UPDATE, 4:45 p.m. ET (3:45 p.m. CT): The National Weather Service Storm Prediction Center (SPC) has issued a “Particularly Dangerous Situation Severe Thunderstorm Watch” for southeast Wisconsin, northeast Illinois, northwest Indiana and southwest Michigan until 9 p.m. central. This includes Chicago and Milwaukee.

Widespread damaging winds to 80 mph, large hail, and one or two tornadoes are possible in this area. SPC says the thunderstorm system is producing a derecho in eastern Iowa which is expected to sweep through this region around between roughly 6 and 9 p.m. central. Significant numbers of downed trees and power outages are likely to occur.



Area under particularly dangerous situation watch outlined in blue. Radar view shown captured at 3:35 p.m. CT

Original, post from 3:10 p.m. ET (2:10 p.m. CT): A complex of thunderstorms erupting over Iowa this afternoon may consolidate into a powerful squall line or derecho that rips through the Upper Midwest this evening.



Storm over Iowa as of 1:45 p.m. ET (NASA)

The National Weather Service’s Storm Prediction Center (SPC) forecasts has highlighted a broad region as having a 45 percent chance of experiencing destructive winds (within 25 miles of any point) over 70 mph. Des Moines, Madison, Milwaukee and Chicago all lie within this zone of elevated risk, which affects almost 19 million people.



Risk of damaging winds. Hatched area indicates elevated likelihood of destructive winds over 70 mph. (NWS SPC)

“Numerous severe thunderstorms capable of large hail, tornadoes and swaths of damaging wind are expected today into tonight over much of the Corn Belt and Midwest,” SPC says.

Already, thunderstorms have exploded over portions of Nebraska and Iowa today, with multiple reports of damaging winds and hail. A tornado touched down in northeast Nebraska, not far from Pilger, where twin tornadoes struck two weeks ago.

One tornado watch covers a large chunk of Iowa and western Nebraska until 6 p.m. central time. A second tornado watch covers eastern Iowa, northwest Illinois and southern Wisconsin until 7 p.m. central.

As the storms exit the Corn Belt, the amount of spin in the atmosphere decreases, lowering the tornado risk some into Illinois and Wisconsin. But strong high altitude winds will continue energizing the complex as it heads east.

A concern is that these thunderstorms will evolve into a derecho, a rapidly moving storm complex featuring damaging winds along at least a 240 mile path.

“[The flow pattern] does support this line of convection that arrives across far western/northwestern Illinois to morph into a bowing segment and possibly transition into a derecho type event,” writes the National Weather Service Forecast Office serving Chicago.



(National Weather Service)

The line of storms could arrive in the Chicago and Milwaukee region between approximately 7 and 10 p.m. tonight (local time). After it crosses Lake Michigan, models generally dissipate the complex.

You can track the storms by following these links:

High Plains regional radar

Great Lakes regional radar

Latest storm watches (severe thunderstorm and/or tornado)

Latest severe thunderstorm warnings

Latest tornado warnings