Mid-December Tornadoes, Derecho, and Damaging Cold Front--December 15-16, 2021

This story may take weeks to be in final form. Any statistics reflect current understanding as of the date posted at the bottom.

A line of destructive thunderstorms tracked over south-central and southeastern Minnesota, as an extraordinary weather system produced severe thunderstorm hazards never before recorded during December in the state, along with other record-breaking and highly unusual weather phenomena.

These severe thunderstorms, with winds frequently exceeding 75 mph and occasional embedded tornadoes, traveled hundreds of miles from Nebraska and Kansas, through Iowa, southern Minnesota, and eventually into Wisconsin on Wednesday December 15th, 2021.

The storms entered southwestern Minnesota around 5:30 PM, and exited far southeastern Minnesota by 8:30PM. Moving at speeds of  65 to 80 mph, the storms damaged churches, barns, outbuildings, small businesses, and some homes. The winds uprooted and snapped trees, and blew down power lines as well.

The thunderstorms produced 22 confirmed tornadoes (as of Friday March 11, 2022) in Minnesota. The strongest hit the town of Hartland in Freeborn county, and was rated EF-2, with winds estimated at 115 mph. The National Weather Service confirmed over a dozen additional EF-1 tornadoes, with winds of 90-110 mph, in Fillmore, Freeborn, Goodhue, Houston, Mower, Wabasha, and Winona counties. At least six EF-0 tornadoes with 80-85 mph winds were confirmed in Fillmore, Freeborn, Steele, and Winona counties, including one that went through Preston.

In addition to the tornadoes, the thunderstorms also produced damaging downburst  winds (that is, unrelated to tornadoes), damaging Kasson, Austin, Rochester, and other towns in the southeastern part of the state. The Rochester airport recorded a wind gust of 77 mph at 7:45 PM, and the La Crosse National Weather Service received reports of gusts of up to 85 mph from personal weather sensors in southeastern Minnesota. A man was killed by non-tornadic winds in the Rochester area, when a tree fell on him.

(The National Weather Service may continue to add details about the severe weather reports in the weeks following the storm.)

This event was not only remarkable for occurring in December, but also for the fact that virtually all of the damage occurred after dark. Moreover, it also had an unusually large number hurricane-force wind gusts reported across the region. The NOAA Storm Prediction Center tweeted that this event produced more wind gust reports of at least 75 mph than any other event at any time of year, anywhere in the US, back to 2004, when consistent modern record-keeping began.

The long path length, the nearly-continuous nature of strong winds along that path, and the abundance of high-end and destructive wind gusts, appear to qualify this event as a "Derecho," (pronounced "deh-REY-cho"). Derechos are long-lasting, damaging windstorms caused by concentrated thunderstorms that travel long distances. They often produce pockets or swaths of tornado-like damage. This particular type of derecho is called a "serial derecho," forming from a "quasi-linear convective system", or QLCS. These are long lines or arcs of thunderstorms, with many cells producing damaging gusts as the complex races forward, typically ahead of a cold front.    

If the system had only produced damaging thunderstorms and tornadoes, it would have been enough to qualify it as one of the most significant Minnesota weather events on record. However, when the final severe thunderstorms cleared southeastern Minnesota Wednesday evening, it marked the approximate half-way point on the timeline of damaging weather affecting the state. Indeed, a hand-off to a whole other wave of intense winds was taking place at about that time. 

As the the severe thunderstorms raced away, a cold front plowed west-to-east across the state, ushering in a larger, longer-lasting, and equally intense rush of winds across much of southern Minnesota. These winds were not thunderstorm-related, but instead were the "pressure gradient winds" that equalize the pressure differences between the outer and inner parts of a low-pressure system. In this case, the low-pressure system was quite intense, and was able to generate an area of roaring winds about the size of Kansas. Arriving with bursts of rain, sleet, and snow, these winds eventually crossed the entire state, and produced gusts in excess of 60 mph across the southern third of Minnesota on and off for a period of five hours. The Redwood Falls airport recorded a gust to 78 mph, Jackson recorded a gust to 77 mph, and Fairmont recorded a 70 mph wind gust. These winds also led to damages scattered widely across rural Minnesota, and between the two separate high-wind events, the state experienced tens of thousands of power outages.

