Chicago sizzles in 2023’s hottest air Friday—thermometer temps to approach 100 degrees first time in 11 years with dangerous heat indices—but a stormy break is in sight

From Tom’s National Weather Service-Chicago colleagues: Southern Lake Michigan water temps from the National Weather Service Chicago/Romeoville, IL at 9:08 AM CDT, Thursday, July 27, 2023

The Michigan City water temperature sensor is located at a water intake one mile offshore and 60 feet below the water surface. It is read every day of the year.

• Severe weather and drenching, thundery downpours Friday night is to introduce a markedly cooler, less humid weekend. Heat advisories remain in effect in all but Lake County, Illinois Friday with excessive heat warnings Friday over many sections of the Chicagoland area.
• The atmosphere is awash in tropical moisture and buoyant heated air which may bubble up into a few isolated predawn and morning t-storms Friday–but also threaten to fuel vigorous late day or Friday night t-storms which may become severe and have the potential of producing HEAVY, POTENTIALLY FLOODING DOWNPOURS OVER A SWATH OF THE CHICAGO METRO AREA. Heat indices are likely to reach 105 to 110 degrees in the city but may surge as high as 106 to 116 with local air 100 degrees or just above possible at hotter locations in Chicago’s southern suburbs.

WHAT TOM IS WATCHING . . .

• Indications of a “back-door-type” front is to settle south into sections of the Chicago area late Friday, particularly on the North Shore, shifting winds “NNE” and dropping temps there by nightfall. The front, which is then expected to stall, will find a wave running along it, which should pull the moisture-rich air into developing t-storms—something likely to occur beginning Friday evening. Conditions appear right for multiple storms to possibly “train” over a portion of the area. This could deliver a real drenching, capable of producing flooding Friday night as we transition to cooler, much less humid air in time over the coming weekend—a period to be dominated by “NNE” winds likely to churn Lake Michigan and producing rough conditions for Saturday and Saturday night mariners and potentially producing rip currents which could threaten shoreline swimmers on Chicago’s beaches. SO AN INTERESTING FRIDAY AND FRIDAY NIGHT—A PERIOD WHOSE WEATHER WILL CONTINUE TO BE MONITORED

WEDNESDAY’S FAST-MOVING THUNDERSTORMS

• Note the 12-hour lightning plot which tracks all the late day and Wednesday evening discharges from storms which flared southeast of Chicago, impacting Lower Michigan, sections of northern Indiana east into Ohio.  
• Wednesday’s fast-moving thunderstorms—some racing along at 55 mph earlier in the day—were a prolific lightning producer

CHICAGO IS UNDER A HEAT ADVISORY FOR EXCESSIVE HEAT WARNING UNTIL 9 PM FRIDAY NIGHT

PHOENIX, ARIZONA ECLIPSES YET ANOTHER HOT TEMPERATURE BENCHMARK THURSDAY AFTERNOON

The official high of 115 degrees (while not a record) marked the 15th time this year the temp has reached or exceeded 115 degrees, the greatest number of 115-plus days since records began there in 1895.