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Portions of northwestern and north central Minnesota experienced one of the most significant precipitation events in Minnesota's post-settlement history on June 9 and 10, 2002. While not unprecedented, the event was extraordinarily rare in its intensity and geographical extent.
Rainfall totals for the 48-hour period beginning during the early-morning hours of Sunday, June 9 exceeded six inches over a broad multi-county area. Rainfall accumulations topped eight inches in portions of Norman, Mahnomen, Marshall, Kittson, Roseau and Koochiching counties. All of Lake of the Woods county fell within the eight inch contour. An incredible twelve inches of rain doused portions of Roseau, Lake of the Woods, and Koochiching counties. The largest rainfall report was a 14.55 inch total near Lake of the Woods on the Roseau/Lake of the Woods county border. Anecdotal reports of fifteen or more inches were received in some areas.
To place the event in historical context, a 48-hour rainfall total of six and one half inches is considered to be a one percent probability occurrence in these areas. The map above indicates that hundreds of square miles exceeded this threshold. Additionally, some communities received more than one half of their total normal annual precipitation during this two-day period. Rainfall events of similar intensity and spatial extent have occurred only twice in the last 30 years in this region. The June 2002 event is on par with the July 1972 "Grand Daddy" of flash floods, and the 1975 flood episode affecting southeastern North Dakota and northwestern Minnesota.
In the southern portion of the affected area, thunderstorms began to drop heavy rains shortly after midnight on June 9 and continued into the morning hours. Six or more inches of rain fell in less than twelve hours in northeastern Clay, southeastern Norman, and western Mahnomen counties. More than nine inches of rain was reported near Twin Valley in Norman county. Northern sections of the affected area received rain during the morning of the 9th, but the rainfall was not nearly as heavy as that experienced by their southern neighbors. However, thunderstorms redeveloped along the Canadian border during the afternoon of the 9th, and moderate to heavy rain continued to fall nearly continuously for the remainder of June 9, into the day on June 10, and finally tapered off in the early morning hours of the June 11.
The thunderstorms produced damaging wind, hail, and some small tornadoes. However, the heavy downpours led to the greatest amount of damage. Many tributaries of the Red River left their banks, creating serious flooding in cities such as Ada in Norman county and Roseau in Roseau county. General urban flooding was reported in many other communities, resulting in flooded basements and blocked streets. Major and minor highways in northwestern and north central Minnesota were closed due to flooding. Widespread inundation of farm fields occurred and will significantly impact agricultural production.
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. Multi-day total images were assembled by the State Climatology Office. 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.
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 270 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 (June 9 and 10) in inches. The data cover the entire state of Minnesota, but the vast majority of the entries are from northwestern and north central 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 DNR Forestry and the National Weather Service. We thank Minnesota's many volunteer precipitation monitors, whose diligent efforts make detailed analysis of storm events possible.