The strong gradient winds could have been associated with any similarly powerful low-pressure system, but the destructive thunderstorms were possible only because this low-pressure system was able to pull out-of-season warmth and humidity into the region. Temperatures reached well into the 60s F in southern and southeastern Minnesota, including 66 F at Caledonia and 65 F at Albert Lea and Preston. High temperature records were set at four of Minnesota's five major climate stations, including 64 F at Rochester, 58 F in the Twin Cities, 54 F in St. Cloud, and 49 F in Duluth. The Twin Cities recorded an hourly dew point reading of 54 F, breaking the record for the date, and marking the latest value of 50 F or higher on record. Previously no such values had been recorded between December 5th and February 20th. 

The unseasonably muggy air was followed by a cold front that sent temperatures tumbling by 30-45 degrees in a matter of 12 hours in some areas. Back in the colder air, a blustery winter storm raged in western and northwestern Minnesota, with 2-5 inches of snow falling into winds gusting over 50 mph at times. Travel was nearly impossible in open areas late Wednesday night and early Thursday.

Forecasters had anticipated an extremely active weather day, and Minnesotans woke up Wednesday morning to find that the Storm Prediction Center had placed southeastern Minnesota in a "Moderate Risk" for severe thunderstorms, with the chance for wind gusts exceeding hurricane force and isolated strong tornadoes. Despite the tame-sounding name, Moderate Risks are actually quite rare and generally signify unusual severe weather damage potential. Minnesota typically sees this level of severe weather risk just once or twice per year on average, and has never had one issued during December; not even during November!

The Moderate Risk was followed by two Tornado Watches issued for southern Minnesota. No part of Minnesota had ever been in a Tornado Watch during December. The Tornado Watches were soon overrun by a flurry of Severe Thunderstorm and Tornado Warnings, and nothing even remotely like this had ever happened during any winter month.

The low-pressure system responsible for everything that happened was very strong, with pressure bottoming out near 975 millibars, but not nearly the strongest on record in Minnesota. The snows were also dangerous but somewhat typical. Nearly everything else, from the high temperatures and dew points, to the severe convective storms and tornadoes, to the destructive gales, was unprecedented, record-breaking, or unusual in some way.

Indeed, this mid-December destructive weather event will go down as one of the greats, and certainly as one of the most bizarre and oddly-timed episodes on record in Minnesota. 

Below are some reports from National Weather Service forecast offices around the region. Note: these summaries may not be up-to-date and may be missing some details or known events.

