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  • Tim McGill

Autumn Is Almost Here

About To Finally Feel Like Fall

Meteorological fall began back on September 1st but it has often felt more like July than September. Eight of the past ten days have seen highs in the 80s. It was especially steamy on Sunday with several spots hitting 90° or more. Rockford had the hottest high with 93°. Aurora and Romeoville hit 91°. O'Hare's high hit 89°. So far O'Hare is running just under 4° above average for the month.

Autumnal equinox arrives at 2:20 pm on Wednesday. Mother nature may be waiting for that day to finally bring in some real fall weather. Highs on Wednesday could be the coolest we have seen in nearly four months.

The GFS Model has our highs reaching into the upper 70s to near 80° today. Average highs for today's date are around 74°.

The GFS Model has our highs sliding into the 60s on Wednesday when the autumnal equinox arrives.

The coolest part of this week will be Thursday morning. Lows overnight Wednesday into early Thursday will fall into the 40s in the outlying areas and into the 50s in and around the city.

Today will be the warmest day of the week as highs reach well into the 70s with a few spots possibly tagging 80°. The hourly forecast for O'Hare has highs sliding into the upper 60s on Tuesday as dew points drop into the 50s. This corresponds to a wind shift early Tuesday that marks the arrival of a cold front and the end of our summer-like streak of weather.

Our winds will gust up to and perhaps over 30 mph today and they stay breezy through tomorrow.

The best chances of rain will be through the middle of this afternoon and then overnight into early Tuesday morning when some of the storms could be severe.

The National Blend Of Models has highs near 80° today but 10° cooler tomorrow and only 60s for highs Wednesday and Thursday. Highs settle back into the 70s by the end of this weekend and stay there well into next week.

Our cool down this week may not last long. The long term temperature outlooks have us moving from near normal temperatures back to above normal by the start of October. The 6-10 day forecast has outlooked Chicago for about average temperatures overall from September 25th through September 29th. The longer range 8-14 day forecast favors us for above average temperatures overall. It covers the period from September 27th through October 3rd. Average highs for this period are in the lower to middle 70s.

The even longer term 30 day forecast contradicts these cooler forecasts. It favors us for warmer than average temperatures overall from now through the middle of next month. The Global Ensemble Forecast System (GEFS) is a weather forecast model made up of 21 separate forecasts, or ensemble members The model tries to give an idea of the amount of uncertainty in a forecast by running an ensemble of multiple forecasts which are each tweaked a bit with their initial conditions. The GEFS has the Midwest and most of the country outlooked for above average temperatures overall all the way into the middle of October.

Some Welcome Rain Early This Week

There is some real hope for help with the ongoing drought impacting a large portion of northern Illinois. O'Hare is now running 1.59" of rain below average for September. It has only received about three quarters of an inch of rain in the past 25 days. Some rain is coming early this week though that will help raise those rainfall numbers.

Here is a recap of my post from Friday with the latest US Drought Monitor report:

The latest US Drought Monitor analysis is in and it is mostly good news for the Midwest but there is some bad news locally.

The US Drought Monitor analysis shows a slight increase in the area under a moderate drought ( 1 one out of 4 levels). The area grew to 10.11% from 9.81% last week. The area in a severe drought (level 2 out of 4 levels) did not change and remains at .79%. The area considered "abnormally dry" (the precursor to a moderate drought) nearly doubled and now stands at 32.94%.

For the Midwest the news was better. The areas in a moderate, severe and extreme drought all dropped slightly. The area in an exceptional drought (level 4 out of 4 levels) is unchanged at .99%. The most parched portion of the Midwest is northern Minnesota.

An analysis of the past two weeks of rainfall (ending on September 19th) shows why our drought numbers haven't budged much. Most of the northern Illinois seen just 10% to 25% of average rainfall during this period. Well west and also south of the city in northern Indiana there has been just 5% to 10% of average rainfall.

It could be a soggy start to the week, at least on and off today and tomorrow. The latest GFS model's meteogram has scattered showers and thunderstorms likely at times today and tomorrow.

The Storm Prediction Center has the Chicago area in a marginal risk (level 1 out of 5) area for severe weather today and tonight. The strongest storms would most likely be after midnight tonight into early Tuesday. There is a slight risk (level 2 out of 5) for severe storms across western Illinois. The biggest threat from our storms would be damaging winds.

A marginal risk is level 1 out of 5 possible risk levels. It is the low end of the risk categories but means "an area of severe storms of either limited organization and longevity, or very low coverage and marginal intensity".

A slight risk is level 2 out of 5 possible risk levels. It means the possibility of "an area of organized severe storms, which is not widespread in coverage with varying levels of intensity".

The GFS model total precipitation forecast next weekend hints at some decent rainfall for portions of the area. The heavier amounts would be well north and northwest of the city with about one to two inches of rain. Similar amounts or more possible south and southeast of the city. The vast majority of this rain would come today and tomorrow.

We need to get all the rain we can this week because the longer range precipitation outlooks don't look promising. The latest 6-10 day forecast has the Chicago area favored for below average rainfall from September 25th through September 29th. The longer range 8-14 day forecast has us outlooked for below average precipitation too from September 27th through October 3rd.

Here is my 7 day forecast:

Today: Mostly cloudy, scattered showers & t-storms, breezy High: 79

Tuesday: Showers early then partly to mostly cloudy, cooler Low: 64 High: 72

Wednesday: Partly to mostly cloudy, cooler Low: 56 High: 66

Thursday: Partly to mostly sunny Low: 49 High: 67

Friday: Mostly sunny Low: 52 High: 75

Saturday: Mostly sunny Low: 53 High: 70

Sunday: Mostly sunny Low: 53 High: 74


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