Chicago Weather Forecast (O'Hare Official) 5:17 pm CDT Sep 30, 2017 with Forecast Discussion

Tonight: Mostly clear, with a low around 48. South southeast wind 5 to 10 mph.

Sunday: Sunny, with a high near 70. South southeast wind 5 to 10 mph, with gusts as high as 20 mph.

Sunday Night: Partly cloudy, with a low around 56. South southeast wind 5 to 10 mph.

Monday: Mostly cloudy, then gradually becoming sunny, with a high near 82. South wind 10 to 15 mph, with gusts as high as 25 mph.

Monday Night: Mostly clear, with a low around 62. South southeast wind around 10 mph.

Tuesday: Mostly sunny, with a high near 87.

Tuesday Night: A 30 percent chance of showers and thunderstorms after 1am. Partly cloudy, with a low around 64.

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

Wednesday Night: A chance of showers, mainly after 1am. Mostly cloudy, with a low around 57.

Thursday: A slight chance of showers. Partly sunny, with a high near 69.

Thursday Night: A slight chance of showers. Partly cloudy, with a low around 53.

Friday: Mostly sunny, with a high near 69.

Friday Night: A chance of showers. Mostly cloudy, with a low around 54.

Saturday: A chance of showers. Mostly cloudy, with a high near 75.

Amf Ohare IL
 41.98°N 87.9°W

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


241 PM CDT Sat Sep 30 2017


Through Sunday...

Splendid early fall conditions will remain in place today and into the first day of October. High pressure centered over Lake Huron will shift to the eastern Great Lakes tonight and into Sunday. Low pressure is still well removed from the area, but will begin to organize in lee of the Rockies. This pressure pattern will allow winds to shift to southeasterly, which will bring modest warming for Sunday. Plentiful sunshine today will give way to some increase in high clouds on Sunday. Highs will bump up to the upper 60s far NE to the lower to maybe mid 70s in Lasalle/Livingston counties.




Sunday night through Saturday...

A highly amplified upper pattern will be in place across the country for Monday into Tuesday as troughing dominates in the western CONUS and anomalous ridging dominates in the east. Overall, this will pattern will result in our area residing within a region of deep south-southwesterly flow through Tuesday, setting up a two day period of much above average temperatures. The latest thinking continues to support the potential for lower to perhaps some mid 80s on Monday, followed by mainly mid 80s on Tuesday, with some upper 80s not out of the question. As mentioned in the previous discussion, lower level moisture should increase across the area on Tuesday, and this may limit high temperatures from warming much above the middle/upper 80s, especially with the possibility for at least some clouds. For these reasons, I have made little change to the temperature forecast for Monday and Tuesday.

Following Tuesday, it appears that the weather pattern will begin to change across the region. Forecast guidance is now in a bit better agreement with the eastward ejection of an upper disturbance emanating from the western U.S. trough. This disturbance is expected to shift eastward across the Upper Midwest and Upper Great Lakes region by midweek. As it does so, it is likely to flatten the upper ridge, and should help drive a surface cold front southward into northern Illinois by early Wednesday. Because of this, it appears that Wednesday will be a cooler day (highs mainly in the 70s), especially with more cloud cover and possible scattered showers/storms around.

Uncertainty remains quiet high later in the week, however, as forecast guidance continues to show large disagreements with the timing and evolution of the western U.S upper trough/low. There are also questions as to how far south the cold front will get later Wednesday before stalling out. These details will ultimately play a major role on the actual precipitation chances over the area for the later half of the period. With all of this uncertainty, however, it does appear that storm system could impact the central CONUS sometime next weekend. This system may result in another decent chance of much needed precipitation for the area.




For the 18Z TAFs...

Concerns are fairly low this period. East winds courtesy of high pressure over Lake Huron will shift to southeasterly tonight as the high will shift to the eastern Great Lakes. This should hold winds this afternoon in the 8-10 kt range for ORD/MDW. Low pressure will strengthen over the far northern plains on Sunday, which will lead to increasing southeast winds through the day, likely above 10 kt. Otherwise, VFR conditions will prevail with some increasing high clouds expected through the day Sunday.




Surface high pressure over the western Great Lakes is currently resulting in light and somewhat variable flow over the lake. However, expect the flow to begin to increase a bit while shifting southerly tonight as the surface high shifts towards New England. It still appears likely that a couple of days of breezy southerly winds up to around 30 kt will occur Sunday night through Tuesday. During this period low pressure is expected to dominate over south central Canada, while high pressure remains in place over New England. Given the slow progression of weather systems during this period, the surface pressure gradient will be supportive of these breezy southerly winds for much of this period. Some low end gale force winds also cannot be ruled out, especially over the northern half of the lake on Tuesday. A surface cold front is then forecast to shift over the lake either late Tuesday night or Wednesday morning. With the passage of this front, lighter northerly flow is likely.




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