Chicago Weather Forecast (O'Hare Official) 8:17 am CDT Aug 17, 2017 with Forecast Discussion

Today: A 40 percent chance of showers and thunderstorms, mainly between 4pm and 5pm. Mostly cloudy, with a high near 83. Breezy, with a south southwest wind 15 to 20 mph, with gusts as high as 30 mph.

Tonight: A 10 percent chance of showers before 7pm. Partly cloudy, with a low around 65. West wind 10 to 15 mph, with gusts as high as 30 mph.

Friday: Mostly sunny, with a high near 81. West wind around 15 mph, with gusts as high as 20 mph.

Friday Night: A chance of showers, with thunderstorms also possible after 1am. Mostly cloudy, with a low around 64. West northwest wind 5 to 10 mph. Chance of precipitation is 40%.

Saturday: Mostly sunny, with a high near 82. North northwest wind around 5 mph.

Saturday Night: Mostly clear, with a low around 65.

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 after 1am. Mostly cloudy, with a low around 70.

Tuesday: A chance of showers and thunderstorms. Mostly cloudy, with a high near 84.

Tuesday Night: 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


557 AM CDT Thu Aug 17 2017


Through tonight...

A potent area of low pressure (for August standards) over southern MN, will shift northeastward over the Upper Great Lakes by tonight. As it does so, an associated cold front will shift eastward across the area later this morning and into the afternoon. Once this front passes, expect some gusty westerly winds (up around 30 MPH) to set up across the area this afternoon and into the evening.

It appears that the main threat of any strong thunderstorm development this afternoon ahead of the cold front may occur east of the area in Indiana and lower Michigan. We are not out of the woods yet for precipitation, however. There is still likely to be another period of showers, and possibly a few thunderstorms early this morning. While there is an area of widely scattered showers/thunderstorms over north central IL at this time, there is also another area of mainly showers shifting northeastward over west central Illinois. This activity appears to be associated with a surge in the low-level jet being induced just ahead of the approaching main upper trough. As this activity continues to shift northeastward, it will overspread much of eastern IL and northwestern Indiana early this morning. Additional widely scattered activity is also likely to continue over far northern Illinois as well, at least through daybreak. All of this activity should end by mid morning as it shifts out of the area. A few scattered showers may redevelop over the area this afternoon, especially over northern IL, where cyclonic flow aloft will drive another weak disturbance across southern WI.




Friday through Wednesday...

Only two periods look favorable for any sort of active weather from Friday through the middle of next week.  The first would be Friday night in association with a weak low center dropping out of the northern Plains, visible early this morning as a disturbance on IR and WV satellite imagery moving from Alberta into Montana.  The likely timing of this appears to be overnight Friday into the predawn hours of Saturday which hopefully should limit its impacts on outdoor activities Friday evening and Saturday morning.

The second period of active weather appears to be in the Tuesday timeframe as a cold front moves across the area.  The ECMWF is faster than the GFS with this feature by about 12 hours which actually is not a vast disagreement five or six days out.  To address the differences in timing we will maintain a mention of precip chances from Monday evening through Tuesday evening, though the actual event likely would not be of that duration.

Beyond these two periods, the big focus for the extended period is on the eclipse Monday.  Assuming there is not a big acceleration in the activity mentioned for Monday night or Tuesday, daytime Monday appears to be dry and warm under the influence of southwesterly flow. There are no strong signals yet to nudge sky cover forecasts higher or lower, so will maintain fair weather partly cloudy conditions for now.



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

Seasonally strong surface low pressure centered near KMSP early this morning will shift northeastward over the Upper Great Lakes by this evening. As it does so, an associated cold front will shift eastward across northern IL through the late morning, with a gusty westerly wind expected to develop in its wake. Expect gusts up around 26 KT this afternoon following the frontal passage.

Scattered rain showers continue to fester across northeastern Illinois early this morning in association with disturbance moving over the area. Expect a period of showers at the Chicago area terminals early this morning (through about 14z) before the focus shifts to the east later this morning. While a lightning strike or two cant be ruled out with this activity, it appears that any such activity will be few and far between. The main threat from these showers will be brief bursts of moderate to heavy rain, which could result in MVFR to IFR visibilities. In addition, a period of some lower MVFR CIGs will be possible over the area this morning due to high lower-level moisture, but it does appear likely that any lower CIGS will quickly improve following the cold frontal passage by late morning.




The next two days will be fairly breezy over the lake with the approach and passage of a cold front.  Southerly flow will increase today and prompt headlines for small craft but fortunately waves in the IL/IN nearshore will not grow too large with the offshore winds. Passage of a cold front toward evening will turn winds westerly but maintain a fairly tight pressure gradient, which with the cooler air over warm water will sustain higher speeds into at least the late evening to overnight hours.  Weak high pressure moving over the lake for the weekend will gradually reduce wind speeds through the day on Friday.



.LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-      LMZ745...11 AM Thursday to 3 AM Friday.

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Hazardous Weather Outlook National Weather Service Chicago/Romeoville IL 505 AM CDT Thu Aug 17 2017

ILZ003>006-008-010>014-019>023-032-033-039-INZ001-002-010-011-019- 181015- 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- 505 AM CDT Thu Aug 17 2017 /605 AM EDT Thu Aug 17 2017/


.DAY ONE...Today and Tonight.

WEATHER HAZARDS EXPECTED:   Limited Thunderstorm Risk.


Thunderstorms are possible early in the day ahead of a passing cold front, and gusty winds to 30 mph are possible behind it this afternoon, but severe weather is not expected today.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Friday and Saturday...   Limited Thunderstorm Risk. Monday and Tuesday...   Limited Thunderstorm Risk.


Thunderstorms are possible Friday night and again Monday night into Tuesday.


Spotter activation is not anticipated.


Moving from the southwest at 35-45 mph.