The most acute risk covers the eastern portions of the Dakotas and western Minnesota, where the National Weather Service Storm Prediction Center has predicted a level 4 in 5 threat of severe weather.
“Severe thunderstorm gusts (some near 75 mph), large hail and a few tornadoes are expected,” the center wrote.
For the third time in four days, Minneapolis finds itself under an “enhanced” Level 3 of 5 storm threat on Thursday. a few tornadoes in the area. Tens of thousands of people lost power.
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Inside the Wednesday night storms
Thunderstorms erupted in eastern Nebraska and western Iowa on Wednesday afternoon, quickly developing into a large drop with reduced tornado risk. As night fell, however, a strengthening low-level jet stream helped connected storms fan out into a “bow echo,” or a backward C-shaped line of squalls that looks like an archer’s bow.
This curved squall line was propelled forward by strong jet-stream winds descending to the surface behind the initial blast of precipitation. Windom in Cottonwood County in southwestern Minnesota reached 76 mph, and Morristown, 50 miles south of Minneapolis, reported a gust of 79 mph. Carver, a southwest suburb of Minneapolis, saw a gust of 77 mph.
There were also a number of tornado warnings pasted on the subway, though the National Weather Service had yet to dispatch meteorologists to distinguish straight-line wind damage from that of brief tornadoes. There were a few short-lived circulations that showed up on radar, but it’s unclear how many tornadoes may have touched down.
Record heat fueling storms
Contributing to the storms was the exceptional heat. A prominent high-pressure thermal dome, flanked by a pair of counterclockwise-rotating lows, is stationed over the Great Lakes and southeastern Canada, spreading over central the United States. Meteorologists call this weather pattern an “omega block,” the trio of systems making interlocking, meshing gears and refusing to budge.
The term “omega block” comes from the appearance of the jet stream, as it tends to trace the shape of the Greek letter above the northern edge of the high during these types of patterns. This allows weather systems to roll along the northern periphery of the high pressure – that is, the northern plains – and bring windy thunderstorms. The heat dome is then free to cook the central and southern plains with sunlight.
Heat advisories are in effect for eastern Iowa and northwestern Illinois on Thursday, where the combination of heat and humidity will cause it to be as hot as the upper 90s to about 100 for the third consecutive day.
The National Weather Service predicts dozens of records will be set Thursday from southeast Texas to parts of the interior of Wisconsin and Michigan. High temperatures are expected to be near or above 90 degrees as far north as Minneapolis and Wausau, Wisconsin, while extending south to the Gulf Coast.
In many parts of the Upper Midwest, temperatures will be 15 to 25 degrees above normal.
Chicago reached 90 degrees for the first time this year on Wednesday, breaking a record for the date, and is expected to be at least as hot on Thursday.
Some of the other records set on Wednesday include:
- Imperial, Neb., and Hill City, Kan. : 97
- Omaha: 96
- Greenwood, Mississippi, and Springfield and Lincoln, Illinois: 94
- Memphis: 92
- Madison, Nashville and Colorado Springs: 91
- South Bend, Ind., Denver and Jackson, Tennessee: 90
- Rochester, Minnesota, and La Crosse, Wisconsin: 88
- Milwaukee: 86
Still, Wednesday’s scorching temperatures were tepid compared to the scorching readings tabulated earlier this week.
Readings reached an astonishing 107 degrees in Oklahoma and San Angelo and Abilene, Texas on Monday, setting records. A community in the Rio Grande Valley reached 112 degrees on Saturday. More off-season heat appears to be looming in the southern United States next week.
Human-caused climate change is intensifying the frequency, intensity and duration of these extreme heat events.
Thursday has the potential for even more damaging winds in a more westerly area than Wednesday.
Strong storms are expected to develop along a cold front near the Highway 281 corridor or in the James River Valley in eastern North Dakota and South Dakota. More than 650,000 people are included in the 4 out of 5 risk level reported by the Storm Prediction Center, including residents of Fargo, ND, and Brookings and Watertown, SD
A broader Level 3 of 5 “increased risk” covers areas from the Twin Cities to Sioux Falls, SD, and all the way south following the front in northern Kansas. Omaha and Lincoln, Neb., are under this risk. Lesser risk extends north to Wichita.
Any initial storms could be supercell, meaning they will spin around and be isolated from their neighbors. Supercells pose the greatest risk of large hail and tornadoes. However, this can last an hour or two at most, as the storms will eventually “grow” into a line.
This line will become an arc echo again and mix the momentum up to the surface in the form of strong gusts of wind. The storms will eventually outgrow the disturbances aloft and surface conditions that caused them, weakening as they move through west-central Minnesota around 9 or 10 p.m.
After Thursday, the Upper Midwest and northern Plains are expected to take a break from storms and heat, but already next week looks active again for severe weather in the region.