Upper Midwest Severe Thunderstorms: Slight Risk of Large Hail, Damaging Wind, and Tornadoes September 11, 2026

Upper Midwest severe thunderstorms are the headline threat on September 11, 2026. The NOAA Storm Prediction Center’s Day 1 Convective Outlook, issued at 12:26 UTC (8:26 am EDT / 7:26 am CDT), places Duluth, International Falls, Bemidji, Brainerd, Hibbing, and St. Cloud, Minnesota, and Superior, Wisconsin, inside a Slight Risk, the second of five severe-weather categories. Scattered storms capable of large hail, with isolated stones two inches or larger, damaging winds, and a couple of tornadoes are expected late this afternoon into tonight across northern and western Minnesota.

Key points:

• Slight Risk (level 2 of 5) covers far northeast South Dakota and northern and western Minnesota, including Duluth, Bemidji, International Falls, Brainerd, Hibbing, and St. Cloud. The outlook is valid from 13Z (9:00 am EDT / 8:00 am CDT) September 11 through 12Z (8:00 am EDT / 7:00 am CDT) September 12.
• A 15% damaging wind probability from Upper Midwest severe thunderstorms covers 50,442 square miles and about 1.3 million people, matching the Slight Risk outline.
• A 15% large hail probability covers 35,997 square miles across western and north-central Minnesota, with a hatched Intensity Level 1 area where isolated hail of two inches or larger is possible.
• A 5% tornado probability is drawn over Bemidji, Brainerd, Hibbing, and Alexandria, with a 2% tornado probability reaching St. Cloud, Willmar, Fergus Falls, and International Falls.
• Storm development is expected between 22Z and 01Z (6:00 pm to 9:00 pm EDT / 5:00 pm to 8:00 pm CDT), with the wind threat waning across western Wisconsin late this evening.

Where Upper Midwest Severe Thunderstorms Are Most Likely

The Slight Risk for Upper Midwest severe thunderstorms runs from Milbank, South Dakota, across western and northern Minnesota to the Lake Superior shore at Duluth and Superior, Wisconsin, and north to the Canadian border at International Falls and Baudette. Cities inside it include Duluth, Superior, International Falls, Bemidji, Brainerd, Hibbing, Grand Rapids, Ely, Cloquet, Alexandria, Fergus Falls, Detroit Lakes, Park Rapids, Willmar, Little Falls, and St. Cloud.

The SPC puts the 15% damaging wind area at 50,442 square miles and 1,299,435 people, naming Duluth, St. Cloud, Superior, Willmar, and Hibbing. That 15% wind polygon matches the Slight Risk outline to within 3% of its area, so the wind threat alone earned the category across the whole zone.

Large hail is the second hazard from Upper Midwest severe thunderstorms, and it splits the Slight Risk in two. A 15% large hail probability covers 35,997 square miles and 694,055 people across western and north-central Minnesota, including Bemidji, International Falls, Brainerd, Hibbing, Alexandria, Fergus Falls, Willmar, Detroit Lakes, Park Rapids, and Sartell.

A hatched Intensity Level 1 area, the lowest of the SPC’s three conditional intensity groups, covers 45,454 square miles and reaches past the Slight Risk into the Marginal over Sioux Falls, Watertown, and Brookings, South Dakota. The SPC discussion ties that hatching to isolated hail of two inches or larger from the initial supercells. Duluth, Superior, St. Cloud, Ely, and Cloquet sit in the 5% hail area, so damaging wind is the lead hazard for those cities.

Two separate Marginal Risk areas, the first of five SPC categories, flank the Slight Risk for Upper Midwest severe thunderstorms. The northern band wraps the Slight Risk on its south and east sides, from Ashland, Hayward, Rice Lake, and Spooner in northwest Wisconsin through Minneapolis, St. Paul, Hutchinson, and Mankato, then southwest through Sioux Falls, Mitchell, Watertown, Brookings, and Yankton, South Dakota, and Sioux City, Iowa, to Norfolk, Columbus, Lincoln, Grand Island, Kearney, and Hastings, Nebraska. Every city in that band carries a 5% damaging wind probability.

A 5% large hail probability, 103,914 square miles and 5.2 million people, covers most of it, including Minneapolis, St. Paul, Sioux Falls, and Sioux City. Lincoln and the northwest Wisconsin cities sit outside the hail line and carry wind only.

