heat of the summer, heat based watches, warnings and advisories are issued as well. Need information you can rely on beyond the scope of watches and warnings? Our WinterRisk™ product allows you to keep an update on potential systems 6-14 days out, so you can knowledgeably plan out your operations!
