Chicago Weather Forecast (O'Hare Official) 6:17 pm CDT Aug 27, 2017 with Forecast Discussion

Tonight: A 50 percent chance of showers and thunderstorms, mainly after 9pm. Mostly cloudy, with a low around 62. South wind around 5 mph.

Monday: A chance of showers and thunderstorms before 7am, then a chance of showers between 7am and 1pm, then a chance of showers and thunderstorms after 1pm. Mostly cloudy, with a high near 76. South wind around 5 mph. Chance of precipitation is 50%.

Monday Night: A chance of showers and thunderstorms before 1am, then a slight chance of showers. Mostly cloudy, with a low around 62. East wind around 5 mph becoming calm in the evening. Chance of precipitation is 40%.

Tuesday: A chance of showers and thunderstorms before 1pm, then a slight chance of showers. Partly sunny, with a high near 77. Northeast wind 5 to 10 mph. Chance of precipitation is 30%.

Tuesday Night: Partly cloudy, with a low around 61. Northeast wind around 5 mph becoming north northwest after midnight.

Wednesday: Mostly sunny, with a high near 80.

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

Thursday: Partly sunny, with a high near 74.

Thursday Night: Mostly clear, with a low around 57.

Friday: Sunny, with a high near 75.

Friday Night: Mostly clear, with a low around 58.

Saturday: Mostly sunny, with a high near 80.

Saturday Night: Partly cloudy, with a low around 65.

Sunday: A chance of showers and thunderstorms. Mostly sunny, with a high near 81.

 Amf Ohare IL  41.98°N 87.9°W

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


655 PM CDT Sun Aug 27 2017


Through Monday...

The first batch of rain from this morning continues to weaken as it moves east over the lake and enters drier air. More moisture will shift east into northern IL and northwest IN later this afternoon and evening. Additional shortwave energy around the base of the deep upper low across MN will pivot eastward leading to several periods of expanded shower coverage. The first batch will focus across our southwestern counties and will initially be void of thunder, but given some heating ahead of this current activity, this would be the area that could see some thunderstorms later this afternoon. Expect a gradual expansion of shower coverage from southwest to northeast more so later this evening and overnight. With decreasing heights aloft and more moisture there is some thunder potential later this evening into the overnight as well.

If we had to pick another dry period beyond this ongoing one, then that would generally be Monday morning, though there could be a few showers. The upper low will pivot overhead Monday afternoon when we would likely see some cloud breaks. Expect additional shower and storm chances, though scattered coverage seems appropriate. There is basically no shear through the column, and thus any storms would be pulsy, but could also throw outflow boundaries, and with the steep low level lapse rates, will need to monitor for landspouts and other weaker low level circulations.




Monday night through Sunday...

Upper low is expected to be in place over northern Illinois Monday evening and gradually open up and shift to east central Indiana by Tuesday afternoon. Steep mid and upper lapse rates will persist under the influence of the upper low through midday Tuesday allowing additional chances for scattered showers and thunderstorms. Later in the day Tuesday, mid level height rises will build overhead resulting in subsidence aloft and weakening lapse rates. In response, PoPs should lower through the day and expect most areas to be dry by late afternoon or evening.

By Wednesday, temperatures should moderate some closer to normal as the upper low continues to move away from the area and high pressure builds into the midwest. Meanwhile, to the north, another upper wave is progged to dig into the Great Lakes region dislodging a fairly cold airmass from Canada into the eastern Great Lakes. Locally, we`ll just get a glancing blow of colder air, but do expect temperatures to return to below normal Thursday and Friday. Primarily dry conditions should occur across northern Illinois and northwest Indiana through the latter half of the week, but there is a chance for a few showers and thunderstorms mainly on Thursday as a cold front associated with the aforementioned upper wave drops across the region.

Flattening upper ridge will translate across the region next weekend allowing temperatures to once more moderate to or even warm slightly above normal. As the upper pattern becomes more zonal throughout the weekend, low amplitude shortwaves could bring showers and thunderstorms to the region, but confidence in timing and location at this distance is low.



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

Slow moving upper level storm system will move across the region through tomorrow night bringing periodic bouts of showers and a few t-storms. Challenge this TAF cycle was attempting to pinpoint most likely times for precip and the more likely to be dry hours. It does appear as though this evening will be mostly dry at the Chicago terminals with some SHRA possibly affecting RFD this evening. Heading into the overnight as spoke of energy pivots around the upper system and looks to potentially provide for an uptick in coverage of SHRA and possible TSRA. Given time of day, opted to just include SHRA in the TAFs overnight, but worth noting that ISOLD TSRA cannot be ruled out.

Monday morning should see any lingering overnight SHRA activity decreasing and probably several dry hours before SCTD SHRA & TSRA fire up tomorrow afternoon. Lake breeze boundary will probably be one focus for activity and most likely timing looks to be mid-late afternoon with activity likely to wane Monday evening.

Finally, given the moist environment and rain, hard to rule out some MVFR CIGS, especially overnight into Monday morning, but confidence isn`t high in lower CIGS. Winds should be fairly light through the TAF cycle with a lake breeze likely shifting winds to easterly Monday, possibly nearing or exceeding 10kt at times, though afternoon SHRA/TSRA will likely result in some variability in the wind speeds/direction.

- Izzi



This afternoon, Lake Michigan falls between a broad area of high pressure over Quebec and New England and low pressure centered over western Lake Superior. A moderate southerly gradient is over Lake Michigan with winds gusting to around 25 kt. Winds should relax some tonight and Monday as the low tracks south across Wisconsin falling to 10-15 kt or less Monday night through early Wednesday as high pressure transits the region. A cold front is expected to then move down the lake later in the day Wednesday and Wednesday night with north winds to around 30 kt expected in its wake.




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Hazardous Weather Outlook National Weather Service Chicago/Romeoville IL 345 PM CDT Sun Aug 27 2017

ILZ003>006-008-010>014-019>023-032-033-039-INZ001-002-010-011-019- 282045- 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- 345 PM CDT Sun Aug 27 2017 /445 PM EDT Sun Aug 27 2017/


.DAY ONE...Tonight.

WEATHER HAZARDS EXPECTED:   Limited Thunderstorm Risk.   Elevated Fog Risk.


Isolated to scattered thunderstorms are expected across the entire outlook area this evening and tonight. Severe weather is not expected. Patchy fog is possible after midnight.

.DAYS TWO THROUGH SEVEN...Monday through Saturday.

Monday...   Limited Thunderstorm Risk. Tuesday...   Limited Thunderstorm Risk. Thursday...   Limited Thunderstorm Risk.   High Swim Risk. Friday...   High Swim Risk.


Spotter activation will not be necessary through tonight.


Moving east at 25 to 30 MPH.