Southern Minnesota Mega-Rain, July 25-26, 2020

A moisture-heavy air mass unloaded torrential rains for a period of 6-12 hours across a large swath of southern Minnesota from the evening of Saturday July 25th into the morning of Sunday July 26th.

The thunderstorms formed in an air mass that contained some the highest total moisture measured in southern Minnesota this season. The 2.25 inches of "precipitable water" tallied from the 7 PM weather balloon launch at Chanhassen was just below record record levels for the day and was above the 95th percentile for this time of year. The excessively humid air mass led to dew point temperatures in the middle and upper 70s throughout Minnesota by Saturday afternoon.

Showers and thunderstorms on the morning of the 25th had left a patchwork of clouds and minor boundaries all over Minnesota. Where the sun could peek out of the clouds, temperature rose to near 90 F or higher, but generally remained in the 80s elsewhere. The winds aloft were weak, and the only real cold front was just creeping into far northwestern Minnesota on Saturday afternoon. The stage was set for prolonged intense rains somewhere in Minnesota, but it was unclear exactly where the storms would form.

The main event began as a large thunderstorm developed over Sibley County just after 5 PM. The storm intensified rapidly, and produced a confirmed tornado near Gaylord around 630 PM. With nothing to steer it or drive it away, the thunderstorm moved little while expanding eastward slowly and "back-building" to the west. The thunderstorms reached into the southern Twin Cities metropolitan area by mid-evening, but they remained anchored over the same areas of Renville, Sibley, and Le Sueur Counties even as darkness fell. The growing but almost stationary storm complex produced vivid lightning, nearly constant thunder, and spectacular late-evening optics, even in areas that received little or no rainfall.

By 9 PM, heavy rains had been falling continuously for 2-4 hours from near Fairfax in Renville County to Gibbon and Winthrop in Sibley County, and over into much of Le Sueur County. The thunderstorms eventually pushed southeastward into Nicollet, Blue Earth, Waseca, and Rice Counties, but as they did were replaced quickly by new storms that resumed lashing the areas that had already been drenched by earlier rains. The National Weather Service in Chanhassen had issued Flash Flood Warnings for a multi-county swath of southern Minnesota.

Even at 3 AM, new strong thunderstorms were forming in southwestern and western Minnesota, as the surge of deep moisture into the region continued, and as the cold front inched its way eastward across the state. The heaviest rains began winding down after sunrise, but moderate rains persisted in some areas through 8 AM.

The expansive, regenerating, and long-lasting intense thunderstorms activity resulted in a wide swath of very high precipitation totals. Automated rain gauges and human observers in six different counties reported rainfall totals of six inches or greater.

The highest known total for the storm came from near Winthrop in Sibley County, where a volunteer observer for the county reported 11.50 inches. Other high totals in that area included 10.70 inches near Lafayette, and 9.15 inches in Gibbon. These amounts are near or exceed the 500-year threshold for 24-hour rainfall in that part of Minnesota.

Southeast of the area of most extreme rainfall, a CoCoRaHS observer just south of Mankato, reported 8.65 inches. Other totals of 8 inches or more were found in the area, with numerous 6-7-inch reports near St. Clair, St. Peter, Kasota, and Morristown. A DNR automated rain gauge at Fort Ridgely State Park recorded 7.77 inches, with the same type of equipment reporting 6.37 inches in Morristown. These values are likely "under-catches" by up to 10%, because automated tipping buckets cannot keep up with intense rainfall rates. The State Climatology Office thanks all of the volunteer observers whose reports added significant information about the extent and magnitude of this major event.

This storm produced six inches of rain or more over an area of roughly 1000 square miles, making this event the first "mega-rain" since 2016. Apart from water covering roads and filling many ditches, however, this event produced little in the way of major damages. Most rains of this magnitude produce landslides, wash out roads, and damage public and private property, but fortunately, this one came when river levels had been relatively low, and when area soils had been in good condition after a mostly "normal" summer.

