Chicago Weather Forecast (O'Hare Official) 4:17 pm CDT Aug 23, 2017 with Forecast Discussion

Tonight: A 20 percent chance of showers after 4am. Partly cloudy, with a low around 58. North northwest wind 5 to 10 mph becoming light northwest in the evening.

Thursday: A 30 percent chance of showers, mainly before 10am. Partly sunny, with a high near 72. North northwest wind 5 to 10 mph becoming northeast in the afternoon.

Thursday Night: Mostly clear, with a low around 55. Northeast wind 5 to 10 mph.

Friday: Mostly sunny, with a high near 74. East wind 5 to 10 mph.

Friday Night: Partly cloudy, with a low around 57. East northeast wind 5 to 10 mph becoming light east after midnight.

Saturday: Partly sunny, with a high near 77.

Saturday Night: Mostly cloudy, with a low around 59.

Sunday: A slight chance of showers after 1pm. Partly sunny, with a high near 76.

Sunday Night: A chance of showers and thunderstorms. Mostly cloudy, with a low around 60.

Monday: A chance of showers, with thunderstorms also possible after 1pm. Mostly cloudy, with a high near 77.

Monday Night: A chance of showers and thunderstorms. Mostly cloudy, with a low around 61.

Tuesday: A chance of showers. Partly sunny, with a high near 78.

Tuesday Night: Partly cloudy, with a low around 62.

Wednesday: Mostly sunny, with a high near 79.

Amf Ohare IL
 41.98°N 87.9°W

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


342 PM CDT Wed Aug 23 2017


Through Thursday...

Main forecast concerns/challenges are with scattered showers late tonight into Thursday morning, the majority of which will occur near the lake in far northeast Illinois and northwest Indiana.

In the near term, relatively quiet conditions with below normal temps in place while high pressure is situated just to the west. Partly cloudy to at most partly sunny skies across the area this afternoon will see a diminishing trend through the remainder of the afternoon with only a slight rise in temps expected over the next couple of hours. Lake breeze has developed today and is situated from Cook county into areas in northwest Indiana. While gradient relaxes this afternoon, should see some westward movement of this boundary. Then anticipate mainly clear skies tonight, with cool temps once again anticipated, dropping to the 50s.

Challenge this period will be with coverage of possible scattered showers late tonight into early Thursday morning. Main focus for this development will be tied to the lake, as cooler air over the lake along with southward moving surface trough will provide a setup for lake effect shower development across areas near the lake in northeast Illinois and northwest Indiana. Setup appears to become more favorable late tonight into early Thursday morning, especially as a more defined/stronger shortwave drops southeast through the region. This should support some blossoming of any convective showers already present, with any development likely continuing through midday and early afternoon Thursday. Did increase pops slightly but with some uncertainty as to exact placement, duration, and coverage, have limited to chance pops. In this setup, a chance of waterspouts will be possible and have continued mention in the forecast.

While showers will mainly be confined more towards areas near the lake, continue to monitor for at least scattered sprinkles for remaining areas in northern Illinois and northwest Indiana. Low confidence of this but some guidance is showing the possibility for the current precip shield ahead of the main shortwave of interest currently in far northwest MN to hold together as it pushes through WI late tonight. Currently think that if it were hold together, that primarily sprinkles would be possible. However, will need to monitor for a more solid area of rain to possibly move through portions of the CWA Thursday morning.




Thursday night through Wednesday...

Primary focus is on unseasonably cool temperatures Thursday night into early Friday and then the return of precipitation potential over the weekend into next work week. Stout upper trough axis will be exiting east Thursday night, but sprawling surface high over the Great Lakes region, dry air mass underneath and mostly clear skies until near dawn will yield favorable radiational cooling conditions. Expect unseasonably chilly lows in the upper 40s to lower 50s outside Chicago and mid to upper 50s in and near the city. Some patchy fog is possible as air temperatures fall to the dew points.

