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  • Tim McGill

Behold! "Chicago-Henge" Is Here.

So Long Astronomical Summer

The autumnal equinox arrives at 8:30 Tuesday morning. So today marks the last full day of astronomical summer. The east/west alignment of roads in Chicago combine with the angle and position of the sun on the equinox twice a year to create this spectacular scene captured by photographer Barry Butler. The sun rises and sets between the buildings for some fabulous photographic opportunities.


Meteorological summer ended on August 31st. That's because the three warmest months on average are June, July and August. This summer turned out to be the warmest on record and this fall has started off above average as well. So far September is nearly 1° above average but a warm up is coming this week.


During both the spring equinox and the fall equinox the Earth is not tilted toward or away from the sun. So if the weather cooperates we get to enjoy full sunshine lining up directly with our streets that are laid out east/west. The "henge" in Chicago-henge is a reference to England's Stonehenge which is aligned in the direction of the sunrise of the summer solstice and the sunset of the winter solstice.

Warm Temps To Stick Around Thru September

Just seven of the first 20 days of this month have been below average. That means nearly 70% of September days have been above average.

Monday will be mild with more above average temperatures. Highs this week will be at least in the 70s with a few days flirting with 80°. Saturday should be the warmest of the next seven with highs in the lower 80s.

Look for highs this afternoon in the middle 70s.

Above average today and all week long. Saturday's highs will be 10° above average. Average highs for late September are in the lower 70s.

The 6-10 day temperature probability forecast has us outlooked for above average temperatures overall from September 26th through September 20th. In contrast, the 8-14 day forecast suggests cooler than average temperatures overall from September 28th through October 4th.

That cool pattern might not continue that long into October. The even longer range multi model ensemble temperature anomaly forecast has us outlooked above average for the week ending October 9th.

More "Milky" Skies Today & Tomorrow

Wildfires from western states will continue to send smoke our way. The main impact will be a "milky" and hazy appearance to the sun. The smoke should be high enough aloft that it doesn't pose a health threat here.

The experimental HRRR integrated smoke forecast has western wildfire smoke being carried aloft by the jet stream all the way through the midwest again today. Some of that smoke will even be caught up in the circulation of tropical storm Beta in the Gulf coast.

Drought Will Deepen This Week

Sunday was the 7th straight dry day at O'Hare. We could see another 6 straight rainfall free days. There is a slight chance for rain late Thursday but most of this week should be dry. The portion of northern Illinois that the US Drought Monitor labels "moderate drought" could deepen and expand this week.

The 6-10 day precipitation probability forecast indicates we might get some much needed rain at the end of the month. The forecast is calling for above average rainfall overall from September 26th through the 30th. The 8-14 day has a small sliver south of the city outlooked for above average rainfall for the start of October but most of us are outlooked for normal or near normal rainfall for the period.

Here is my 7 day forecast:

Today: Partly to mostly sunny High: 75

Tuesday: Mostly sunny Low: 56 High: 78

Wednesday: Partly to mostly sunny Low: 58 High: 79

Thursday: Partly to mostly sunny Low: 57 High: 76

Friday: Mostly sunny Low: 58 High: 79

Saturday: Mostly sunny Low: 60 High: 81

Sunday: Mostly cloudy, scattered shower possible Low: 61 High: 75



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