Following is a round-up of rainfall reports from the National Weather Service office in Chanhassen.

Public Information Statement
National Weather Service Twin Cities/Chanhassen MN
1006 AM CDT Sun Jul 26 2020
...July 25-26 Heavy Rain Event...
Location                     Amount    Time/Date       Provider
...Minnesota...
...Anoka County...
Ramsey 3NE                   1.05 in   0837 AM 07/26   COCORAHS
...Benton County...
Sauk Rapids                  1.27 in   0957 AM 07/26   CWOP
Rice 3.8 SSE                 1.27 in   0700 AM 07/26   COCORAHS
...Blue Earth County...
Mankato 3.3 SSW              8.65 in   0800 AM 07/26   COCORAHS
Skyline 2ESE                 8.63 in   0700 AM 07/26   COCORAHS
Mankato 0.2 SSW              7.93 in   0600 AM 07/26   COCORAHS
Mankato 1ESE                 7.75 in   0700 AM 07/26   COCORAHS
Mankato 1.2 SE               7.38 in   0700 AM 07/26   COCORAHS
Mankato 2.9 WSW              7.30 in   0800 AM 07/26   COCORAHS
Mankato 4.0 SW               7.05 in   0815 AM 07/26   COCORAHS
St. Clair                    6.96 in   0645 AM 07/26   HADS
Eagle Lake 0.4 SW            6.90 in   0700 AM 07/26   COCORAHS
Mankato 1.3 SW               6.74 in   0600 AM 07/26   COCORAHS
Mankato 6.4 S                6.69 in   0700 AM 07/26   COCORAHS
Mankato 2.2 NNE              6.58 in   0804 AM 07/26   COCORAHS
4.5 SE Skyline               6.08 in   0900 AM 07/26   HADS
4.6 NE Good Thunder          5.98 in   0900 AM 07/26   HADS
Garden City 2.7 WSW          5.87 in   0750 AM 07/26   COCORAHS
4.4 NE Good Thunder          5.47 in   0900 AM 07/26   HADS
Mankato AP                   5.38 in   0956 AM 07/26   AWOS
1.6 W Garden City            5.21 in   0930 AM 07/26   HADS
Mapleton 4N                  4.55 in   0830 AM 07/26   COCORAHS
Lake Crystal 4.9 N           4.30 in   0630 AM 07/26   COCORAHS
4.8 S Good Thunder           3.19 in   0900 AM 07/26   HADS
...Brown County...
New Ulm 0.2 SSW              5.01 in   0800 AM 07/26   COCORAHS
Sleepy Eye 0.8 SW            2.87 in   0800 AM 07/26   COCORAHS
Comfrey                      2.52 in   0955 AM 07/26   CWOP
4.6 SE Cobden                2.21 in   0830 AM 07/26   HADS
Springfield 1NW              1.92 in   0700 AM 07/26   COCORAHS
Springfield                  1.89 in   0946 AM 07/26   CWOP
4.3 SW Springfield           1.66 in   0930 AM 07/26   HADS
3.1 SE Cobden                1.01 in   0900 AM 07/26   HADS
...Carver County...
Watertown 0.5 NNW            2.07 in   0700 AM 07/26   COCORAHS
1.9 NW Jordan                1.66 in   0930 AM 07/26   USGS
Waconia 0.7 WNW              1.39 in   0800 AM 07/26   COCORAHS
Waconia                      1.38 in   0700 AM 07/26   UCOOP
Carver 3S                    1.28 in   0949 AM 07/26   RAWS
Waconia 1WSW                 1.20 in   0959 AM 07/26   AWS
...Chippewa County...
8 NE Watson                  1.20 in   0955 AM 07/26   AWS
Raymond 4.7 WSW              1.10 in   0800 AM 07/26   COCORAHS
...Chisago County...
Scandia                      1.47 in   1000 AM 07/26   CWOP
Wyoming                      1.07 in   0956 AM 07/26   CWOP