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
1136 AM CST FRI DEC 17 2021
...NWS DAMAGE SURVEY FOR 12/15/21 FREEBORN COUNTY TORNADO EVENT...
.OVERVIEW... HERE ARE THE PRELIMINARY DAMAGE SURVEY RESULTS FOR A
SERIES OF THREE TORNADOES ACROSS RURAL FREEBORN IN SOUTHERN 
MINNESOTA WITH THE MOST SIGNIFICANT DAMAGE IN HARTLAND.
.HARTLAND TORNADO...
RATING:                 EF-2
ESTIMATED PEAK WIND:    115 MPH
PATH LENGTH /STATUTE/:  2.17 MILES
PATH WIDTH /MAXIMUM/:   55 YARDS
FATALITIES:             0
INJURIES:               0
START DATE:             DEC 15 2021
START TIME:             7:10 PM CST
START LOCATION:         1 SW OF HARTLAND
START LAT/LON:          43.7966 / -93.4943
END DATE:               DEC 15 2021
END TIME:               7:13 PM CST
END LOCATION:           2 NNE OF HARTLAND
END LAT/LON:            43.8260 / -93.4751
SURVEY SUMMARY: A FAST MOVING TORNADO TOUCHED DOWN TO THE SOUTHWEST
OF HARTLAND AND MOVED INTO TOWN DAMAGING NUMEROUS BUILDINGS, TREES, 
AND UTILITY POLES. THE DAMAGE IN THE SOUTHERN RESIDENTIAL PART OF TOWN
WAS MAINLY TREE AND ROOF DAMAGE. AS THE TORNADO TRACKED INTO THE CENTER
OF TOWN, MORE SIGNIFICANT DAMAGE WAS OBSERVED TO BUILDINGS ALONG 
BROADWAY STREET. THE MOST SIGNIFICANT DAMAGE WAS THE TWO BUILDINGS 
ALONG BROADWAY AND JOHNSON IN THE CENTER OF TOWN WITH SIGNIFICANT DAMAGE 
TO THE WALLS OF LOW RISE COMMERCIAL BUILDINGS. MORE TREE AND ROOF DAMAGE
WAS OBSERVED TO THE NORTHEAST WITH SOME BRANCHES OBSERVED ALONG STATE
HIGHWAY 13. THE TORNADO DISSIPATED TO THE NORTHEAST IN A FIELD WITH NO 
FURTHER DAMAGE OBSERVED. 
.ALDEN TORNADO 1...
RATING:                 EF-1
ESTIMATED PEAK WIND:    105 MPH
PATH LENGTH /STATUTE/:  3.62 MILES
PATH WIDTH /MAXIMUM/:   90 YARDS
FATALITIES:             0
INJURIES:               0
START DATE:             DEC 15 2021
START TIME:             6:56 PM CST
START LOCATION:         2 ENE OF WALTERS
START LAT/LON:          43.6146 / -93.6292
END DATE:               DEC 15 2021
END TIME:               6:59 PM CST
END LOCATION:           1 SW OF ALDEN
END LAT/LON:            43.6585 / -93.5880
SURVEY SUMMARY: FIRST DAMAGE OBSERVED OF TWISTED ROAD SIGN NEAR TORNADO
ORIGIN POINT. PATH OF TREE AND UTILITY POLE DAMAGE CONTINUED TO THE 
NORTHEAST ENDING JUST SOUTH OF I-90.
.ALDEN TORNADO 2...
RATING:                 EF-1
ESTIMATED PEAK WIND:    100 MPH
PATH LENGTH /STATUTE/:  1.64 MILES
PATH WIDTH /MAXIMUM/:   40 YARDS
FATALITIES:             0
INJURIES:               0
START DATE:             DEC 15 2021 
START TIME:             7:03 PM CST
START LOCATION:         1 E OF ALDEN
START LAT/LON:          43.6728 / -93.5438
END DATE:               DEC 15 2021
END TIME:               7:05 PM CST
END LOCATION:           3 ENE OF ALDEN
END LAT/LON:            43.6932 / -93.5288
SURVEY SUMMARY: AFTER THE DISSIPATION OF THE PREVIOUS TORNADO, A NEW 
TORNADO DEVELOPED TO THE EAST OF ALDEN DAMAGING TREES AND A FARM 
OUTBUILDING. IT THEN CONTINUED TO THE NORTHEAST, DAMAGING ADDITIONAL 
TREES BEFORE DISSIPATING IN A FIELD.
EF SCALE: THE ENHANCED FUJITA SCALE CLASSIFIES TORNADOES INTO
THE FOLLOWING CATEGORIES.
EF0...WEAK......65 TO 85 MPH
EF1...WEAK......86 TO 110 MPH
EF2...STRONG....111 TO 135 MPH
EF3...STRONG....136 TO 165 MPH
EF4...VIOLENT...166 TO 200MPH
EF5...VIOLENT...>200MPH
---
PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE LA CROSSE WI
212 PM CST FRI DEC 17 2021
...NWS DAMAGE SURVEY FOR 12/15/21 TORNADO EVENT - UPDATE 5...
.UPDATE...ADDED TORNADO FROM PLAINVIEW MINNESOTA AREA
.PLAINVIEW AREA TORNADO...
RATING:                 EF1
ESTIMATED PEAK WIND:    93 MPH
PATH LENGTH /STATUTE/:  4.584 MILES
PATH WIDTH /MAXIMUM/:   40.0 YARDS
FATALITIES:             0
INJURIES:               0
START DATE:             12/15/2021
START TIME:             08:02 PM CST
START LOCATION:         3 SE PLAINVIEW / WABASHA COUNTY / MN
START LAT/LON:          44.1313 / -92.1369
