Chicago Weather Forecast (O'Hare Official) 5:17 pm CDT Aug 16, 2017 with Forecast Discussion

Tonight: Showers and thunderstorms likely before 3am, then showers likely and possibly a thunderstorm between 3am and 4am, then a chance of showers and thunderstorms after 4am. Cloudy, with a low around 71. South wind around 10 mph. Chance of precipitation is 60%. New rainfall amounts between a quarter and half of an inch possible.

Thursday: A 30 percent chance of showers and thunderstorms. Mostly cloudy, with a high near 83. South southwest wind 10 to 15 mph, with gusts as high as 25 mph.

Thursday Night: Partly cloudy, with a low around 65. West wind 10 to 15 mph, with gusts as high as 25 mph.

Friday: Mostly sunny, with a high near 81. West wind around 10 mph, with gusts as high as 15 mph.

Friday Night: A 30 percent chance of showers and thunderstorms after 1am. Mostly cloudy, with a low around 64. West northwest wind 5 to 10 mph.

Saturday: A 20 percent chance of showers and thunderstorms before 1pm. Mostly sunny, with a high near 83.

Saturday Night: Mostly clear, with a low around 64.

Sunday: Sunny, with a high near 86.

Sunday Night: Mostly clear, with a low around 67.

Monday: Mostly sunny, with a high near 88.

Monday Night: A chance of showers and thunderstorms. Mostly cloudy, with a low around 70.

Tuesday: Showers and thunderstorms likely. Mostly cloudy, with a high near 83.

Tuesday Night: A slight chance of showers and thunderstorms. Partly cloudy, with a low around 64.

Wednesday: Sunny, with a high near 80.

 Amf Ohare IL  41.98°N 87.9°W

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


636 PM CDT Wed Aug 16 2017


Through Thursday...

Low pressure centered over the mid Missouri Valley this afternoon will lift to northern Wisconsin by midday tomorrow resulting in active weather across northern Illinois and northwest Indiana through tomorrow. This afternoon, a warm front is draped east from the low center across Iowa and northern Illinois where dew points have risen into the low 70s for many areas. Mid level lapse rates aren`t too terribly impressive this afternoon which is helping to keep instability in check to some degree, but still expect moderately unstable conditions of 1000-1500 J/kg MLCAPE to overspread the entire CWA through the afternoon. Broad ascent will gradually overspread the area through the afternoon and evening as upper ridge axis departs east with weak mid level height falls begin moving overhead. An arcing band of showers have already formed from near DVN southeast through PNT and expect intensity to continue to increase this afternoon in response to increasing forcing. 18Z SPC mesoanalysis indicates deep layer shear is less than 30kt across the area which should remain the case through at least the remainder of the afternoon, then slightly stronger winds aloft will move overhead this evening and tonight. Overall, severe threat is low through the rest of the afternoon given the submarginal shear, but cannot rule out an isolated severe storm or two this evening. Widespread severe weather is not expected.

Second area of thunderstorms is expected to develop over Missouri and Kansas this evening in response to strengthening low level jet ahead of a secondary low that forms near Kansas City. Storms should track northeast into central and parts of northern Illinois late this evening and overnight. Many of the high-res models have trended farther south with overnight convection and have lowered PoPs some in response. If trends continue to hold, PoPs may need to be lowered further. Southward trend seems reasonable given the expected orientation and position of the LLJ with the best forcing focused south of our CWA.

Through the day Thursday, surface low will move from northern WI into the Upper Peninsula of MI while a trailing cold front sweeps across northern Illinois. Depending on how quickly the area clears tomorrow and destabilizes, there is some threat for thunderstorms to redevelop along the front across the far eastern tier of counties in northwest IN early tomorrow afternoon. If this happens, there is some potential storms could be severe, but chances appear best just east of the CWA.




Thursday night through Wednesday...

We dry out Thursday night as the upper level trough axis rotates through.  Slightly cooler and drier air moves in leading to a dry day Friday. A secondary trough axis rotates through Friday night through Saturday, and showers and storms may form in response to the trough axis.  Models disagree on how widespread precip will be Friday night. The ECMWF is the most bullish while the GFS and NAM have precip focused south of I-80. Decided to keep a chance of showers and storms across the forecast area.

The upper level pattern becomes more zonal over the weekend and heat builds back into the area. Dry conditions are expected for the beginning of next week, and we may see 90s Monday.

The next significant chance of precip arrives Tuesday with an upper level wave and surface cold front. Cooler air and an expansive region of high pressure move in behind the front for mid week.



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

636 pm...Forecast concerns include a chance of thunderstorms late this evening/early overnight...mvfr cigs Thursday morning and gusty westerly winds Thursday.

Showers and scattered thunderstorms are developing across northwest IL currently and this activity is expected to slowly develop to the southeast as it lifts off to the northeast during the evening hours. Included tempo thunder mention at rfd this evening but confidence further east at the Chicago terminals is low as this activity may be in a weakening phase as it arrives late this evening. However...included vcts and will need to monitor trends over the next few hours. Confidence is very low for any additional precip from the predawn hours through Thursday evening. There is a small chance of showers around daybreak... early Thursday afternoon and again toward Thursday evening. Trends will need to be monitored later in the forecast period for these time periods.

Outside any wind shifts/outflows from thunderstorms this evening...prevailing southeast winds are expected to slowly turn more southerly by midnight and then south/southwesterly overnight. Speeds are likely to increase into the 10-12kt range with some higher gusts possible overnight. Model forecasts differ on how strong low level winds will be overnight...but will need to monitor for possible low level wind shear. Winds will turn southwest Thursday morning and begin to gust then turn more westerly Thursday afternoon. Forecast soundings suggest gusts into the 25-30kt range will be possible and these stronger winds may persist into Thursday evening...with only a slow downward trend after sunset.

Patchy mvfr cigs are possible this evening and with any preci Ptonight. But more widespread low mvfr cigs are possible overnight through sunrise...then begin to lift Thursday morning. Its possible there may be some ifr cigs...likely across northwest IL but overall confidence for cigs heights and how prevailing they may become is low. cms



High pressure over the eastern Great Lakes will reach New England this evening. Meanwhile a low pressure system over Iowa will reach the upper Midwest by Thursday morning and then move over the western Great Lakes Thursday night. Winds will become southeast and increase to 30 kt especially over the northern half of the lake tonight as the low approaches the region.  Gusts to 30 kt are possible over the northern half of the lake Thursday morning.  The low`s cold front will swing across the lake Thursday evening/night, and winds will become west to 30 kt behind it.

Winds will diminish and become northwest Friday, and high pressure builds over the Great Lakes Saturday.  Winds will back to southwest Saturday night into Sunday. Guidance then suggests a cold front will slide down the lake Tuesday or Wednesday next week.




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