...Dakota County...
Northfield 1NNW              2.00 in   0930 AM 07/26   COCORAHS
Farmington 3E                1.52 in   0800 AM 07/26   COCORAHS
West Saint Paul 1.2 NE       1.48 in   0800 AM 07/26   COCORAHS
Farmington 2ESE              1.40 in   0935 AM 07/26   COCORAHS
4.4 SE Lakeville             1.40 in   0815 AM 07/26   HADS
Inver Grove Heights 1.6 NE   1.35 in   0751 AM 07/26   COCORAHS
Hastings 0.7 S               1.25 in   0600 AM 07/26   COCORAHS
Lakeville 2.1 N              1.25 in   0815 AM 07/26   COCORAHS
Hastings 1.6 NW              1.21 in   0700 AM 07/26   COCORAHS
Farmington                   1.12 in   0700 AM 07/26   UCOOP
Miesville 2WSW               1.11 in   0700 AM 07/26   COCORAHS
Lakeville AP                 1.07 in   0955 AM 07/26   AWOS
2.6 SW Coates                1.03 in   0900 AM 07/26   HADS
Hastings                     1.00 in   0530 AM 07/26   COOP
...Faribault County...
Minnesota Lake               3.56 in   0946 AM 07/26   CWOP
Winnebago                    2.20 in   0713 AM 07/26   COOP
Wells                        2.11 in   0946 AM 07/26   CWOP
Wells 3.5 SE                 2.10 in   0900 AM 07/26   COCORAHS
Elmore 1N                    1.88 in   0700 AM 07/26   COCORAHS
Frost 0.3 ENE                1.45 in   0700 AM 07/26   COCORAHS
Blue Earth 0.6 ESE           1.44 in   0700 AM 07/26   COCORAHS
0.6 W Blue Earth             1.31 in   0615 AM 07/26   HADS
0.7 N Blue Earth             1.21 in   0615 AM 07/26   HADS
Bricelyn                     1.01 in   0700 AM 07/26   COOP
...Freeborn County...
Albert Lea 1WSW              2.32 in   0700 AM 07/26   COCORAHS
...Goodhue County...
3 ENE Dennison               1.76 in   0955 AM 07/26   AWS
Cannon Falls 2S              1.44 in   0915 AM 07/26   HADS
Dennison 0.4 SSE             1.43 in   0700 AM 07/26   COCORAHS
7.0 S Cannon Falls           1.43 in   0900 AM 07/26   HADS
Red Wing                     1.37 in   0955 AM 07/26   CWOP
Cannon Falls 5.6 ESE         1.37 in   0830 AM 07/26   COCORAHS
Dennison 2.0 NNE             1.33 in   0700 AM 07/26   COCORAHS
Nerstrand 4E                 1.30 in   0700 AM 07/26   COCORAHS
Cannon Falls 0.5 SE          1.29 in   0800 AM 07/26   COCORAHS
Red Wing                     1.22 in   0945 AM 07/26   CWOP
Stanton AP                   1.20 in   0955 AM 07/26   AWOS
4.4 S Maiden Rock            1.18 in   0830 AM 07/26   HADS
3.5 SW Maiden Rock           1.18 in   0830 AM 07/26   HADS
Zumbrota                     1.12 in   0958 AM 07/26   CWOP
Vasa 5NNE                    1.12 in   0700 AM 07/26   COCORAHS
Zumbrota 0.6 SSE             1.10 in   0952 AM 07/26   COCORAHS
Red Wing 3SE                 1.08 in   0700 AM 07/26   COCORAHS
Cannon Falls 0.7 SSW         1.08 in   0700 AM 07/26   COCORAHS
Dennison 6E                  1.05 in   0700 AM 07/26   COCORAHS
Wanamingo 4SSE               1.03 in   0800 AM 07/26   COCORAHS
...Hennepin County...
Crystal AP                   1.08 in   0953 AM 07/26   ASOS
Minneapolis 3.6 SSE          1.00 in   0900 AM 07/26   COCORAHS
