CHICAGO — An Excessive Heat Watch has been issued for Wednesday and Thursday in Chicago which could break records last seen in the 1940s.

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-deg on Wed for Aug 23 and 100-deg on Thursday for Aug 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-deg at Midway–but low 80-deg dew points, as high as they get in this area, indicated levels of moisture which, combined with heat, shut down our our bodies’ ability to cool themselves. The heat index which resulted in that hot air mass reached 125-deg at Midway.

The heat this week won’t be quite as extreme. And while the 1995 heat lasted more than three days, this week’s will be concentrated on Wed and Thursday. Still, our sad experience with that heat in that episode here in Chicago, has led to us taking very seriously here. Cooling centers are opened 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 Wed and Thu 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 thunderstorm 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 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 “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-deg and peak afternoon heat indices to surge to 110 degrees.

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 beside temp, humidity and wind is 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 “HILARY” & WESTERN MONSOONAL T-STORMS 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 out of the tops of these storms is vented out 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 the heat there.