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Ingredients are coming together across parts of the Midwest and Ohio Valley for a dangerous outbreak of severe weather with widespread wind damage similar to that of a derecho.
Thunderstorms will begin along a corridor from east-central Iowa through northern Illinois late Wednesday afternoon into Wednesday evening. The thunderstorms that develop initially in this location will have the ability to take on some rotation, and a few tornadoes are possible, especially in and around Davenport, Iowa, Rockford, Ill.; Sterling Ill.; Ottawa, Ill. and Chicago. The thunderstorms will then begin to congeal into a bowing, or backward 'C' shaped line as they reach northern Indiana and southwestern Michigan. How much of a bowing shape the thunderstorms can take on will play a role in determining how much wind the storms can produce, and ultimately, whether or not a "derecho" fully evolves.
...SEVERE THUNDERSTORMS EXPECTED OVER PARTS OF THE UPPER MIDWEST INTO THE OHIO VALLEY THIS AFTERNOON AND TONIGHT... THE NWS STORM PREDICTION CENTER IN NORMAN OK IS FORECASTING THE DEVELOPMENT OF WIDESPREAD DAMAGING WINDS...LARGE HAIL AND A FEW TORNADOES OVER PARTS OF THE UPPER MIDWEST INTO THE OHIO VALLEY THIS AFTERNOON AND TONIGHT. THE AREAS MOST LIKELY TO EXPERIENCE THIS ACTIVITY INCLUDE EASTERN IOWA NORTHERN AND CENTRAL ILLINOIS MUCH OF INDIANA WESTERN OHIO ELSEWHERE...SEVERE STORMS ARE ALSO POSSIBLE IN SURROUNDING AREAS...INCLUDING PARTS OF THE MIDDLE MISSISSIPPI VALLEY...SOUTHERN GREAT LAKES...CENTRAL APPALACHIANS AND MID ATLANTIC COAST STATES...AS WELL AS ACROSS PARTS OF THE NORTHERN ROCKIES INTO ADJACENT HIGH PLAINS. THE ENVIRONMENT IS BECOMING VERY MOIST AND UNSTABLE ALONG A STALLED FRONTAL ZONE EXTENDING FROM THE MID ATLANTIC COAST WEST NORTHWESTWARD THROUGH THE OHIO VALLEY...TO A DEVELOPING AREA OF LOW PRESSURE ACROSS THE MIDDLE MISSOURI VALLEY. THIS LOW IS EXPECTED TO BECOME THE FOCUS FOR INTENSE THUNDERSTORM DEVELOPMENT AS IT TRACKS EASTWARD ACROSS EASTERN IOWA INTO NORTHERN ILLINOIS THIS AFTERNOON...WHERE CONDITIONS LIKELY WILL BECOME CONDUCIVE TO THE EVOLUTION OF A LARGE AND ORGANIZED CLUSTER OF STORMS. THIS ACTIVITY IS THEN EXPECTED TO SPREAD RAPIDLY EAST SOUTHEASTWARD ACROSS SOUTHERN PORTIONS OF THE GREAT LAKES REGION...AND SOUTHWARD TOWARD THE LOWER OHIO VALLEY....LATE THIS AFTERNOON AND EVENING. WIDESPREAD STRONG...POTENTIALLY DAMAGING...WIND GUSTS ARE POSSIBLE...ALONG WITH THE RISK FOR LARGE HAIL. A FEW TORNADOES ARE ALSO POSSIBLE...PARTICULARLY WITH EARLY STORMS ACROSS PARTS OF EASTERN IOWA INTO NORTHERN ILLINOIS. STATE AND LOCAL EMERGENCY MANAGERS ARE MONITORING THIS DEVELOPING SITUATION. THOSE IN THE THREATENED AREA ARE URGED TO REVIEW SEVERE WEATHER SAFETY RULES AND TO LISTEN TO RADIO...TELEVISION...AND NOAA WEATHER RADIO FOR POSSIBLE WATCHES...WARNINGS...AND STATEMENTS LATER TODAY.
Its common definition is a thunderstorm complex that produces a damaging wind swath of at least 250 miles (400 km), featuring a concentrated area of convectively-induced wind gusts exceeding 50 knots (58 mph; 93 km/h). According to the National Weather Service criterion, a derecho is classified as a band of storms that have winds of at least 50 knots (93 km/h; 58 mph) along the entire span of the storm front, maintained over a time span of at least six hours.Winds in a derecho can be enhanced by downburst clusters embedded inside the storm. These straight-line winds may exceed 100 miles per hour (160 km/h), reaching 130 miles per hour (210 km/h) in past events. Tornadoes sometimes form within derecho events, although such events are often difficult to confirm due to the additional damage caused by straight-line winds in the immediate area.