END DATE:               12/15/2021
END TIME:               08:05 PM CST
END LOCATION:           4 ENE PLAINVIEW / WABASHA COUNTY / MN
END LAT/LON:            44.1909 / -92.0967
SURVEY SUMMARY: A TORNADO PRODUCED DAMAGE TO NUMEROUS TREES AND
SEVERAL FARM BUILDINGS SOUTHEAST TO NORTHEAST OF PLAINVIEW IN 
WABASHA COUNTY MINNESOTA.
&&
EF SCALE: THE ENHANCED FUJITA SCALE CLASSIFIES TORNADOES INTO THE
FOLLOWING CATEGORIES:
EF0...WEAK......65 TO 85 MPH
EF1...WEAK......86 TO 110 MPH
EF2...STRONG....111 TO 135 MPH
EF3...STRONG....136 TO 165 MPH
EF4...VIOLENT...166 TO 200 MPH
EF5...VIOLENT...>200 MPH
NOTE:
THE INFORMATION IN THIS STATEMENT IS PRELIMINARY AND SUBJECT TO
CHANGE PENDING FINAL REVIEW OF THE EVENT AND PUBLICATION IN NWS
STORM DATA.
---
PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE LA CROSSE WI
608 PM CST THU DEC 16 2021
...NWS DAMAGE SURVEY FOR 12/15/21 TORNADO EVENT - UPDATE 2...
.OVERVIEW...HERE IS A PRELIMINARY DAMAGE SURVEY RESULT FROM
STORMS THAT HIT RURAL PARTS OF WINONA COUNTY, MINNESOTA DURING
THE EVENING HOURS OF DECEMBER 15TH. OTHER SURVEY RESULTS ARE COMING.
.WYATTVILLE TORNADO...
RATING:                 EF-0
ESTIMATED PEAK WIND:    85 MPH
PATH LENGTH /STATUTE/:  0.6 MILES
PATH WIDTH /MAXIMUM/:   40 YARDS
FATALITIES:             0
INJURIES:               0
START DATE:             DEC 15 2021
START TIME:             8:09 PM CST
START LOCATION:         1 MILE N OF WYATTVILLE MN
START LAT/LON:          43.9605 / -91.7913
END DATE:               DEC 15 2021
END TIME:               8:10 PM CST
END LOCATION:           2 MILES N OF WYATTVILLE MN
END LAT/LON:            43.9696 / -91.7892
SURVEY SUMMARY: DAMAGE WAS NOTED TO TREES, A GARAGE, AND SEVERAL
OUTBUILDINGS ALONG AND NEAR COUNTY ROAD 23 BETWEEN LEWISTON AND
WYATTVILLE.
&&
EF SCALE: THE ENHANCED FUJITA SCALE CLASSIFIES TORNADOES INTO THE
FOLLOWING CATEGORIES:
EF0...WEAK......65 TO 85 MPH
EF1...WEAK......86 TO 110 MPH
EF2...STRONG....111 TO 135 MPH
EF3...STRONG....136 TO 165 MPH
EF4...VIOLENT...166 TO 200 MPH
EF5...VIOLENT...>200 MPH
---
PUBLIC INFORMATION STATEMENT  
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN  
546 AM CST THU DEC 16 2021  
..HIGHEST WIND REPORTS SINCE WEDNESDAY EVENING  
SOME REPORTS ARE FROM THUNDERSTORMS, BUT MOST ARE FROM NON-THUNDERSTORM   
WINDS OVERNIGHT.  
LOCATION                     SPEED     TIME/DATE       PROVIDER                         
REDWOOD FALLS AP             78 MPH    1130 PM 12/15   ASOS                               
FAIRMONT AP                  70 MPH    1019 PM 12/15   AWOS                   
EAU CLAIRE AP                67 MPH    0900 PM 12/15   ASOS                   
ELLENDALE 5NNE               65 MPH    1050 PM 12/15   MNDOT                  
EDEN PRAIRIE AP              63 MPH    0211 AM 12/16   ASOS                   
CANNON FALLS 7SSE            63 MPH    0105 AM 12/16   MNDOT                  
ST. JAMES AP                 62 MPH    1155 PM 12/15   AWOS                   
MANKATO AP                   62 MPH    1107 PM 12/15   AWOS                   
OWATONNA AP                  62 MPH    1053 PM 12/15   AWOS                   
TWIN LAKES 5SE               62 MPH    1110 PM 12/15   MNDOT                  
MINNEAPOLIS-ST. PAUL AP      61 MPH    0317 AM 12/16   ASOS                   
RED WING 4W                  61 MPH    1235 AM 12/16   MNDOT                  
WASECA AP                    60 MPH    1135 PM 12/15   AWOS                   
OLIVIA AP                    60 MPH    1255 AM 12/16   AWOS                   
NEW ULM AP                   60 MPH    1135 PM 12/15   AWOS                   
WASECA 1SSE                  60 MPH    1130 PM 12/15   MNDOT                  
RED WING                     59 MPH    0206 AM 12/16   CWOP                   
HANSKA                       59 MPH    0700 PM 12/15   CWOP                   
