Summer Weather Recap and Fall Outlook

A cool last week of August, with our coolest days of the summer all in the last week.

We can thank a big pattern shift over North America for the more comfortable weather for us– but it’s bringing intense heat to the Pacific Northwest and western Canada at the same time. We’re seeing what’s called a “positive PNA pattern”, which describes the jet stream pattern from the Pacific Ocean across North America. When it’s “positive” it’s warm/hot in the west and it forces the jet stream to take a dip in the east– providing cooler than average temperature.

Summer Temperature Trends

Our count of days above 80 and days above average has paused with these cooler than average temperatures this week. 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.

End of Summer and Start of Fall

Labor Day weekend is often considered the unofficial end of summer– but technically astronomical summer doesn’t officially end until September 22nd this year with the fall equinox. We meteorologists go by a slightly different calendar. For easier record keeping– Meteorological Fall consists of the full months of September, October & November.

When today is over and included in our summer averages, these numbers might be tweak slightly. But this summer will go down as wavering 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 in late June and again in early August. Averaging the high and low for each day and then across the whole three-month period– we can say statistically it’s been one of our hotter summers in records that go back to 1871.

Rainfall and Drought

We went into Meteorological Summer this year with a drought holdover from our lack of winter snow, but we made it up with so much rain this summer. We’ve had many days of rainfall this month– but the precipitation this time of year can be both intense and also very spotty. Many times when we see rain this time of year, the air is juiced enough with moisture that we can see these flooding rains without much warning. It’s something folks who saw flooded basements several times this summer know all too well.

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.

Great Lakes Water Temperatures

Water temps in the Great Lakes reached peak heat along the Chicago shore earlier this month when the Chicago lakefront hit 77 degrees. That’s one degree warmer than all of last year.

That will have 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.

September Weather Outlook

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 though, so more heat could be ahead– even if it doesn’t seem likely in the next 6-10 days.

Upcoming Events

Things to mark on your calendar ahead in the coming weeks: the Full Harvest Moon will grace our skies on the 6th and 7th of September. The Harvest Moon rises just as the sun sets since we’re so close to the fall equinox. We’ll have sunsets after 7p until September 16th.

The Autumnal Equinox is on September