Abstract
Results are presented from a systematic study of 386 Iowa tornadoes reported over a 13-year period (1959–71). The tornado occurrences are studied as a function of time of day, date, path length, width, 500-mb flow, and their relationship to associated fronts and low-pressure centers. The data surprisingly reveal that Iowa tornadoes fall naturally into two rather distinct classes: northeast-advancing (NE) and southeast-advancing (SE) tornadoes. There also appear to be subgroups of tornadoes occurring ahead of the front and on or behind the front. Considering all the tornadoes studied, the tornado direction of movement is found to be closely tied to the 500-mb flow (statistically significant at the 99.99% level). Tornado path lengths tend to be longest during the spring.
Some contrasts found between the two main classes of Iowa tornadoes are the following:
NE and SE tornadoes comprise approximately 70% and 30%, respectively, of the total in Iowa.
NE tornadoes exhibit a bimodal annual distribution occurring predominantly in the spring and again in the fall, while SE tornadoes have a peak occurrence in the summer.
NE tornadoes tend to occur ahead of the front, while SE tornadoes tend to occur on or behind the front.
NE tornadoes are more closely associated with a low than SE tornadoes.
SE tornadoes tend to occur slightly later in the day than NE tornadoes.
The relationship between path length and width is rather complicated and is independent of direction of movement but depends upon the location of the tornado relative to the front.
Because of the results of recent tornado-damage studies which showed that first-floor rooms facing the tornado direction of approach were statistically less safe than other areas, application of some of the present results promises to lead to increased public safety in upper-midwest tornadic storms.