LYDIA                        59 MPH    0116 AM 12/16   CWOP                   
EAU CLAIRE                   59 MPH    0108 AM 12/16   CWOP                   
HUTCHINSON AP                59 MPH    1155 PM 12/15   AWOS                   
NEW GERMANY 1NW              59 MPH    1220 AM 12/16   MNDOT                  
GANOE HILL                   59 MPH    0107 AM 12/16   RAWS                   
HOPE                         58 MPH    1109 PM 12/15   CWOP                   
STANTON AP                   58 MPH    1115 PM 12/15   AWOS                   
MADELIA 3ENE                 58 MPH    1150 PM 12/15   MNDOT                  
MINNESOTA LAKE               57 MPH    1045 PM 12/15   CWOP                   
SILVER LAKE 3E               57 MPH    1206 AM 12/16   MNDOT                  
CLARA CITY 2WSW              57 MPH    1110 PM 12/15   MNDOT                  
WINTHROP 2W                  57 MPH    1140 PM 12/15   MNDOT                  
MANKATO US-169 MILE POST 52  57 MPH    0100 AM 12/16   MNDOT                  
EASTON                       56 MPH    1046 PM 12/15   CWOP                   
ALBERT LEA AP                56 MPH    1055 PM 12/15   AWOS                   
WILLMAR AP                   56 MPH    1215 AM 12/16   AWOS                   
CRYSTAL AP                   56 MPH    0436 AM 12/16   ASOS                   
ST. PAUL DWTN AP             56 MPH    0200 AM 12/16   ASOS                   
LITCHFIELD 3S                56 MPH    0211 AM 12/16   RAWS                   
GLENCOE AP                   55 MPH    1215 AM 12/16   AWOS 
---
PUBLIC INFORMATION STATEMENT  
NATIONAL WEATHER SERVICE LA CROSSE WI  
841 PM CST WED DEC 15 2021  
..HIGHEST WIND REPORTS SO FAR TODAY [thunderstorm winds] 
LOCATION                     SPEED     TIME/DATE                              
ROCHESTER INTERNATIONAL AIRP 77 MPH    0745 PM 12/15                          
JCT CR 7 I-90 MILE POST 226  74 MPH    0800 PM 12/15
1 ENE DEXTER                 73 MPH    0740 PM 12/15                                                  
2 E KASSON                   72 MPH    0750 PM 12/15                                                                        
3 N STEWARTVILLE             59 MPH    0750 PM 12/15                          
ROCHESTER                    57 MPH    0756 PM 12/15                          
WABASHA                      57 MPH    0828 PM 12/15                          
1 SSE KELLOGG                57 MPH    0820 PM 12/15                                                   
AUSTIN MUNICIPAL AIRPORT     54 MPH    0735 PM 12/15 
---
PUBLIC INFORMATION STATEMENT  
NATIONAL WEATHER SERVICE LA CROSSE WI  
450 AM CST THU DEC 16 2021  
..HIGHEST NON-THUNDERSTORM WIND REPORTS FROM LAST 12 HOURS  
LOCATION                     SPEED     TIME/DATE       LAT/LON                
..MINNESOTA  
..DODGE COUNTY  
DODGE CENTER AIRPORT         58 MPH    0135 AM 12/16   44.02N/92.82W          
..FILLMORE COUNTY  
PRESTON AIRPORT              60 MPH    1135 PM 12/15   43.68N/92.17W          
..MOWER COUNTY  
AUSTIN MUNICIPAL AIRPORT     58 MPH    1115 PM 12/15   43.67N/92.92W          
..OLMSTED COUNTY  
ROCHESTER INTERNATIONAL AIRP 77 MPH    1125 PM 12/15   43.90N/92.50W          
3 N STEWARTVILLE             65 MPH    1205 AM 12/16   43.90N/92.48W          
MAYO HELIPAD                 61 MPH    1115 PM 12/15   44.02N/92.48W                    
ROCHESTER                    48 MPH    1155 PM 12/15   44.05N/92.53W                   
BYRON                        45 MPH    0216 AM 12/16   44.04N/92.64W          
..WABASHA COUNTY  
1 SSE KELLOGG                63 MPH    1150 PM 12/15   44.28N/91.99W          
LAKE CITY                    60 MPH    0145 AM 12/16   44.32N/92.44W          
..WINONA COUNTY  
WINONA MUNICIPAL AIRPORT     52 MPH    1155 PM 12/15   44.07N/91.70W                                                    
---

Modified March 11, 2022