Summer 2023’s hottest spell of weather is due in Chicago Wednesday into Thursday, prompting an “excessive heat watch”

Heat-limiting easterly lake breezes, responsible for cooling lakeside areas Sunday and Monday, to linger one more day. But, well developed southwest flow to shut down that cooling Wednesday and pushing heat up to and out over the lake, allowing temps to soar. Record to near record heat expected Wednesday and Thursday with temps approaching 100 degrees for the first time in 12 years. The northern flank of heat domes can be finicky and prone to impacts like nearby thunderstorms and their outflows and lake breezes, the possibility of each of which will be monitored Thursday afternoon and evening.
Temps at or above 100 in Chicago are rare. Since official records began in the city 153 years ago in 1871, there have only been 62 days with highs at or above 100 degrees. And at the Midway Airport site on the city’s South Side where weather observations have been archived since 1928, on only 92 occasions over that 95 year period has a reading of 100 degrees or higher occurred.
There’s a reason for the scarcity of such EXTREME HEAT. The fact is, nature finds ways to derail the development of such intense heat. That’s why as forecasters, we’re careful about predicting such readings and must ALWAYS MAKE IT CLEAR there are forces which have been around over the term of official weather observations which work to keep a 100-degree readings from happening. The best evidence currently available suggests a 99-degree high is a strong possibility Wednesday — a reading just one degree shy of a triple-digit reading and, if it occurs, a record breaker which would exceed the old August 23rd record Chicago high of 97 degrees. Thursday could see a 100-degree high, but the potential for a front to sag into the area and turn winds off the lake at some point in the afternoon isn’t completely off the table. ALSO, though not expected at the moment, the development of thunderstorms — even if close-by — can send a cooling outflow of air into the area aborting a 100-degree temperature.
What’s interesting is even if a cold front comes into the picture Thursday, the convergence of winds along that front can lead to heating which would be capable of sending Chicago-area highs to 100, which would tie the record for August 24 and become a windshift behind the front that would send temps falling.
So there are variables which have to be monitored. But, there’s enough evidence that we have some real heat on the way married, with steamy mid 70-degree dew points that the level of heat which may occur as evidenced by HEAT INDEX READINGS which could surge to 110 to 115 degrees Wednesday and Thursday, that it’s prudent to indicate the evidence such heat may occur in coming days. That includes Chicago.

AN EXCESSIVE HEAT WATCH HAS BEEN ISSUED LATE MONDAY MORNING FOR WEDNESDAY AND THURSDAY IN CHICAGO

Near record to record-breaking heat on the way. New heat records could be set across sections of 20 states this week. Dangerous levels of heat on the way to Chicago Wednesday and Thursday. 76-year-old records will be approached or exceeded here. Heat-moderating lake breezes possible at shoreline locations along Lake Michigan but not until later Thursday. This monster hot dome — nudged east in part by “Hilary” and monsoonal thunderstorms out west — is contributing to the heat which could see heat indices hitting 110 degrees here and even higher west of Chicago.
Temps are to surge to within reach of records for each of those days set 76 years ago in 1947. The records currently on the books are 97 degrees on Wednesday for August 23 and 100 degrees on Thursday for August 24.
We know something about heat here in Chicago. Far from being simply a nuisance, we learned tragically in July, 1995 that heat can be deadly. More than 700 of our fellow citizens died in the July, 1995 episode of record heat. Temps in that air mass reached 105 degrees at Midway — but low 80-degree dew points (as high as they get in this area), indicated levels of moisture which, combined with heat, shut down our bodies’ ability to cool themselves. The heat index which resulted in that hot air mass reached 125 degrees at Midway. The heat this week won’t be quite as extreme. And, while the 1995 heat lasted more than 3 days, this week’s will be concentrated on Wednesday and Thursday. Still, our sad experience with that heat in that 1995 episode here in Chicago has led us to take this heat very seriously. Cooling centers are open for those without air conditioning, and the city springs into action asking citizens to check on the elderly and neighbors who may be impacted and offering help in transporting those unable to get to cooling centers to those locations. You will be hearing more on this in news coverage in coming days.

THE METEOROLOGICAL SET-UP FOR THIS WEEK’S HEAT

What’s ahead this week won’t be quite as extreme. But the point may be an academic one. What’s ahead Wednesday and Thursday will be a formidable spell of heat. There’s a potential for a lake breeze to develop later Thursday and mitigate lakeside heat at that time, but much of Thursday will be hot until that occurs.
Domes of hot air are regions in the atmosphere in which air SINKS ON A LARGE SCALE. “Sinking air” descends into regions of the atmosphere with higher air pressures, and this leads to COMPRESSIONAL WARMING which further heats the air. And, the heat extends though such a deep swath of the atmosphere it has a “CAPPING EFFECT,” IN OTHER WORDS, IT SHUTS DOWN T-STORM DEVELOPMENT which might otherwise cool the air.
We’re not alone in Chicago dealing with this hot-air mass. The National Weather Service estimates 80 million in our country of 330 million will be exposed to dangerous levels of heat. And, I see heat records at risk of falling across 20 states in the coming week.

RELIEF IN SIGHT LATE THIS WEEK

The good news is heat relief seems on the horizon Thursday night and Friday. For the moment in Chicago, the passage of a “back door” cold front has introduced “northeast winds” off Lake Michigan which is tempering the heat. Those winds will ease Tuesday allowing temps to surge to 90, and a southwest flow is set to overcome lake cooling Wednesday and much of Thursday, allowing temps to reach to within striking distance of 100 degrees and peak afternoon heat indices to surge to 110 degrees in an air mass.

RECORD DOME OF HOT AIR DEVELOPING

Our models have been predicting a record hot-air dome developing over the central U.S. the past two weeks. The atmosphere expands when heated. When weather balloons are launched each day, among the parameters measured besides temp, humidity and wind are the height at which certain air pressure are found. These “heights” offer a measure of heat’s intensity. Rare is the air mass in which the atmosphere has been heated quite like the air mass which is already broiling a wide swath of the central U.S.

HERE’S HOW HOT DOMES WORK

HERE’S HOW “HILARY” AND WESTERN MONSOONAL THUNDERSTORMS MAY BE PLAYING A ROLE IN THE EASTWARD SHIFT OF HEAT INTO THE MIDWEST

When t-storms/heavy rain showers blossom over the Rockies and West, air is vented out of the tops of these storms. That air migrates away from the storms producing the updrafts, often hundreds of miles. As it sinks, it warms and dries under the higher pressures of the lower atmosphere contributing to the heat there.