Chicago Weather Forecast (O'Hare Official) 7:17 am CDT Sep 25, 2017 with Forecast Discussion

Today: Mostly sunny, with a high near 91. South wind 5 to 10 mph.

Tonight: Mostly clear, with a low around 69. East southeast wind around 5 mph becoming south after midnight.

Tuesday: Mostly sunny, with a high near 89. Light south southwest wind becoming southwest 5 to 10 mph in the morning.

Tuesday Night: A 20 percent chance of showers. Mostly cloudy, with a low around 63. Northwest wind 5 to 10 mph.

Wednesday: Mostly sunny, with a high near 72. North northwest wind around 10 mph, with gusts as high as 15 mph.

Wednesday Night: Mostly clear, with a low around 53.

Thursday: Sunny, with a high near 72.

Thursday Night: Mostly clear, with a low around 52.

Friday: A slight chance of showers before 1pm. Mostly sunny, with a high near 66.

Friday Night: Partly cloudy, with a low around 49.

Saturday: Mostly sunny, with a high near 65.

Saturday Night: Mostly clear, with a low around 49.

Sunday: Sunny, with a high near 68.

Amf Ohare IL
 41.98°N 87.9°W

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National Weather Service Chicago/Romeoville, IL
GO Current Forecast Discussion


610 AM CDT Mon Sep 25 2017


Through Tonight...

Another hot one by not only late September criteria but also mid- summer standards is in store today. If 90 is reached at Chicago it would be the first time with six 90 degree days in a row since July 2012 and the first time in September since 1978.

The longstanding upper ridge continues across the eastern half of the country today. There is a weak short wave circulation and associated 700mb cool pocket (+5C) seen on water vapor imagery over southern Illinois this morning. This feature will drift north today in the highly amplified pattern, and patches of high/mid cloud with it as well as some more cumulus will be seen. Forcing from this wave provides an outside chance of a couple showers this afternoon mainly in northwest to north central Illinois. This is where the cap is forecast to be neared or just barely overcome -- and that is factoring boundary layer mixing of lower dew points. None of the CAM solutions are overly excited but have simulated reflectivity "blips" in that area.

Temperatures early this morning are running about 1-3 degrees cooler than previous mornings thanks to lower dew points and 850-925mb temperatures down about 1C in 24 hours. Taking this into account, have high temperatures forecast around 90 which would be near or right at record. The slightly lower dew points of lower 60s keep forecast heat index readings just a degree or two above the temperature. A lake breeze again will develop by noon and advance inland in lake adjacent counties in a similar way to yesterday. Expecting downtown to peak in 80s.




Tuesday through Sunday...

A couple small chances for rain -- which has been a rarely used word locally this month -- present themselves on Tuesday and then again late in the week. All in all though, dryness should prevail much of if not all of this week. While some locations especially east could be up to the 90 mark Tuesday, a noticeable change in temperatures will occur during this period. The first drop back to around normal is by Wednesday and the one to 5-10 degrees below normal on Friday.

The surface cold front over the Plains this morning will make very gradual progress into the area Tuesday evening. Overall forcing along this boundary is not impressive, especially for much or any thunder given marginal lapse rates. The speed of this boundary may be even slower than what guidance is indicating given the strength of the inherent ridge as well as the parent surface low being fairly weak in pressure (1012mb at 00Z Wed). The slightly better probability of some showers Tuesday afternoon and evening is west of I-55.

Northwest winds behind the front on Tuesday night into Wednesday will bring a seasonable air mass into the area, with highs in the 70s for Wednesday and Thursday. Both of those midweek days look to be mainly sunny.

As the upper ridge collapses, a modestly strong upper trough drops southeast into the Great Lakes on Friday. The associated cold front looks to be strengthening as it moves through, meaning breezy conditions on Friday, possibly with some showers along the front. Lake effect clouds look likely though do not see a strong enough signal for lake effect showers at this time. Highs look to dip to the mid to upper 60s for a good part of the CWA Friday and Saturday with a slight rebound into early next week.



.AVIATION... For the 12Z TAFs...

Similar aviation weather to the past several days is in store today. There will be some clouds -- few to scattered cumulus and some mid and high cloud -- but all VFR. A lake breeze will develop again and likely not arrive to ORD/MDW/GYY until after 21Z. Confidence in a lake breeze arriving at the airports is medium, and confidence is also medium in the forecast specific hour of arrival.




A similar wind pattern will be seen over the lake today as in recent days, with an onshore lake breeze developing in the Illinois nearshore and to a lesser extent the Indiana nearshore. A cold front will move across the lake on Tuesday night turning winds northwest. Small craft criteria waves on Wednesday look probable for part of and maybe the entire northwest Indiana shore. A second stronger cold front, more typical of early autumn, will move southward across the lake late Thursday night into early Friday. This has potential for some gales over the open water depending on the low strength and exact track as it dives southeast over Ontario and Great Lakes region.




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Arlington Heights Weather Forecast
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Chicago respiratory weather

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Air Quality Alert
McHenry-Lake IL-Kane-DuPage-Cook-Kendall-Grundy-Will-
Including the cities of Woodstock, Waukegan, Aurora, Elgin, Wheaton,
Chicago, Oswego, Morris, and Joliet
314 PM CDT Sun Sep 24 2017


The Illinois Environmental Protection Agency has determined that an
air pollution action day will be declared for the greater Chicago
Metropolitan Area through midnight CDT Monday night...September 25th
for the following counties:

Cook...DuPage...Kane...Kendall...Grundy...Lake...McHenry and Will.

An air pollution action day is declared when weather conditions are
such that widespread ozone and or particulate levels are expected to
be at or above the unhealthy for sensitive groups category of the
air quality index.

Active children and adults especially people with pulmonary or
respiratory disease such as asthma should limit prolonged outdoor

Area residents are urged to reduce pollution levels. Air quality
forecasts and tips on how you can reduce your emissions can be found