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  • Tim McGill

Weather Whiplash Coming As Summer Ends and Fall Begins

Updated: Sep 1, 2020

Sunday was a rare summer day. With a high of 79° and a low of 60° it was just a bit below average. It was the first below average day in over three weeks and one of only four below average days in August. Only 8 days this entire summer have been below average. 91% of days this summer have been above average.

As those stats suggest, it has been remarkably warm this summer. June ended up 5° above average. July ended up 5.2° above average. August is running 4.5° above average. So with just one day left of meteorological summer (June, July & August), the summer of 2020 is the warmest on record for Chicago.

No surprise then that summer will end on a warm note. Highs today should reach into the lower to middle 80s. 80° is the average high for late August.

A cooler pattern will kick in almost on cue next week as we dive into meteorological fall. The longer range forecasts have consistently suggested a shift to below average temperatures for the start of September. The 6-10 day and 8-14 day temperature probability forecast has us outlooked for below average temperatures from September 5th through September 13th. It's a fairly strong signal that much cooler air is coming soon.

The GFS temperature anomaly forecast for a week from Tuesday has a dramatic drop in temperatures. Most of Illinois will be anywhere from -15° to -30° below average. That could mean highs fail to get out of the 50s for parts of the area. Talk about weather whiplash!

The looooooonger range temperature probability forecast suggests the early September cool pattern won't last. Chicago is outlooked for above average temperatures overall (along with all of the US) for September, October and November.

Our drought could deepen this week where there is a moderate drought already and expand into new areas that are now abnormally dry. The good news is the 6-10 day precipitation probability forecast has us outlooked for above average precipitation from September 5th through the 9th. The 8-14 day forecast has us near normal for the second week of September.

Our best chances of rain this week will come later tonight and Tuesday.

The GFS total rainfall accumulation forecast through early Wednesday morning calls for around a half inch of rain to three quarters of an inch. Welcome rain but we need a bigger dent in the drought that has developed across parts of the area. O'Hare is down almost 4 inches of rain compared to average for August and nearly 5 inches since July 1st.

Here is my 7 day forecast:

Today: Partly to mostly sunny High: 85

Tuesday: Mostly cloudy, sct. showers/t-storms possible Low: 65 High: 78 (74 Lakefront)

Wednesday: Partly to mostly sunny Low: 66 High: 85

Thursday: Partly to mostly sunny, stray shower possible Low: 65 High: 83

Friday: Mostly sunny Low: 58 High: 75

Saturday: Partly to mostly sunny Low: 59 High: 81

Sunday: Partly cloudy, sct. shower possible Low: 62 High: 82



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