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Saturday, March 26, 2011

Jeff Masters: Spring snowstorm adds to flooding potential for the Midwest

Spring snowstorm adds to flooding potential for the Midwest

by Jeff Masters, wunderblog, March 24, 2011

A major spring snowstorm dumped heavy snow in excess of six inches over a wide swath of the Upper Midwest this week, adding to a snowpack that is already near or in excess of record levels over Minnesota, North Dakota, and South Dakota. This is bad news for residents in flood-prone areas of the Upper Midwest, as the new storm added more than half an inch of melted rainfall equivalent to the record wet snowpack. When all that snow melts in April, we can expect major and possibly record flooding for North Dakota, South Dakota, Minnesota, and the Upper Mississippi River north of St. Louis, according to the National Weather Service (NWS). Their March Spring Flood Outlook released last week warned: 

A large swath of the North Central United States is at risk of moderate to major flooding this spring. Heavy late summer and autumn precipitation have left soils saturated and streams running high before the winter freeze-up. National Weather Service models show this year's snowpack contains a water content ranked among the highest of the last 60 years, which is similar to the past two years. This threat area extends from northeastern Montana through Wisconsin and along the Mississippi River south to St. Louis. For the third consecutive year, forecasters predict major flooding along the Red River of the North, which forms the state line between eastern North Dakota and northwest Minnesota. Other areas of the Midwest primed for major flooding include Devils Lake in North Dakota, the Milk River in Northeastern Montana, the James and Big Sioux Rivers in South Dakota, the Minnesota River, and the Mississippi River from its headwaters near St. Paul, Minnesota, downstream to St. Louis. 

Figure 1. U.S. spring 2011 flood risk. Image credit: NWS.

Figure 2. NOAA's latest significant river flood outlook shows that spring flooding is already occurring over South Dakota and on the Mississippi River near its junction with the Ohio River, but is not yet widespread across the Upper Midwest.
There is a huge amount of snow on the ground in North Dakota along the tributaries of the Red River, thanks to fall precipitation that was 150%-300% of normal, and winter snows that have dumped up to 400% more precipitation than usual. If one were to melt this snow, it would amount to 4-6 inches of rain. If heavy rains occur at the same time that the snow melts, there is the potential for the greatest flood in history to affect the cities of Fargo and Grand Forks, the largest and third largest cities in North Dakota. NWS is giving a 35% chance that Fargo will see its greatest flood in history this spring, up from the 20% chance they gave in their February spring flood outlook.

The situation is similar in Minnesota, which has received about double its normal precipitation over the past 3-4 months, resulting in the 5th snowiest winter on record in Minneapolis. Snow depths are in excess of 20 inches over wide swaths of of the state, and this snow has a very high water content equivalent to 4-6 inches of rain. NWS is giving a 95% chance that the Mississippi River at St. Paul will exceed major flood stage this spring.

In South Dakota, heavy snows this winter have also left a snowpack with a high water content over the northeast corner of the state. The NWS is predicting a 25% chance that the the James River at Huron, SD will reach its highest flood height in history, and a 50% chance for the Big Sioux River at Brookings, SD.

Figure 3. The snow water equivalent of the Upper Midwest's snowpack as of March 24, 2011. Large sections of Minnesota and North Dakota have the equivalent of 3.9-5.9 inches of rain (purple colors) stored in their snowpack. Image credit: NWS/NOHRSC.

When will all this flooding occur?
The latest guidance from the GFS model predicts winter-like conditions will persist over the Upper Midwest for the next week, with no major storms for the region through early next week. Late next week, there is the potential for a snowstorm that could bring an additional 0.5-1.0" of melted equivalent snow, though this is very uncertain at this point. The first chance of a major thaw will not occur until Sunday, April 3. This will give some time for the current pulse of flood waters generated during last week's warm spell over South Dakota and southern Minnesota to move downstream, and makes the peak of this year's spring flood unlikely to occur until at least the second week of April. 

Looking back at past great floods in the Upper Midwest, the record 2009 Red River flood peaked on March 28 in Fargo. The great 1997 Red River flood that devastated Grand Forks, causing $3.5 billion in damage, crested on April 18. St. Paul's greatest flood in history crested on April 19, 1965. I expect this year's peak flood will most likely arrive during the 3rd week of April.

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