The southeastern Marginal band runs from Jonesboro, Arkansas, and Memphis through Jackson, Clarksville, Nashville, and Chattanooga, Tennessee, Florence and Huntsville, Alabama, Atlanta, Athens, and Augusta, Georgia, Greenville, Spartanburg, and Columbia, South Carolina, Charlotte, Winston-Salem, Greensboro, Raleigh, and Fayetteville, North Carolina, and Danville, Lynchburg, Roanoke, Charlottesville, and Richmond, Virginia. Unlike the Upper Midwest severe thunderstorms zone, it carries a 5% damaging wind probability and no hail or tornado probability. The two 5% wind areas together cover 260,428 square miles and 31.9 million people, with the SPC naming Memphis, Charlotte, Nashville, Atlanta, and Minneapolis. Knoxville, Asheville, Birmingham, Little Rock, Norfolk, Virginia Beach, and Wilmington sit in the general thunderstorm area with no severe probability.

The tornado threat from Upper Midwest severe thunderstorms is narrower than the categorical outline. A 5% tornado probability covers 14,556 square miles and 367,386 people in north-central Minnesota, with the SPC naming Hibbing, Brainerd, Bemidji, and Alexandria. Point-in-polygon checks against the frozen outlook also place Grand Rapids, Park Rapids, Walker, Little Falls, Wadena, Staples, and Sauk Centre inside it.

A surrounding 2% tornado probability covers 34,757 square miles and 660,912 people, taking in St. Cloud, Sartell, Willmar, Fergus Falls, Detroit Lakes, Morris, Montevideo, International Falls, Baudette, Ely, and Cloquet. It spills past the Slight Risk line to Marshall, Minnesota, which sits in the Marginal Risk. Duluth, Superior, and Two Harbors carry no tornado probability.

Meteorologist’s Take on Upper Midwest Severe Thunderstorms

The setup for Upper Midwest severe thunderstorms begins with large-scale upper troughing parked over central Canada. An embedded mid-level shortwave trough is forecast to eject eastward over the northern Plains into the Upper Midwest through the period. The SPC expects a modest surface response across South Dakota, with most guidance developing a sub-1000 mb low into eastern South Dakota by 00Z (8:00 pm EDT / 7:00 pm CDT). A cold front tied to that low and to the primary low over central Canada will push east-southeastward across the Upper Midwest, with a dryline extending south across the central Plains.

Ahead of the front, a narrow corridor of 60s surface dewpoints will combine with daytime heating and steepened mid-level lapse rates to produce moderate destabilization for Upper Midwest severe thunderstorms. Recent NAM and RAP runs show minimal capping along and ahead of the front in far northeast South Dakota and western Minnesota by 22Z to 01Z (6:00 pm to 9:00 pm EDT / 5:00 pm to 8:00 pm CDT). The SPC calls convective development likely in that window, driven by low-level convergence along the front and ascent from the shortwave trough.

Forecast soundings show strongly veering and gradually strengthening winds with height, and 35 to 45 knots of deep-layer shear will support organized convection. Initial supercells pose the large hail threat, including isolated stones of two inches or larger. Storm mode should trend toward clusters fairly quickly because of the cold front and a strengthening low-level jet this evening. Any supercells that stay at least semi-discrete into the early evening may also pose a tornado threat as low-level shear increases, but the SPC rates confidence in that scenario as fairly low because of the expected mode transition.

Scattered damaging winds from Upper Midwest severe thunderstorms are expected with any clusters or bowing line segments that develop and spread east through the evening. The SPC expects the wind threat to wane with eastward extent across Minnesota into western Wisconsin by late this evening because the zone of destabilization is narrow. That is why the Marginal Risk, not the Slight Risk, covers Ashland, Hayward, and Rice Lake, Wisconsin.

Farther south, from southeast South Dakota and southwest Minnesota into the central Plains, storm initiation and coverage are highly uncertain and conditional this afternoon and evening. The better forcing stays across western and northern Minnesota, along and northeast of the surface low. A favorable thermodynamic environment and 25 to 35 knots of deep-layer shear would still support severe hail and wind with any storm that develops and is sustained. Model guidance shows some signal for initiation across southern Nebraska into northern Kansas late this afternoon and evening where surface temperatures run hotter, and the SPC extended the southern end of the Marginal Risk to cover that possibility.

