Extreme Heat and Big Storms, July 19, 2019

Cumulonimbi
Distant thunderstorms north of Otsego, which baked in the heat with the rest of southern Minnesota during the afternoon of Friday July 19, 2019.
 
Photo credit: Zach Moore

An exceptionally muggy air mass brought some of hottest-feeling weather in years to southern and central Minnesota, while a sharp cold front ignited an impressive round of damaging thunderstorms across central Minnesota.

Before daybreak, forecasts had highlighted the potential for both the heat and the storms to make headlines, but the storm potential got more attention, at least initially. By 8 AM, NOAA's Storm Prediction Center had indicated a "Moderate Risk" for damaging thunderstorms over east-central Minnesota and into Wisconsin. Despite its common-sounding name, this level of risk is rare, and has not been forecast for any part of Minnesota since July of 2016.  The concern was for strong tornadoes, unusually high winds, and very large hail. Minnesota was fortunate to escape without any confirmed tornadoes, but large hail and damaging winds affected a west-to-east swath through the middle of the state.   

Thunderstorms erupted by mid-morning in western Minnesota and began producing quarter to golf ball-sized hail before noon over Traverse and Stevens counties, and just after noon to the east in Grant, Douglas, and Todd counties. As initial storms weakened, new ones formed nearby, gradually delineating a boundary between storm-cooled air and much hotter and more humid air to the south.

The main event of thunderstorms finally kicked off along that strengthening boundary around 2 PM, with Ottar Tail, Todd, and Wadena counties getting lashed by severe weather for the fourth time in a week, with hail up to the size of ping pong balls. The powerful thunderstorm cells multiplied and intensified, producing hail the size of tennis balls, baseballs--and even larger-- from Morrison into Pine and Chisago counties. The hail broke windows, pounded on vehicles stranded on I-35, pierced through a garage roof near Rock Creek, and flattened crops near West Rock in Pine County.

As the storms took a slight turn to the southeast, they began producing strong straight-line wind damage, first north of Ogilvie in Kanabec County, then into Mora where trees and power lines blew over, and into Pine and Chisago counties, where extensive damage to trees and some buildings occurred. The storms continued intensifying into Wisconsin, where tens to possibly hundreds of thousands of trees were knocked down. In anticipation of extreme winds, the Storm Prediction Center issued an unusually-worded Severe Thunderstorm Watch, with the "Particularly Dangerous Situation" designation. 

This damaging thunderstorm wind event appears to fit the definition of a derecho (pronounced "deh-rey-cho"), because it produced a nearly continuous swath of damage reports exceeding 250 miles in length. The Duluth National Weather Service produced a comprehensive write-up of this event.

South of the severe thunderstorms, a surge of extreme heat pushed heat index values to some of the highest levels on record at many stations. The heat reached its peak as many Minnesotans headed out of town to cool off for the weekend, leading to numerous stalled and overheated vehicles on area highways and roads.

Temperatures rose into the mid 90s, but it was the dew points climbing into the upper 70s and low 80s that made for the oppressive conditions. The Twin Cities International Airport recorded a dew point of 80 degrees at three different hourly observations; combined with a temperature of 95 F, this gave a maximum heat index value of 115 F. Although there are gaps in the dew point record for the Twin Cities that make it impossible to know what the true all-time highs may have been, this value is tied for the second-highest known heat index on record at MSP, behind the 119 F reading on July 19, 2011.

Public Information Statement
National Weather Service Twin Cities/Chanhassen MN
658 PM CDT Fri Jul 19 2019
...MAXIMUM HEAT INDEX REPORTS FROM TODAY...
Heat index values of 100 degrees or more are listed below. 
Location                     Heat Idx. Time/Date       Provider             
Crystal AP                   116 F     0400 PM 07/19   ASOS                 
Mayer 1NE                    116 F     0521 PM 07/19   MNDOT                
Winthrop 2W                  116 F     0445 PM 07/19   MNDOT                
South St. Paul 1WSW          116 F     0411 PM 07/19   MNDOT                
Carver 3S                    116 F     0249 PM 07/19   RAWS                 
Fairmont AP                  115 F     0356 PM 07/19   AWOS                 
Hutchinson AP                115 F     0636 PM 07/19   AWOS                 
Minneapolis-St. Paul AP      115 F     0453 PM 07/19   ASOS                 
Silver Lake 3E               115 F     0420 PM 07/19   MNDOT                
Ellendale 5NNE               115 F     0511 PM 07/19   MNDOT                
Eden Prairie AP              114 F     0353 PM 07/19   ASOS                 
Granite Falls AP             114 F     0536 PM 07/19   AWOS                 
Glencoe AP                   114 F     0256 PM 07/19   ASOS                 
Owatonna AP                  114 F     0459 PM 07/19   AWOS                 
Morton MN-19 Mile Post 78    114 F     0320 PM 07/19   MNDOT                
Cannon Falls 7SSE            113 F     0300 PM 07/19   MNDOT                
Red Wing 4W                  113 F     0520 PM 07/19   MNDOT                
Canby AP                     112 F     0156 PM 07/19   AWOS                 
Redwood Falls AP             112 F     0353 PM 07/19   ASOS                 
Blue Earth 2NW               112 F     0635 PM 07/19   MNDOT                
Hanley Falls MN-23 Mile Post 112 F     0610 PM 07/19   MNDOT                
South St. Paul AP            111 F     0359 PM 07/19   AWOS                 
Stanton AP                   111 F     0635 PM 07/19   AWOS                 
Litchfield 3S                111 F     0611 PM 07/19   RAWS                 
New Ulm AP                   110 F     0335 PM 07/19   AWOS                 
Waseca AP                    109 F     0635 PM 07/19   AWOS                 
St. Paul Dwtn AP             109 F     0553 PM 07/19   ASOS                 
Buffalo AP                   108 F     0515 PM 07/19   AWOS                 
Olivia AP                    106 F     0157 PM 07/19   AWOS                 
Red Wing AP                  105 F     0415 PM 07/19   AWOS                 
New Richmond AP              105 F     0435 PM 07/19   AWOS                 
Montevideo AP                103 F     0235 PM 07/19   AWOS                 
Cambridge AP                 102 F     0356 PM 07/19   AWOS                 
Eau Claire AP                101 F     0634 PM 07/19   ASOS                 
Willmar AP                   100 F     0636 PM 07/19   AWOS                 
Rice Lake AP                 100 F     0455 PM 07/19   AWOS                 
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.

Last modified July 23, 2019