TORNADO WATCH; Storm Forecast Assessment for Chicagoland, Heat Index Fizzled

Today’s weather was a battle between a cool lake outflow boundary pushing cool air southwest vs. a drawing of warm Gulf air sucked up from the counterclockwise rotation of a Low Pressure System to the southwest over central Kansas, and a clockwise push of warm air from the south where a High Pressure System was over southwest Tennessee.

As has been common this summer and last summer, the cold outflow boundary from Lake Michigan pushed cooler air into the area. The clash caused unstable air that mixed with moist air that measured dew points as high as 76°F at O’Hare, and as high as 79°F in Kankakee, and as high as 82°F in Streator.

The most severe weather today occurred in McHenry County and Lake County, where a funnel or tornado was spotted near Route 14 and Oak Grove Road near Harvard at about 2:19 p.m.

The worst of the power outages appear to be east of I-94 in Lake County, where at least five areas of 100 ComEd customers or more were without electricity.

No significant thunderstorms were visible on radar at 5:45 p.m., but heating with clear skies could create conditions for severe thunderstorms to return Saturday evening.

Not quite sure why the National Weather Service didn’t expire the Heat Advisory early. The NWS Heat Advisory was expired sometime between 5:00 p.m. and 6:00 p.m.

Today’s high temperature was 87°F at 1:51 pm at O’hare. The Heat Index High at the same time was 96°F.

The Heat Advisory criteria is Maximum Heat Index > 105°F for at least 3 hours (HI=Heat Index).

After the storms and rainfall, the warm air temperature has not recovered above 76°F, and has in fact dropped one degree to 75°F from 3:51 p.m. to 4:51 p.m. at O’Hare International Airport. At 5:51 p.m. the air temperature at O’Hare was 78°F with a Heat Index of 80°F.


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