...Kandiyohi County...
Pennock 6NNE                 1.93 in   0700 AM 07/26   COCORAHS
New London 6WSW              1.25 in   0700 AM 07/26   COCORAHS
2 NW Pennock                 1.17 in   0955 AM 07/26   AWS
5 SW Atwater                 1.15 in   0955 AM 07/26   AWS
4.7 W New London             1.00 in   0915 AM 07/26   HADS
...Lac qui Parle County...
Madison 3.6 NW               1.13 in   0800 AM 07/26   COCORAHS
...Le Sueur County...
2.6 SW Kasota                6.25 in   0900 AM 07/26   HADS
3 N Madison Lake             3.74 in   0955 AM 07/26   AWS
Le Center 5NNW               2.82 in   0800 AM 07/26   COCORAHS
New Prague 1ESE              2.55 in   0725 AM 07/26   COCORAHS
Montgomery 3E                2.45 in   0700 AM 07/26   COCORAHS
Montgomery 2.1 N             2.24 in   0800 AM 07/26   COCORAHS
New Prague 1.0 S             1.86 in   0700 AM 07/26   COCORAHS
...Martin County...
4 N Ceylon                   1.34 in   0920 AM 07/26   AWS
6 SSE Granada                1.34 in   0955 AM 07/26   AWS
Fairmont 0.7 E               1.34 in   0700 AM 07/26   COCORAHS
Fairmont AP                  1.30 in   0956 AM 07/26   AWOS
3 NNE Granada                1.12 in   0955 AM 07/26   AWS
...McLeod County...
Brownton                     1.58 in   0845 AM 07/26   HADS
Brownton 0.2 SSE             1.30 in   0800 AM 07/26   COCORAHS
...Mille Lacs County...
Milaca                       2.18 in   0951 AM 07/26   CWOP
Foreston                     1.80 in   0956 AM 07/26   CWOP
Foreston 2.1 NW              1.77 in   0645 AM 07/26   COCORAHS
1.3 NW Princeton             1.05 in   0900 AM 07/26   HADS
Princeton AP                 1.02 in   0955 AM 07/26   AWOS
...Nicollet County...
4.9 S Fairfax                7.77 in   0615 AM 07/26   HADS
North Mankato 0.8 SSW        7.50 in   0730 AM 07/26   COCORAHS
Saint Peter 0.6 NW           6.95 in   0800 AM 07/26   COCORAHS
Kasota 4WSW                  6.71 in   0700 AM 07/26   COCORAHS
0.6 E St. Peter              6.67 in   0900 AM 07/26   UCOOP
St. Peter 6WSW               6.57 in   0700 AM 07/26   COCORAHS
St. Peter                    6.52 in   0600 AM 07/26   COOP
3.7 SW Kasota                5.89 in   0900 AM 07/26   HADS
Judson                       5.74 in   0800 AM 07/26   UCOOP
Nicollet                     4.98 in   0958 AM 07/26   CWOP
New Ulm 3SE                  4.52 in   0700 AM 07/26   COOP
North Mankato                3.81 in   1000 AM 07/26   AWS
...Pope County...
8 WNW Sunburg                1.16 in   0955 AM 07/26   AWS
...Ramsey County...
St. Paul Dwtn AP             1.31 in   0953 AM 07/26   ASOS
3 S Maplewood                1.05 in   1000 AM 07/26   AWS
St. Paul 1N                  1.00 in   0800 AM 07/26   COCORAHS
...Redwood County...
Redwood Falls 0.6 NE         5.39 in   0700 AM 07/26   COCORAHS
1.1 NE Lamberton             4.75 in   0900 AM 07/26   HADS
Redwood Falls                2.82 in   0700 AM 07/26   COOP
Redwood Falls AP             2.64 in   0953 AM 07/26   ASOS
Lamberton                    2.05 in   0956 AM 07/26   CWOP
