The only difference between a blizzard warning and a blizzard watch is time. A blizzard watch is issued when a blizzard is expected within the next 36 hours. A blizzard warning is issued when a blizzard is on your doorstep. You could say the watch is the heads-up so you can plan ahead without being surprised at the last minute. However, sometimes weather can change in unexpected ways within 36 hours, so a blizzard watch isn’t always followed by a blizzard warning.
What is a blizzard?
A blizzard is a very strong snow storm in which heavy snow is accompanied by winds strong enough to reduce visibility. The windspeed and the amount by which visibility is reduced are the other two characteristics which define a blizzard.
Temperature and windchill values aren’t used any more to classify a blizzard. That’s not to say it won’t be bitterly cold in the wind! The combination of freezing temperatures plus blizzard-level winds always creates wind chills of at least -18 degrees Celsius, even when the temperature outside the wind is only just below freezing. It’s a really good idea to stay out of the wind during a blizzard!
All these conditions must last for at least four hours in Canada, or three hours in the US. Up in the Canadian north (Yukon and Northwest Territories, Nunavut), conditions have to stay at blizzard level for at least six hours to be considered a “real” blizzard, so if you’re moving up north to work in the diamond fields, take that difference into account.
The duration difference between the Canadian north and the rest of Canada’s because up north, the wind doesn’t really ever stop blowing below blizzard level at all during November and December. On the other hand, the Canadian north really doesn’t get a lot of snow compared to more southerly parts of Canada, so the blizzard warning up north is mostly about how long the snow lasts and the amount of snow in the storm.
Taken altogether, it makes for the kind of really nasty storm you don’t ever want to be caught in by surprise!
When is a blizzard warning issued in Canada?
In Canada, Environment Canada issues blizzard warnings when hazardous amounts of snow are accompanied by winds of at least 40 kilometres per hour. The warning’s only issued after visibility falls below 400 metres, but don’t go by that if you’re expecting to go outside during the blizzard. In most blizzards, you can’t even see across the street.
The amount of snow that’s considered hazardous is different in different parts of the country, depending on what each place is used to. The lowest amount is 25 centimetres of snow within 24 hours. If Canada had gotten the snow part instead of the rain part of Hurricane Sandy, it might have qualified as a blizzard.
Storms which don’t meet the wind plus blowing snow limits aren’t officially blizzards. However, if they have the right amount of snowfall plus at least one of freezing rain, strong winds, blowing snow or extreme windchill, they still get their own watch and warning, a winter storm warning.
If it’s only wind without snow, a blowing snow warning is issued. If it’s just a lot of snow without the blowing snow and wind, or if it doesn’t last for four hours, a heavy snowfall warning is issued instead.
For lake-effect snow, no matter how much or how long, Environment Canada issues a snow squall warning. Lake-effect snow is too localized to be considered a blizzard, even though all the normal criteria are usually met, and then some!
When is a blizzard warning issued in the US?
The National Weather Service issues blizzard warnings when hazardous amounts of snow are accompanied by winds of at least 35 miles per hour, reducing visibility to below a quarter of a mile. As in Canada, the amount that’s considered hazardous depends on where you are.
Winter storm warnings are issued for winter storms which have at least two of heavy snow, freezing rain, sleet or strong winds. If it’s only wind without snow, a blowing snow advisory is issued. If it’s just a lot of snow without the blowing snow and wind, a heavy snow warning is issued instead.
For lake-effect snow, the NWS issues a lake-effect snow warning. Lake-effect snow is too localized to be considered a blizzard, even though all the normal criteria are usually met.