In the Mid-South and Southeast, two weak mid-level perturbations, one over Arkansas and one along the central Gulf Coast, will track east-northeastward through the day. A stalled surface front from the ArkLaTex into the Tennessee Valley and northeastward to Virginia gives storms a focus. The SPC expects mostly disorganized, pulse-type convection with occasional damaging winds as heating steepens low-level lapse rates. Modestly enhanced mid-level flow across parts of the Mid-South and Virginia could promote loosely organized clusters with some damaging wind threat, which is why the 5% wind band extends from Memphis to Richmond.

How to Stay Safe During Upper Midwest Severe Thunderstorms

Large hail and damaging wind are the primary hazards from Upper Midwest severe thunderstorms in the Slight Risk. Large hail means stones at least one inch in diameter, the National Weather Service severe threshold, and the SPC notes isolated stones of two inches or larger are possible within the 15% hail area from Bemidji and International Falls to Alexandria and Willmar. Move vehicles into a garage or under a hard-sided roof before storms arrive. Hail that size cracks windshields and damages siding and roofs.

Damaging wind, a gust of 58 mph or greater, is the one hazard drawn across the entire Slight Risk and both Marginal bands. Go indoors when thunder is heard, stay away from windows, and secure outdoor furniture and anything else wind can lift. If you are driving when a storm hits, pull off the road and park away from trees and power lines. Keep a close watch on active weather alerts so you know the moment a Severe Thunderstorm Warning or Tornado Warning is issued for your county.

The 5% tornado probability over Bemidji, Brainerd, Hibbing, and Alexandria, and the 2% probability reaching St. Cloud and International active weather alerts so you know the moment a Severe Thunderstorm Warning or Tornado Warning is issued for your county.

The 5% tornado probability over Bemidji, Brainerd, Hibbing, and Alexandria, and the 2% probability reaching St. Cloud and International Falls, mean rotating storms cannot be ruled out in the early evening. Identify a lowest-floor interior room now, before Upper Midwest severe thunderstorms develop, and keep a way to receive warnings after dark. SPC uses a five-tier system to communicate severe weather potential; see the iAlert guide to SPC outlook risk categories for a full explanation of each level, and check your local weather forecast for storm timing in your area.

Upper Midwest Severe Thunderstorms FAQ

Where is the highest risk of severe thunderstorms on September 11?

The Slight Risk covering far northeast South Dakota and western and northern Minnesota is the highest category today, with Duluth, Superior, International Falls, Bemidji, Brainerd, Hibbing, Alexandria, Willmar, and St. Cloud inside it. The SPC confined the Slight Risk to where it has the greatest confidence in organized thunderstorm development, and the 15% damaging wind probability covers all of it. The highest combined threat from Upper Midwest severe thunderstorms, with 15% wind, 15% hail, hatched hail intensity, and 5% tornado probabilities stacked together, sits over Bemidji, Brainerd, Hibbing, and Alexandria.

How large could hail get in today’s Upper Midwest severe thunderstorms?

The SPC discussion says initial supercells should pose a threat for large hail, with isolated stones of two inches or larger possible. That threat maps to the 15% hail probability and the hatched Intensity Level 1 area across western and north-central Minnesota, which the SPC puts at 45,454 square miles and about 1 million people, naming Sioux Falls, Watertown, Willmar, Brookings, and Hibbing.

Are tornadoes expected with Upper Midwest severe thunderstorms today?

The SPC forecasts a couple of tornadoes and draws a 5% tornado probability over Bemidji, Brainerd, Hibbing, and Alexandria, with 2% reaching St. Cloud, Willmar, Fergus Falls, and International Falls. The discussion ties the tornado threat to supercells that stay at least semi-discrete into the early evening, and it rates confidence in that scenario as fairly low because storms are expected to cluster quickly along the cold front. Duluth and Superior carry no tornado probability.

When will storms develop?

The SPC expects Upper Midwest severe thunderstorms to develop between 22Z and 01Z (6:00 pm to 9:00 pm EDT / 5:00 pm to 8:00 pm CDT) in far northeast South Dakota and western Minnesota, then spread east through the evening as clusters and bowing line segments. The wind threat should wane across Minnesota into western Wisconsin by late this evening. The outlook is valid through 12Z (8:00 am EDT / 7:00 am CDT) September 12.

Sources:

• NOAA Storm Prediction Center, Day 1 Convective Outlook, issued 12:26 UTC (8:26 am EDT / 7:26 am CDT), September 11, 2026
• NOAA Storm Prediction Center, About the SPC Convective Outlooks: probability-to-category conversion and Conditional Intensity Groups