Climatological Characteristics of Summertime Extreme Rainstorms in Minnesota.

Each year flash floods cause considerable property damage and loss of life, yet the heavy precipitation events responsible for flash floods remain extremely difficult to forecast, especially during summer. This paper presents synoptic analog models for use in the shod-range prediction of summertime extreme rainstorms in Minnesota. To develop these analog models, forty-three heavy rain events that occurred from 1974-78 first were classified into four similar categories on the basis of the spatial properties of their rainfall distributions. Next, composite fields of surface and upper-air parameters were prepared for each of the four categories, and features on the composite maps that contributed most to the occurrence of heavy rainfall were incorporated into synoptic analog models. The analog models reveal that the typical circulation patterns for the four rainfall types vary significantly and that the spatial characteristics of extreme rainstorms are determined by the synoptic-scale circulation regime in which the rainstorms form. This interrelationship between the spatial characteristics and atmospheric circulation can be used to improve subjective forecasts of location and areal extent of extreme rainstorms. The results of this study also indicate that the preparation of synoptic analog models for local areas can improve our understanding of the climatology of heavy precipitation. [ABSTRACT FROM AUTHOR]