Record heat fueling violent storms in central US

The most acute risk covers the eastern parts of the Dakotas and western Minnesota, where the National Weather Service Storm Prediction Center has forecast a Level 4 out of 5 threat of severe weather.

Early Thursday evening, a destructive line of thunderstorms, unleashing winds of 80 to 100 mph, was roaring through eastern South Dakota and northeast Nebraska at 85 mph. “This is an EXTREMELY DANGEROUS SITUATION,” the Weather Service warned.

The violent storm compex, taking on the characteristics meteorologists call a “derecho,” was pointed at Sioux Falls, SD and eventually southwest Minnesota Thursday evening. Tornadoes were also possible embedded within the thunderstorms.

This area was under a particularly dangerous situation severe thunderstorm watch until 10 pm Central time for widespread wind gusts up to 105 mph.

For the third time in four days, Minneapolis finds itself under a Level 3 out of 5 “enhanced” storm threat Thursday. Violent storms ripped through the Twin Cities on Wednesday night, unleashing 70 mph to 80 mph winds, hail and a few tornadoes in the region. Tens of thousands lost power.

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Thursday has the potential for even more damaging winds in an area farther west compared with Wednesday

Heavy storms are expected to erupt along a cold front near the Highway 281 corridor, or in the James River Valley of eastern North Dakota and South Dakota. More than 650,000 people are included in the Level 4 out of 5 risk reported by the Storm Prediction Center, including residents of Fargo, ND, and Brookings and Watertown, SD

A broader Level 3 out of 5 “enhanced risk” covers areas from the Twin Cities through Sioux Falls, SD, and all the way south trailing the front into northern Kansas. Omaha and Lincoln, Neb., are under that risk. A lesser slight risk stretches down to north of Wichita.

Any initial storms might be supercellular, meaning they’ll be rotating and isolated from their neighbors. Supercells pose the greatest risk of large hail and a tornado. That may last an hour or two at most, however, since storms will eventually “grow upscale” into a line.

That line will once again become a bow echo and mix momentum down to the surface in the form of strong wind gusts. The storms will eventually outrun the upper-air disturbance and surface conditions that gave rise to them, weakening as they plow through west central Minnesota by 9 pm or 10 pm

After Thursday, the Upper Midwest and northern Plains should catch a break from both storms and the heat, but already next week is looking active again for severe weather in the region.

Record heat fueling storms

Contributing to the storms has been the exceptional heat. A major heat dome of high pressure, flanked by a pair of lows spinning counterclockwise, is parked over the Great Lakes and southeast Canada, and sprawled over the central United States. Meteorologists refer to that weather pattern as an “omega block,” the trio of systems making interlocking, meshed gears and refusing to budge.

The term “omega block” comes from the jet stream’s appearance, since it tends to trace the shape of the Greek letter over the top of the northern edge of high pressure during these sorts of patterns. That allows weather systems to ride along the northern periphery of high pressure — ie the northern Plains — and bring windy thunderstorms. The heat dome is then free to bake the central and southern Plains with sunlight.

Heat advisories are in effect for eastern Iowa and northwest Illinois Thursday, where the combination of heat and humidity will make it feel as hot as the upper 90s to around 100 for the third straight day.

The National Weather Service predicts dozens of record highs will be set Thursday from southeastern Texas to parts of interior Wisconsin and Michigan. High temperatures are forecast to be near or above 90 degrees as far north as Minneapolis and Wausau, Wis., while extending south all the way to the Gulf Coast.

In many parts of the Upper Midwest, temperatures will be 15 to 25 degrees above normal.

On Wednesday, Chicago hit 90 degrees for the first time this year, breaking a record for the date, and is predicted to be at least that hot again Thursday.

Some of the other record highs set Wednesday include:

• Imperial, Neb., and Hill City, Kan.: 97
• Omaha: 96
• Greenwood, Miss., and Springfield and Lincoln, Ill.: 94
• Memphis: 92
• Madison, Nashville and Colorado Springs: 91
• South Bend, Ind., Denver, and Jackson, Tenn.: 90
• Rochester, Minn, and La Crosse, Wis.: 88
• Milwaukee: 86

Yet the toasty temperatures Wednesday were tipped compared with the scorching readings tabulated earlier this week.

Readings spiked to an astonishing 107 degrees in Oklahoma and San Angelo and Abilene, Tex., on Monday, records setting. One community in the Rio Grande Valley made it to 112 degrees on Saturday. More unseasonable heat looks to be in the offing across the southern United States next week.

Human-caused climate change intensifies the frequency, intensity and duration of such extreme heat.

Inside Wednesday night’s storms

Thunderstorms sprouted across eastern Nebraska and western Iowa during the afternoon hours Wednesday, quickly merging into a large blob with a diminished tornado risk. By nightfall, however, a strengthening low-level jet stream helped the connected storms to fan out into a “bow echo,” or a backward C-shaped squall line that resembles an archer’s bow.

That curved squall line was propelled forward by strong jet stream winds descending to the surface behind the initial blast of rainfall. Windom, in Cottonwood County in southwest Minnesota, gusted to 76 mph, and Morristown, 50 miles south of Minneapolis, reported a gust to 79 mph. Carver, a suburb on the southwest side of Minneapolis, saw a gust to 77 mph.

There were also a number of tornado warnings plastered about the metro, though the National Weather Service has yet to dispatch meteorologists to disentangle straight-line-wind damage from that of brief tornadoes. There were a few short-lived circulations that appeared on radar, but it’s unclear how many tornadoes may have touched down.