WOW! What a way to send Sept on its way and welcome the new month of October.

Monday has recorded the third straight day of 80s and is likely to end up with an official high of 86-deg after an 87 Sunday and an 84-deg high Saturday (which was the closing day of Sept 2023)–and there are another TWO DAYS OF UNSEASONABLE 80s to go which will bring the streak to 5 consecutive 80s.

Today (Mon, Oct 2) marked the third consecutive day of 100% of our possible sun. In fact—here are the percentages of possible sun we’ve logged here in Chicago as measured officially by veteran NWS observer at Midway Airport Frank Wachowski:

THU (9/28): 23%.  FRI (9/29): 81%  SAT (9/30): 100%.  SUN (10/1) 100%. MON (10/2) 100%

The Greater Chicago area’s warmest Monday highs included 91 at Tinley Park: 90 at Roselawn and Hebron IN and 90 at Elwood in Will county plus highs of 88 at New Lenox, Oswego, Starved Rock and at Midewin Tallgrass Prairie—–while  VERY LOCALIZED LAKE BREEZES held immediate lakeshore locations in the mid to upper 70s.

There AREN’T RECORDS—but the city’s airport highs in the mid 80s at both O’Hare and Midway are  within 5 to 6 -deg of the the RECORD OF 91 for the date set back in 1971—and they’re 15-16-deg ABOVE THE NORMAL HIGH OF 69 for Oct. 2nd.

The persistence of sun and warmth has, as expected, been the product of a huge dome of unseasonably warm air which has parked over the central U.S.—hemmed in by an atmospheric blocking pattern which has all but stalled its movement.  

BUT BIG CHANGES LOOM—with a MAJOR JET STREAM REALIGNMENT which is to have these critical upper air steering winds guiding MUCH COOLER AIR FROM CANADA INTO THE CHICAGOP AREA starting Thursday and Friday.

The transition is to be a wet one beginning with showers and possible t-storms as early as Wed afternoon and evening and continuing Wed night and into Thursday. Current model rainfall projections fall in the 0.30″ to 0.90″ range. A few totals topping 1″ aren’t out of the question.

Beyond the wet weather—THE COOLEST AIR IN 5 MONTHS (since May) with the first daytime 50s to NEAR 60 and first nighttime 40s since early May expected by Friday, Saturday and Sunday.

While this past weekend’s weather was a late season outdoor bonanza—with highs of 84 Saturday and 87 Sunday and nighttime lows of 61 and 63 deg Sat and Sunday mornings respective—next weekend is to average more than 20-deg cooler. THAT’S A HUGE CHANGE!  Highs Sat and Sun may well struggle to reach 60-deg—likely spending the weekend in the 50s.  Nighttime lows  will drop into the 40s in the city if current forecast trends continue for the first time since early May last spring.

Having talked about the coming chill—it’s appears transient. We’ll already see Chicago’s weather warming next week. In fact, high temps may well be flirting with 70-deg a number of days next week.

A FEW INTERESTING NOTES ON FINICKY OCTOBER HERE. The month has historically been our 2^nd fastest cooling month of the year. And its a month with many meteorological faces. It runs us through a gamut of meteorological extremes.

Most notably, 46% of Octobers sicne 1942 when official observations first shift to the airports here in the city have brought us Chicago’s first flurries of snow and 21% of Octobers have even managed measurable snow—though it most often melts quickly.

At the other end of the spectrum, 73% of Octobers—clearly this one included—have produced 80s and 6% have produced 90s.  So HOWS’S THAT FOR A SPREAD??? 80s at one end of the spectrum—yet snowflakes can occur. 

HERE’S LATEST MONDAY CHICAGO METRO FORECAST (10/2/2023):

TONIGHT: Clear, hazy and continued mild for the season. Some patchy ground fog late outlying areas. Low 64—with some mid to upper 50s cooler inland locations.

TUESDAY:  Sunny, hazy and unseasonably warm. Some clouds appear later in the day filtering the sun a bit. High 88—but low 80s immediate shoreline.  An 88-deg high is nearly 20-deg above normal!

TUESDAY NIGHT: Clouding over, mild and hazy. Low 68—well above the 50- considered normal.

WEDNESDAY: Partly cloudy to cloudy, breezy and warm. Growing prospects for a few scattered showers or t-storms impacting 20-30% of the area in the afternoon.  Better coverage showers and t-storms at night.  High Wednesday 83.

THURSDAY: Opens cloudy—some showers possible. The breaks for mixed sun, breezy and not as warm.  High 71.

FRIDAY: Turning showery and markedly cooler with a good deal of cloudiness with gusty NW winds. High 60.

SATURDAY: Partly cloudy to cloudy, windy and much cooler. The coolest daytime times in the 5 months since early last May. A shower or two can’t be ruled out.  High 56.

SUNDAY: Mixed sun—but clouds build and a shower or two is possible in the afternoon or evening. High 59.

MONDAY: Partly cloudy and breezy. Slight chance of a shower. High 64.