Weather Update: A Cool End to August and a Chilly Start to September

A cool end to August and very chilly start to September.

This morning some communities in far western Chicagoland had their coldest temps since last spring. Rochelle hit a very chilly 34 degrees. Not quite frosty– but it’s within striking distance.

The Cause of the Chilly Weather

A persistent pattern that’s only now actually showing signs of finally breaking down is the culprit. Meteorologists describe the jet stream pattern over North America as being “positive” or “negative” depending on where the jet stream is. Called the “PNA patten” where the P stands for Pacific and the NA is for North America.

When it’s warm in the west and chilly in the east it’s considered a “positive” pattern and when it’s cold in the west and warm in the east it’s considered “negative”. It’s not a judgement call on how the weather feels– it’s just what they’ve named the particular patterns. When it’s “positive” the hot weather out west basically forces the jet stream to take a dip in the east– providing cooler than average temperatures. And it has been MUCH cooler than average so far this month.

Weather Patterns and Temperatures

Our count of days above 80 and days above average has paused with these cooler than average temperatures this week. But we might go back to counting them as soon as Monday. Last year we saw more than 100 of those warm days– when about 80 is average.

And we’re seeing our break from the 90s continue. So far, we’ve seen more than our fair share of 90s this summer–though not quite record setting. An average August sees about 4 (ORD) or 5 (MDW) days of 90 or warmer. The most 90 ever in a summer in Chicago was in 1988 (our hottest summer) when we saw 47 days of temps at or above 90. The latest on the calendar we’ve seen a 90-degree or warmer temp in Chicago is October 6th.

End of Meteorological Summer

The end of August means the end of the meteorological summer, which consists of the full months of June, July and August. This past summer was a bit warmer than average and a bit wetter than average. So let’s look back…

But I think the summer of 2025 will go down as being remembered for flipping abruptly between three types of weather: hot & very humid, comfortable but smoky, or flooding rains. Our hottest temp for this summer was 95 degrees that we hit several times this summer. Technically, right now it’s still late summer–considered “astronomical summer” which doesn’t officially end until the fall equinox, which this year is on September 22nd.

Drought Monitor and Rainfall

The USDA’s Drought Monitor product that’s released on Thursdays shows we’re no longer in a drought for now– just pockets of “abnormally dry”. August historically is considered our second wettest month of the year with an average of 4.25″ of rain.

It’s only surpassed by May with 4.49″ of rain. As we take a look back at all 31 days of August, we had more than five inches of rain which is about 3/4 of an inch more than average. September is our 7th wettest month of the year with 3.19″ of rain over the 30-day period.

Water Temperatures and Fall Weather

Water temps in the Great Lakes reached peak heat along the Chicago shore in August when the Chicago lakefront hit 77 degrees. That’s one degree warmer than all of last year. Warm lake water has a big impact on our fall weather. Water hangs onto heat much more effectively than air does– so the lake will stay warm even when the air starts to get cold in late fall. That combination can make for some big lake effect snow later this year– and in the fall, it can bring lake effect showers in rain form too.

The 9th month of the year for Chicagoland, sees average high temps that start at 80 degrees and fall to the average of 70 by the end of the month. The average low goes from 62 on September 1st to a nippy 51 degrees by the 30th. September is one of the months that darkens the fastest, we lose 80 minutes of daylight over the 30-day period. Our sunset by the end of September is down to 6:35p. We can see 80-degree temps into October, historically.

Upcoming Events

Things to mark on your calendar ahead in the coming weeks: We’ll have sunsets after 7p until September 16th. The Autumnal Equinox is on September 22nd this year.

The new season officially starting