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A series of disturbances moving along a stalled frontal boundary dropped extremely heavy rain on much of northern Iowa and southern Minnesota on September 14 and 15. Nearly all of northwest and north central Iowa, and south central and southeast Minnesota received over four inches of rain. Six inch totals were common in the region and a large section of north central Iowa and south central Minnesota was inundated by more than eight inches of precipitation. The heaviest Minnesota rainfall reports were from Faribault and Freeborn counties where more the 10 inches of rain fell in a 36 hour period. The largest 36 hour total was a 13 inch value reported by an unofficial weather observer located five miles southwest of Alden in Freeborn county.
The rain began falling during the early morning hours of the 14th. The rain tapered off during the mid-day of the 14th and then reintensified by early evening. Heavy rains continued through the night, finally ending during the morning of the 15th.
The deluge led to numerous reports of stream flooding, urban flooding, mudslides, and road closures. The National Weather Service issued flash flood warnings and flood warnings for 13 Minnesota counties during the episode: Jackson, Martin, Blue Earth, Faribault, Waseca, Freeborn, Steele, Mower, Dodge, Olmsted, Fillmore, Winona, and Houston.
The combination of huge rainfall totals and a very large areal extent, make this episode one of the most significant "flash floods" in Minnesota's climate history. A six inch rainfall total for a given location in this region over a 24 hour period is said to be a "100-year storm". The area receiving six or more inches during this sequence of storms encompasses over 4000 square miles in Minnesota alone. September flash floods are not without precedent. Over a 16 year period from 1970 through 1985, September flash floods made up 16% of all flash flood events studied. For more information on flash floods see: Minnesota Flash Floods: 1970-2000.
The color-coded map seen above is also available as a black-and-white labeled contour map. This Windows Metafile (wmf) is readily imported into word processing documents. A black-and-white version of the map of often preferred when inexpensive photocopying is desired. A list of the 161 data points used to generate this map is available as a text file. The values are x and y utm coordinates (NAD83, zone 15), followed by a 36-hour precipitation total (September 14-15) in inches.
The State Climatology Office thanks the Soil and Water Conservation Districts, the National Weather Service, and all of the diligent volunteer precipitation observers who make analyses of these events possible.