Extraordinarily
Heavy Rains
Fall on
Northwestern and North Central Minnesota
June 9 and 10, 2002
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.
WSI radar-based
precipitation estimate - June 9
WSI radar-based precipitation estimate -
June 10
WSI radar-based
precipitation estimate - June 9/10 combined
WSI radar-based
precipitation estimate - June 11
WSI radar-based
precipitation estimate - June 9 - 11 combined
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.
Related sites:
 Return to the Climate Journal
mcwg@soils.umn.edu
URL:
http://climate.umn.edu/doc/journal/flash_floods/ff020609-10.htm
Last modified: June 14, 2002
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