Short-wave impulses over the Plains could kick off convection to our west and northwest Friday through Saturday, but the dry surface high and northwest flow from troughing over the eastern CONUS will likely shear this out and keep precipitation largely at bay. While mid and high clouds will likely be the main effect on sensible weather, can`t completely rule out isolated showers into parts of north central and far northern Illinois on Saturday. Daytime temperatures will remain below seasonable for late August in the 70s, but quite comfortable.

From Saturday night into Sunday and Monday, models are keying on short-wave energy over the northern Plains merging with a trough dropping south toward the northern Lakes, with a weak elongated surface trough reflection. The trough then looks to close off into an upper low that will meander slowly southward and eventually eject eastward and weaken into an open wave by mid week. This will present our next opportunity for showers and possibly isolated embedded thunder with some weak elevated instability advecting eastward.

However, the main uncertainty is tied to how quickly the surface high moves east and the upper low and weak surface trough move toward our area. At this time, have higher chance category PoPs focused on Sunday night and Monday, but confidence is low. Closed upper lows are typically quite challenging for guidance to handle. Most recent suite of global guidance keeps Harvey/Harvey`s remnants well south of the region through mid next week, but it`s possible some of the moisture from it could get entrained into the system that may affect our area depending on how the pattern evolves at the mid and upper levels. Temperatures in the 70s by day will continue to be the rule, near to slightly below normal for the end of August.



.AVIATION... For the 18Z TAFs...

VFR conditions expected across the terminals through the period, with generally quiet weather expected this afternoon into this evening. Steady northwest winds will begin to diminish this afternoon, with the already present lake breeze making some westward movement. More northerly winds across GYY at this time as it has already pushed through, with MDW likely observing this wind shift later this afternoon. Winds around 10 KT possible with this passage and although do think this lake breeze will then move across ORD, think it won`t occur until this evening and when speeds should be well below 10 KT. Variable winds tonight will then return back to a north northeast wind on Thursday. Monitoring some possible scattered precip development late tonight through midday Thursday. This will be mainly tied to possible showery development off Lake Michigan, with some additional sprinkles possible elsewhere. Don`t anticipate any impacts, and have included only a VCSH at this time.




Waves hazardous for small craft over the Indiana nearshore will subside early this evening. The main marine focus then shifts to the chance for waterspouts late tonight through Thursday morning on the southern half of the lake, including the Illinois and Indiana nearshore. A surge of unseasonably cool air will move south down the lake tonight accompanied by steady north winds, with an upper disturbance kicking off scattered showers. Given the unseasonably cool air over the warm lake waters, parameters support waterspout potential through about mid day. The moderate northerly winds could build waves back into the 3 to 4 foot range again on the south half on Thursday.

High pressure will spread over the western Great Lakes late Thursday and linger through Saturday, resulting in lighter and likely variable at times winds. The next period of concern is on Sunday night into Monday. High pressure will shift east on Sunday, yielding steady southeast winds. A weak surface trough will then approach from the west, strengthening the east- southeast winds Sunday night into Monday with the potential for gusts up to 25 kt and possibly as high as 30 kt on the south half depending on the track of the trough.



.LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...Beach Hazards Statement...INZ001-INZ002 until 8 PM Wednesday.

LM...Small Craft Advisory...IN nearshore waters until 8 PM Wednesday.

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Hazardous Weather Outlook National Weather Service Chicago/Romeoville IL 351 PM CDT Wed Aug 23 2017

ILZ003>006-008-010>014-019>023-032-033-039-INZ001-002-010-011-019- 242100- Winnebago-Boone-McHenry-Lake Illinois-Ogle-Lee-De Kalb-Kane- DuPage-Cook-La Salle-Kendall-Grundy-Will-Kankakee-Livingston- Iroquois-Ford-Lake Indiana-Porter-Newton-Jasper-Benton- 351 PM CDT Wed Aug 23 2017 /451 PM EDT Wed Aug 23 2017/


.DAY ONE...Tonight.


  High Swim Risk.


Dangerous swimming conditions will continue at northwest Indiana Lake Michigan beaches through early this evening.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

Thursday...   Limited Fog Risk. Sunday and Monday...   Limited Thunderstorm Risk.   High Swim Risk possible for Illinois Lake Michigan beaches.


Spotter activation will not be necessary through tonight.