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Climate of North Dakota

Blizzards


In popular usage, the term blizzard denotes a severe weather condition characterized by low temperatures and strong winds bearing a great amount of snow, most of which is fine, dry snow picked up from the ground. The National Weather Service definition for a blizzard requires a wind speed greater than 32 mph, temperature less than 20° F, and visibility not greater than 500 feet. Further, a blizzard is classified as severe when winds exceed 45 mph, temperature not greater than 10° F, and visibility less than or equal to 100 feet.

Blizzards in the Great Plains and the North Central states are generally caused by deep or deepening low pressure systems (3). The low pressure systems which cause blizzards usually result from eastward migration of low pressure systems from the lee side of the Rocky Mountains. The most favored position for low pressure systems to originate that cause blizzards in North Dakota is either southern Alberta (Alberta lows) or southern Colorado (Colorado lows).

Only a small number of the lows migrating out of the formation area and crossing the state in winter develop into blizzards. About 39 Colorado lows move out onto the plains during the winter season, and in an average year only two or three develop into blizzards somewhere in the plains or North Central States (3). About 42 Alberta lows develop during the blizzard season, but usually only two of them develop into storms intense enough to cause blizzards in North Dakota. The complexity of the blizzard forecasting problem is easily understood when it is realized that it is basically trying to identify which one out of every 20 lows will develop into a blizzard.

Alberta lows generally move more rapidly than Colorado lows. Once away from their origin, there is little tendency for them to stagnate or meander.

The more rapid movement plus less tendency to stagnate makes blizzard conditions associated with Alberta lows generally of shorter duration than Colorado low blizzards. Blizzards originating from Alberta lows probably pose more of a threat to loss of life because blizzard conditions frequently develop very rapidly and are accompanied by sub-zero temperatures.

Data in Table 15 show that North Dakota with 24 blizzards and South Dakota with 23 blizzards had the greatest number of blizzards in the 1957-67 period. More than half of the blizzards in North Dakota were caused by Alberta lows, while one-third of the blizzards were caused by Colorado lows. About one-eighth of the blizzards in North Dakota stem from sources other than Alberta or Colorado lows such as rapidly moving cold fronts or low pressure systems.

Table 15. Frequency of blizzards by states for years 1957 through 1967 (after Black, 3).

State Colorado Low Alberta Low Other Total frequency of occurence
Colorado 0 0 1 1
Illinois 2 0 1 3
Iowa 10 0 2 12
Kansas 4 0 2 6
Minnesota 8 3 3 14
Missouri 2 0 0 2
Montana 1 8 2 11
Nebraska 14 0 3 17
North Dakota 8 13 3 24
South Dakota 12 6 5 23
Wisconsin 6 0 2 8
Wyoming 2 0 0 2

Colorado lows cause more blizzards in North Dakota than Alberta lows in November and March, while Alberta lows cause more blizzards in December, January and February (Table 15). The reason for this is that during December, January and February the Canadian storm track for Alberta lows is farthest south affecting the area along the Canadian border more frequently than in other months. In November and March, the Colorado low storm tracks are in more northerly positions and thereby affect North Dakota more frequently than in the other months.

In many parts of the U.S.A., the words "North Dakota" and "blizzards" are virtually synonymous. However, blizzards are not a frequent wintertime event in North Dakota, averaging only slightly more than two per winter season (Table 15). Contrary to popular belief of people not familiar with North Dakota, blizzard conditions seldom last more than a day or two and frequently only a few hours. Blizzards are dangerous storms, possibly second only to tornadoes and hurricanes, and they demand the respect of all who must deal with them. Blizzards occasionally strike suddenly, filling a previously calm air with snow driven by strong winds which reduce visibility from miles to feet in minutes. It was this feature of blizzards that brought terror to the pioneers and thereby furnished credence to the North Dakota blizzard legends. The pioneers feared blizzards because they would strike without a hint of warning, frequently on a mild day, catching them miles from home or shelter on the open prairie. In nearly an instant after a blizzard struck, all vestiges of landmarks needed to guide them home would disappear, and death by freezing was the all-too-often result in those early days. Now, the advance warning of impending blizzards given by the National Weather Service has greatly lessened the chances of being surprised by a blizzard, or even being stranded in one, by those who observe the forecasts and warnings. Now, nearly all North Dakotans, while respectful of the consequences, think of blizzards more as minor and temporary inconveniences to their normal life-style than as potential killers to be feared.


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