Belview 0.3 S                1.92 in   0630 AM 07/26   COCORAHS
Wabasso 3E                   1.76 in   0800 AM 07/26   COCORAHS
...Renville County...
Fairfax 4.9 SSE              8.00 in   0700 AM 07/26   COCORAHS
Bird Island 7SSE             3.27 in   0800 AM 07/26   COOP
Bird Island 7SSE             3.27 in   0800 AM 07/26   COCORAHS
Morton                       2.59 in   0845 AM 07/26   HADS
Delhi 5NE                    2.20 in   0700 AM 07/26   COCORAHS
Bird Island 0.3 SW           2.02 in   0650 AM 07/26   COCORAHS
Danube 2WNW                  1.79 in   0700 AM 07/26   COCORAHS
4.1 W Sacred Heart           1.44 in   0845 AM 07/26   HADS
Sacred Heart 5.8 S           1.15 in   0700 AM 07/26   COCORAHS
Lake Lillian 5SE             1.05 in   0830 AM 07/26   COCORAHS
...Rice County...
Morristown                   6.37 in   0815 AM 07/26   HADS
Faribault 1.6 S              3.29 in   0700 AM 07/26   COCORAHS
Faribault                    3.28 in   0957 AM 07/26   CWOP
Faribault 1SSW               3.11 in   0959 AM 07/26   AWS
Faribault 1.7 N              3.07 in   0800 AM 07/26   COCORAHS
Faribault 1.1 E              2.95 in   0800 AM 07/26   COCORAHS
Faribault 3.5 ESE            2.83 in   0700 AM 07/26   COCORAHS
Veseli 2E                    2.30 in   0700 AM 07/26   COCORAHS
Montgomery 3ENE              1.97 in   0600 AM 07/26   COCORAHS
Northfield 3.9 SSW           1.75 in   0700 AM 07/26   COCORAHS
Northfield                   1.50 in   0845 AM 07/26   UCOOP
Northfield 2.1 SSW           1.42 in   0715 AM 07/26   COCORAHS
Northfield 1.2 ESE           1.38 in   0700 AM 07/26   COCORAHS
Veseli 1W                    1.25 in   0500 AM 07/26   COCORAHS
Northfield 0.4 NW            1.25 in   0600 AM 07/26   COCORAHS
...Scott County...
New Prague 1.0 NNW           2.22 in   0700 AM 07/26   COCORAHS
Jordan 1SSW                  2.20 in   0530 AM 07/26   COOP
New Prague 3.8 NE            1.92 in   0745 AM 07/26   COCORAHS
New Prague 0.9 NNE           1.90 in   0638 AM 07/26   COCORAHS
2 WNW Elko New Market        1.78 in   0955 AM 07/26   AWS
Jordan 4.2 SSE               1.65 in   0800 AM 07/26   COCORAHS
Lydia                        1.32 in   0946 AM 07/26   CWOP
Lakeville                    1.17 in   0956 AM 07/26   CWOP
Shakopee 1NW                 1.16 in   0957 AM 07/26   CWOP
...Sherburne County...
Elk River                    2.58 in   0950 AM 07/26   CWOP
Elk River 3.2 W              1.72 in   0700 AM 07/26   COCORAHS
Elk River 1.6 S              1.59 in   0700 AM 07/26   COCORAHS
...Sibley County...
4 E Gibbon                   7.64 in   0955 AM 07/26   AWS
Gibbon                       6.61 in   0946 AM 07/26   CWOP
...Stearns County...
1.4 NW St. Martin            1.78 in   0845 AM 07/26   HADS
Clearwater 3.2 SW            1.54 in   0800 AM 07/26   COCORAHS
Belgrade 0.4 WNW             1.45 in   0900 AM 07/26   COCORAHS
Sauk Centre AP               1.17 in   0955 AM 07/26   AWOS
Kimball 3N                   1.12 in   0600 AM 07/26   COOP
Sartell 2.6 SW               1.00 in   0800 AM 07/26   COCORAHS
...Steele County...
