Chicago Weather Forecast (O'Hare Official) 9:57 pm CDT Aug 24, 2017 with Forecast Discussion

Tonight: Mostly clear, with a low around 53. North wind around 5 mph.

Friday: A slight chance of sprinkles before 10am. Mostly sunny, with a high near 74. Calm wind becoming east around 5 mph in the morning.

Friday Night: Partly cloudy, with a low around 57. East northeast wind around 5 mph becoming calm in the evening.

Saturday: Partly sunny, with a high near 77. Southeast wind 5 to 10 mph.

Saturday Night: Mostly cloudy, with a low around 61. South southeast wind 5 to 10 mph.

Sunday: A 40 percent chance of showers and thunderstorms after 1pm. Mostly cloudy, with a high near 78.

Sunday Night: Showers likely and possibly a thunderstorm before 1am, then a chance of showers and thunderstorms after 1am. Mostly cloudy, with a low around 61. Chance of precipitation is 60%.

Monday: A chance of showers and thunderstorms. Mostly cloudy, with a high near 75.

Monday Night: A chance of showers and thunderstorms before 1am. Mostly cloudy, with a low around 61.

Tuesday: A chance of showers and thunderstorms. Partly sunny, with a high near 77.

Tuesday Night: A slight chance of showers. Mostly cloudy, with a low around 61.

Wednesday: Mostly sunny, with a high near 78.

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

Thursday: Mostly sunny, with a high near 78.

 Amf Ohare IL  41.98°N 87.9°W

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


911 PM CDT Thu Aug 24 2017


Forecast largely on track, with main story the very cool overnight and early morning readings outside of Chicago. Widespread upper 40s are favored and even a few mid 40s in favored cool spots such as near KARR and KRPJ, with mid to upper 50s in and immediately near the city. A secondary shot of cool air aloft should result in patchy lake induced clouds after midnight. However, observed and forecast soundings indicate an inversion from roughly 800-750 mb that will likely preclude true shower activity due to low equilibrium levels, despite steep lapse rates and around 100 j/kg of lake induced CAPE below the inversion. Maintained slight chance sprinkle mention near the lake.




Through Friday...

Latest radar imagery depicting only isolated showers remaining across portions of northwest Indiana, and the southern end of Lake Michigan. Large scale ascent has diminished with the departure of the mid level wave, with low level convergence also weakening over the southern end of the lake. Other than a few light showers over the next 1-2 hours, expect generally dry conditions. Sky cover in place will also diminish, however, setup would favor for additional scattered clouds to continue to spill off the lake late this afternoon into the evening. This is already evident, with cu still noted over much of the lake at this time. Did increase cloud cover to keep at least partly cloudy mention into this early evening, but may need to increase more than currently forecast.

High pressure will spread south across the region through the period, with generally dry/quiet conditions expected and with another cool night of below normal temps likely tonight. A few items that will need to be monitored will be for lake effect clouds to possibly continue spilling into areas near the lake tonight into Friday morning, as well as the possibility for at least sprinkles. Guidance not really showing much of either at this time, but with another push of slightly cooler air over the lake, thermally induced surface trough looks to develop once again over the southern end of the lake. This could support some enhanced cloud development along with isolated light showers, especially late tonight into early Friday morning. Lower confidence with this solution, but did maintain partly cloudy skies with a sprinkle mention for areas near the lake. This possibility should end by mid/late morning, with temps in the 70s expected Friday.




Friday night through Thursday...

Surface ridge axis will shift to our east Friday night through Saturday night allowing return flow to overspread the region resulting in slightly warmer temps into the mid to upper 70s and dew points creeping back up to around 60. Expect dry weather through this time frame, but we should see increasing cloud cover Friday into Saturday as a wave takes shape upstream.

Through the day Sunday, strong upper wave is expected to dig into the western Great Lakes, closing off and slowly rotating across the local area Monday night into Tuesday. Mid level lapse rates will steepen as this occurs resulting in renewed precip chances Sunday through Tuesday. Best chances still appear to be Sunday afternoon and evening as a weak surface trough advances through the area, and chances appear more diurnally favored Monday and Tuesday.

Closed upper low is progged to shift off to our east around midweek. Precip chances will diminish behind the upper wave, but expect temperatures to remain fairly steady state with highs in the mid to upper 70s still under the influence of longwave troughing in place across the Great Lakes and eastern CONUS.



.AVIATION... For the 00Z TAFs...

Concerns are around lake effect cloud development through early Friday with maybe a few sprinkles, otherwise NE winds will continue through the period.

Last looks at visible satelitte show some VFR lake effect clouds still streaming into NE IL, but we have been observing a decreasing trend. Forecast soundings suggest thinning tonight but some redevelopment later tonight into early Friday. A strong inversion suggests just some lower VFR clouds. High pressure will remain anchored over northern Lake Michigan, resulting in a continued ENE wind that will trend more E.




High pressure is stretched from Hudson Bay south across Wisconsin and into the lower Ohio River Valley. This high will slowly move eastward and cross Lake Michigan through the weekend. Steadiest winds will be into tonight but speeds will begin to slacken late then be on the light side into Saturday. Direction through tonight will primarily be north turning northeast and east but may see another weak low pressure circulation set up over the southern part of the lake some time tonight. South to southeast winds will gradually increase later Saturday as the high shifts east and low pressure moves from the central Plains into Minnesota. Speeds will further increase into Sunday as the low moves into northern Wisconsin. The low then begins to weaken and meanders southward down the lake early next week. Confidence in forecast details lowers through the weekend as Hurricane Harvey will be interacting with larger scale features which may lead to differences in track and timing of features.



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