Heavy Rains Douse Central and Southern Minnesota
August 20-21, 2002

A succession of heavy thunderstorms drifted across central and southern Minnesota on August 20 and 21, leaving a stripe of heavy rain that spanned the width of the state. The heaviest precipitation fell upon southwestern, west central, and central Minnesota where some rainfall totals exceeded five inches. Five inch totals were reported in sections of Lac Qui Parle, Yellow Medicine, Chippewa, Meeker, and Rock counties. The highest rainfall total was 6.30 inches in Montevideo. An anecdotal report of 8.30 inches came from a location one mile east of downtown Montevideo. ff000620.gif (7317 bytes) In west central and central Minnesota, the rains began during the mid-afternoon hours of August 20 and continued into the early morning of August 21. A second rain complex formed during the morning hours of August 21 in southwestern Minnesota and finally cleared southeastern Minnesota by late afternoon.

In the affected areas of west central Minnesota, a 24-hour rainfall total of five inches is considered to be a one percent probability occurrence. Elsewhere in southern Minnesota, a one percent probability event is six inches in 24 hours.

In spite of the intense rainfall, relatively few problems with urban or rural flooding were reported. In west central Minnesota, antecedent conditions were relatively dry, meaning that soils were receptive to the heavy rain.

National Weather Service radar imagery can be used to estimate precipitation totals in near real-time. A private vendor (WSI) purchases radar data from the National Weather Service and utilizes the data to prepare the precipitation estimates offered in the hyperlinks below. Precipitation estimates from radar returns are often fraught with errors. Attenuation from heavy rain, enhanced reflection due to hail, and distance from the radar site, can cause radar-based estimates to over or under estimate true precipitation totals. For these reasons, this technology may never replace the need for ground-based precipitation measurements.

WSI radar-based precipitation estimate - 24 hour totals ending 7:00 AM, August 21
WSI radar-based precipitation estimate - 24 hour totals ending 7:00 AM, August 22

A labeled-line, black-and-white Windows Metafile (wmf) version of the color map can be obtained by right-clicking on this link and choosing "Save Target As" or "Save Link As". This version of the map will be useful for insertion into documentation that requires inexpensive photo-reproduction.

A list of the 156 data points used to generate the map is offered here. The values are x and y utm coordinates (NAD83, zone 15), followed by a 48-hour precipitation total (August 20 and 21) in inches. The data cover the entire state of Minnesota, but the majority of the entries are from the southern half of Minnesota.

The State Climatology Office thanks Soil and Water Conservation Districts in the affected areas for their prompt and thorough response to our request for precipitation data. Data were also provided by the National Weather Service. We thank Minnesota's many volunteer precipitation monitors, whose diligent efforts make detailed analysis of storm events possible.

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URL: http://climate.umn.edu/doc/journal/flash_floods/ff020820-21.htm
Last modified: August 22, 2002