A large percentage of North Americans have to endure at least one large winter storm every year, but a blizzard is another thing altogether. That's a superstorm that can down power lines, bury houses and strand you in your car for an extended period. If you're planning on traveling or engaging in outdoor activity in winter, knowing the warning signs of an impending blizzard can be a matter of life and death.
What Constitutes a Blizzard
The National Oceanic and Atmospheric Administration defines a blizzard as a large storm with winds in excess of 56 kilometers per hour (35 miles per hour), blowing snow or large amounts of snow and visibility of less than 0.4 kilometer (1/4 mile) that lasts for at least three hours. It isn't hard to see such a large storm brewing, but if you don't have access to a means of electronic communication, you may mistake it for a smaller event, such as a snow squall. You probably won't do this, however, if you understand how large storms form. The dynamics vary somewhat according to the part of the country in which you live.
The Geography of Blizzards
In general, winter storms in North America form as cold air coming south from the North Pole meets warm air headed north from the Gulf of Mexico, creating a front. In the West, cold air that blows in the from Pacific Ocean can create blizzard conditions on the windward slopes of mountains. The cold air streams that produce storms in the Midwest often originate on the lee side of the Rocky Mountains, which blow eastward toward the Great Lakes and beyond. Cold air blowing in from the Atlantic in the form of nor'easters is usually responsible for blizzards that form on the Eastern Seaboard. In the Great Lakes region, storms occur as cold winds blow over warm, moist air from the lakes.
Recognizing When a Blizzard Is Likely
Conditions at a cold front favor a blizzard when ground temperatures are already cold, humidity is high and the air is moving quickly, creating heavy winds. All three conditions must be met, and that helps you determine whether the stormy conditions you see developing have the potential to turn into a blizzard. High winds alone can't produce a blizzard if ground temperatures are above freezing; any snow that falls will turn to rain before it hits the ground. Similarly, if the humidity is low, you may be in for a wind storm, but it isn't likely to snow. Finally, snow and cold temperatures may create a snowstorm, but if conditions aren't windy, it won't become a blizzard.
When a Blizzard Is Imminent
Blizzards don't occur suddenly -- they develop over a period of days, and the National Weather Service can predict one several days before it strikes. It accordingly issues a blizzard warning for the affected areas, and once you're aware of such a warning, it's time to make preparations, including canceling any travel plans you have made. If you're already traveling, or you're hiking in the mountains, you should seek shelter whenever a storm develops, but especially when the three conditions for a blizzard are present: high winds, high humidity and near-freezing ground temperatures. It's important to remember that high winds can also create a "ground blizzard" after a recent snowfall. This is a low-visibility event caused by blowing snow.