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  • Tim McGill

Winds Of Change

Updated: Sep 22, 2021

Gusty Northerly Winds Bring Cool Change


Meteorological fall is now three weeks old. 12 of the first 20 days of the month had a high of 80° or warmer. This includes Monday's high of 82° at O'Hare. So far O'Hare is running just over 4° above average for the month.


Autumnal equinox arrives tomorrow at 2:20 pm and right on cue we could see the coolest day in nearly four months. Highs Wednesday will only climb into the middle 60s. The good news is the weekend looks warmer and still wonderful so far.


The HRRR Model has our temperatures holding steady in the middle to upper 60s most of today. The high for today actually occurred just after midnight when O'Hare registered a temperature of 76°. Average highs for today's date are around 74°.



The GFS Model has our highs sliding into the middle 60s on Wednesday. This cool down coincides with the arrival of the autumnal equinox.



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



Our temperatures this afternoon and Wednesday afternoon will be around 10° below average. Gusty northerly an northeasterly winds will add insult to injury.


Winds will gust near 30 mph today and could frequently gust over 30 mph tomorrow.


Lake effect rain showers and even thunderstorms will develop later tonight and linger into early Wednesday. The best chance of rain is from around midnight to noon tomorrow.


Gusty northerly winds today and northeasterly winds tomorrow will whip up big waves and generate dangerous rip currents. A beach hazards statement is in effect from 3 pm today for Lake & Cook counties in Illinois until 7 pm Thursday. Lake & Porter counties in Indiana have a beach hazards statement in effect from 10 AM today until 4 AM Friday.


The high lake levels and big waves combined with a "storm surge rise" from winds running the length of the lake will cause some flooding and beach erosion on the southern shores of Lake Michigan. A lakeshore flood advisory is in effect along Illinois' shoreline in Cook County from 7 AM Wednesday to 4 AM Thursday. A lakeshore flood advisory is in effect along Indiana's shoreline from 7 AM Wednesday to 3 PM Thursday.


Wave heights will be building tonight into Wednesday and then diminishing later Thursday or early Friday. At their peak waves could build to between 10 to 14 feet along the Illinois shoreline and 12 feet to 16 feet along Indiana's shoreline.



Looking Longer Range At Temperatures

While we may be cooling off for a couple of days there is still some warmer weather in the longer range forecasts.


The National Blend Of Models has highs in the 60s tomorrow and Thursday but warming up Friday to near 80°. More 70s next week and at least one day with a high of 80° is possible.


The warmest day of next week will be Monday with highs around 80° or about 7° above average. The temperature anomaly forecast for Monday evening shows the vast majority of the country and most of the Midwest will be well above average. Portions of the central plains and Midwest could as much as 20° above.


The long term temperature outlooks suggests our current cool down won't last very long. The 6-10 day outlook has Chicago favored for above average temperatures overall from September 26th through September 30th. The longer range 8-14 day forecast also favors us for above average temperatures overall. It covers the period from September 28th through October 4th. Average highs for this period are in the upper 60s to lower 70s.





The even longer term 30 day forecast still has most of the country (and nearly all of the Midwest) favored for above average temperatures through just beyond 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.








Welcome Lake Effect Rain


O'Hare picked up a little less than a tenth of an inch of rain yesterday and is now running 1.62" of rain below average for September. Cooler air on gusty northerly and northeasterly winds running over relatively warmer water in Lake Michigan means lake effect showers tonight into tomorrow and some additional welcome rain. This might help some of the parched portions of northern Illinois.


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.



Most of today's showers should be south and southeast of the city. Tomorrow the focus for lake effect showers will be closer to the shoreline but parts of Lake County, much of Cook County and all the way south to even Kankakee County will benefit from some rain. After tomorrow the GFS model's meteogram for O'Hare has a chance for a shower overnight Friday into Saturday morning but keeps most of the weekend dry. It hints at more active weather for the middle of next week.



The GFS model total precipitation through this weekend squeezes out a wide range of rainfall. Most of the rain this week will fall later tonight into Wednesday. The heaviest amounts on this model run target areas south and southeast of the city with as much as an inch of rain or more.




A discouraging long range rainfall forecast considering the drought situation. The latest 6-10 day forecast has the Chicago area favored for below average rainfall from September 26th through September 30th. The longer range 8-14 day forecast has us outlooked for below average precipitation too from September 28th through October 4th. In both cases we have fairly high probabilities of being dry compared to average.






Here is my 7 day forecast:


Today: Mostly cloudy, scattered showers & t-storms mainly early, breezy High: 69


Wednesday: Mostly cloudy, showers, isolated t-storm possible, windy Low: 56 High: 66


Thursday: Partly to mostly cloudy, spotty shower possible Low: 49 High: 67


Friday: Mostly sunny Low: 52 High: 76


Saturday: Mostly sunny Low: 53 High: 70


Sunday: Mostly sunny Low: 53 High: 75


Monday: Mostly sunny Low: 59 High: 80

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