Northern Prairie Wildlife Research Center
Of all the winds that sweep this planet, tornadoes are the most violent. Tornadoes are local storms of short duration consisting of winds rotating at very high speeds, usually counter-clockwise. These small, severe storms form several thousand feet above the earth's surface, usually during warm, humid, unsettled weather, and usually in conjunction with a severe thunderstorm. Sometimes a series of two or more tornadoes is associated with a parent thunderstorm, such as the series of tornadoes which struck the Fargo vicinity in 1957.
In the period from 1953 to 1970, 232 tornadoes were reported in North Dakota for an average of 12.9 tornadoes per year (Table 13). Yearly occurrence of tornadoes has varied widely, ranging from only two in 1961 to 41 in 1965. July is the peak month for tornado activity, and in June and July nearly two-thirds of all tornadoes occur. No part of the state is safe from a tornadic event, although statistics indicate that tornado frequency is higher in the southeast than in other areas of the state. Since 1916, 19 persons have lost their lives in tornadoes and 182 have been injured. The most notable tornado in the state's history slammed into Fargo on June 20, 1957 causing more than $5 million damage and 10 fatalities.
Table 13. North Dakota tornado statistics.
No reliable statistics are available concerning the frequency of windstorms in North Dakota. The main reason for this is that reporting windstorms is based almost entirely on damage to property. In large areas of the state the potential for wind damage is small because of the sparsely settled countryside. Therefore, many vigorous thunderstorms in the state with high winds go unreported which would be detected by property damage in more populous states. The same argument may be true in part for tornadoes, but it is believed in this era of easy and rapid communication that nearly all tornadoes are reported because of their spectacular nature.
Severe thunderstorms with outflow winds strong enough to cause widespread property and tree damage leave their signature at many places about the state every year. Statistics are not available but it would be reasonable to assume that the seasonal frequency distribution of wind storms would parallel that of tornadoes, although the number of occurrences would be much higher. Probably the most destructive windstorm to strike the state happened on July 12, 1943. The windstorm was said to have destroyed 1,847 buildings with damage to 5,678 others. It seems likely that North Dakota windstorms in nearly all years cause several times more property damage than do tornadoes.
A climatology of hail in the central United States was recently published (4). Figure 53, showing the hail climatology for North Dakota, was redrawn from that publication.
Over a 20-year period, the number of days with hail ranged from less than 30 days in the central portion of the state to more than 70 days around Selfridge in the south central. July is the major hail month in North Dakota as it is for most states in the upper Great Plains, followed by June and then August. The lower number of hail days in August, when the state is considered as a whole, was a consequence of about a 50 percent decrease in hail days in many parts of the west and north.
The diurnal frequency of tornadoes, windstorms with winds more than 57 miles per hour (mph), and hailstones 3/4- inch diameter or larger is given in Table 14. The data were compiled by the Severe Local Storms Unit at the National Severe Storms Forecast Center in Kansas City, Missouri (12).
Table 14. Frequency distribution by hour of day of tornadoes, windstorms with winds 58 mph or over, and hailstorms with hailstones 3/4-inch diameter or larger.
|Data for years 1955-1967*|
Tornadoes, windstorms and hailstorms have well-developed daily cycles in North Dakota. Peak activity occurs between 2:00 p.m. and 8:00 p.m. During these hours, about 75 percent of the tornadoes, 62 percent of the windstorms, and 83 percent of the hailstorms take place. It should be emphasized that although three-fourths of the tornadoes are reported between the hours of 2:00 p.m. and 8:00 p.m., a scattering of tornadoes has occurred for nearly all hours of the day during the 12-year period for which the data were processed. Fewer windstorms with winds greater than 57 mph occurred during the six-hour peak activity period than tornadoes or hailstorms, but the windstorm activity continued at a substantially higher rate between the hours of 8:00 p.m. and 1:00 a.m. than the other two types of severe storms. Hailstorms producing large hail appear to be rare between the hours of 5:00 a.m. and 2:00 p.m. However, after 2:00 p.m. hailstorm activity increases sharply and continues at a high rate until dropping off at 8:00 p.m.