Hope                         4.18 in   0958 AM 07/26   CWOP
Owatonna 1.8 SSE             3.89 in   0630 AM 07/26   COCORAHS
Owatonna 1.5 S               3.59 in   0700 AM 07/26   COCORAHS
Owatonna                     3.31 in   1000 AM 07/26   AWS
Owatonna 1.0 ENE             3.15 in   0710 AM 07/26   COCORAHS
Owatonna 0.8 E               2.94 in   0700 AM 07/26   COCORAHS
1.9 NE Owatonna              2.92 in   0845 AM 07/26   HADS
Ellendale 0.3 SW             2.35 in   0700 AM 07/26   COCORAHS
Owatonna                     2.20 in   0800 AM 07/26   COOP
...Swift County...
1.2 W Benson                 1.37 in   0715 AM 07/26   HADS
...Waseca County...
Morristown 2SW               5.35 in   0651 AM 07/26   COCORAHS
Waseca 6.0 N                 5.28 in   0800 AM 07/26   COCORAHS
Waseca 1NNE                  4.81 in   0700 AM 07/26   COCORAHS
Waseca 2.0 N                 4.70 in   0700 AM 07/26   COCORAHS
Waseca AP                    1.13 in   0459 AM 07/26   AWOS
...Washington County...
2 ENE Mahtomedi              2.15 in   0955 AM 07/26   AWS
Stillwater                   1.98 in   0955 AM 07/26   CWOP
Stillwater 2 SW              1.94 in   0900 AM 07/26   COOP
Woodbury                     1.30 in   0741 AM 07/26   CWOP
Afton 1E                     1.29 in   0850 AM 07/26   COCORAHS
Denmark Township             1.28 in   0950 AM 07/26   CWOP
3 NNE Stillwater             1.27 in   0955 AM 07/26   AWS
Stillwater 0.8 SW            1.25 in   0725 AM 07/26   COCORAHS
Lakeland 1.8 S               1.20 in   0920 AM 07/26   COCORAHS
Forest Lake 1SE              1.17 in   0959 AM 07/26   AWS
Lakeland                     1.15 in   0955 AM 07/26   CWOP
Stillwater 0.2 E             1.12 in   0615 AM 07/26   COCORAHS
Stillwater 5.7 SSE           1.07 in   0754 AM 07/26   COCORAHS
...Watonwan County...
2 N Madelia                  4.04 in   0955 AM 07/26   AWS
Madelia                      3.87 in   0950 AM 07/26   CWOP
Saint James 0.5 SSW          3.65 in   0700 AM 07/26   COCORAHS
2.9 W Madelia                3.64 in   0745 AM 07/26   HADS
3.5 E La Salle               3.27 in   0830 AM 07/26   HADS
...Wright County...
Clearwater 4SE               2.00 in   0700 AM 07/26   COCORAHS
Clearwater                   1.87 in   0915 AM 07/26   HADS
Delano                       1.70 in   0730 AM 07/26   HADS
Maple Lake                   1.22 in   0945 AM 07/26   CWOP
Winsted                      1.22 in   0956 AM 07/26   CWOP
Golden Valley                1.20 in   1000 AM 07/26   CWOP
Monticello                   1.18 in   0958 AM 07/26   CWOP
Otsego 1.8 ENE               1.08 in   0748 AM 07/26   COCORAHS
4.0 NE Cokato                1.00 in   0945 AM 07/26   HADS
...Yellow Medicine County...
Granite Falls                1.51 in   0900 AM 07/26   UCOOP
4.4 S Hazel Run              1.21 in   0900 AM 07/26   HADS
Observations are collected from a variety of sources with varying
equipment and exposures. We thank all volunteer weather observers
for their dedication. Not all data listed are considered official.

Modified July 27, 2020