Chicago Weather Forecast (O'Hare Official) 3:17 am CDT Sep 26, 2017 with Forecast Discussion

Today: A 20 percent chance of showers after 4pm. Partly sunny, with a high near 90. Light south wind becoming southwest 5 to 10 mph in the morning. Winds could gust as high as 15 mph.

Tonight: A 20 percent chance of showers before 1am. Mostly cloudy, with a low around 60. Northwest wind 5 to 10 mph, with gusts as high as 15 mph.

Wednesday: Mostly cloudy, then gradually becoming sunny, with a high near 72. North northwest wind around 10 mph, with gusts as high as 15 mph.

Wednesday Night: Mostly clear, with a low around 51. North northwest wind 5 to 10 mph.

Thursday: Sunny, with a high near 73. North northwest wind around 5 mph.

Thursday Night: Mostly clear, with a low around 53.

Friday: Mostly sunny, with a high near 67.

Friday Night: Partly cloudy, with a low around 49.

Saturday: Sunny, with a high near 65.

Saturday Night: Mostly clear, with a low around 47.

Sunday: Sunny, with a high near 70.

Sunday Night: Partly cloudy, with a low around 53.

Monday: 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


328 AM CDT Tue Sep 26 2017


Through Tonight...

One more day - this week - of temperatures making a solid run to near record levels mainly for locations along and east of I-55. There remains a minimal chance for isolated showers today and this evening, but overall forcing for any storms/high impact weather looks low.

The main cold front this morning is draped from western Wisconsin through Kansas City and slowly inching eastward. Outflow emanating from showers last night reached northwest/north central Illinois, triggering a few showers. Spotty light showers could pop in this same area into the morning as 20-25 kt south-southwest flow continues over the old outflow.

The challenge with temperatures today will be clouds. Early morning satellite imagery depicts high clouds spreading over northern Illinois. Some of this looks to thin later this morning into early afternoon before re-increasing later, although predicting mid-high cloud trends is challenging. The approach of the boundary and increasing precipitable waters to 1.5 inches should mean some off and on high clouds for the forecast area today, with north central Illinois likely seeing more clouds than sun. Warm advection areawide and a mild starting point will allow temperatures to at least reach well into the 80s, and eastern locations if enough sun will have a fair chance to reach 90 given low-level thermal profiles. Boundary layer southwest wind speeds should be enough to prevent lake breeze cooling today, so lakefront areas including downtown Chicago should see just as warm as inland.

As the front moves through the area later today and early this evening, isolated showers along the general convergence zone are possible. Ahead of the front in eastern areas, afternoon temperatures may reach the convective temperatures, but will depend on dew points. Cannot rule out a stray earlier shower, but the best chance...which is still quite low...should be along the boundary. Thunder chances are even lower given the general synoptic pattern of clouds shielding the convergence zone as well as weak elevated lapse rates.

Northwest winds will pick up some tonight behind the front steering in gradually cooler and drier air.




Wednesday through Monday...

Noteworthy items for this part of the forecast are another cold front moving through late Thursday night with a more seasonable shot of air, but that looks quite brief with increasing potential for another warm-up into the first week of October.

Northwest winds of 10-15 mph will filter in cooler air on Wednesday. Morning cloud cover looks to ease in the afternoon due to a stout subsidence inversion and drying. Lake effect stratocumulus, at least patches, are likely into northwest Indiana. Both Wednesday and Thursday should see highs in the lower-mid 70s for most.

A strong upper short wave trough will drop southeast over Ontario and the Great Lakes region Thursday night into Friday. Looking at the model trend of this, it has been further northeast in global solutions in the past 24 hours. For our area, that means even less chance for any rain with the system`s cold front, as well as a quicker passage of the cool air.

Friday looks breezy behind the cold front with lake effect stratocumulus very likely into northwest Indiana and possibly northeast Illinois. Moisture depths are shallow, but some sprinkles or light showers may need to be added to lakeside areas. Highs are forecast to fall short of 70 in northern areas Friday and for most on Saturday. Depending on precise high placement, Friday and Saturday nights should see decent radiational cooling. Some outlying favorable locations could dip into the upper 30s if dew points are low enough.

Global solutions are trending warmer for at least the first half of next week, with upper ridging building back into the area. The forecast pattern on both the 00Z GFS and EMCWF, as well as the locally dry ground and increased sensible heat flux, could favor highs reaching or topping 80 already by Monday.



.AVIATION... For the 06Z TAFs...

Light southerly winds will become southwest after daybreak with an increase in speed to 9-10 kt by midday. Isolated showers are possible around RFD this afternoon with the approach of a cold front. This front will pass the RFD area during the late afternoon and the Chicago airports during the early evening, turning winds to the northwest. Centered around this passage time is a window for all TAF sites where there could be an isolated shower, but overall the forcing along this front is meager. Behind the frontal passage, 4000-6000 ft cloud bases are expected. Chances for MVFR look small through tonight.

Confidence in TAF details is medium-high.




A cold front passage tonight will end the quieter marine weather, at least temporarily. Winds will turn northwest behind the front and increase in speed with gusts around or a little above 25 kt for much of the open water. A Small Craft Advisory will be needed for Wednesday along the Indiana nearshore for winds and waves and possibly the Illinois nearshore for winds. Winds and waves will ease some Wednesday night and early Thursday.

A second and more stout cold front will move south across the lake on Thursday night. This will bring northwesterly 30 kt gusts for the open water for Friday, and these winds will turn more toward due north by Friday evening it appears. This means quickly building waves for the Indiana nearshore by Friday morning and the Illinois nearshore shortly after. Winds and waves look to diminish by Saturday morning or early afternoon at this time.




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Hazardous Weather Outlook National Weather Service Chicago/Romeoville IL 342 AM CDT Tue Sep 26 2017

ILZ003>006-008-010>014-019>023-032-033-039-INZ001-002-010-011-019- 270845- 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- 342 AM CDT Tue Sep 26 2017 /442 AM EDT Tue Sep 26 2017/


.DAY ONE...Today and Tonight.

WEATHER HAZARDS EXPECTED:   Limited Thunderstorm Risk.


There is a slight chance of thunderstorms across mainly north central Illinois this afternoon. The risk of any severe weather is low.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday.

Wednesday...   High Swim Risk. Friday...   High Swim Risk.


Spotter activation will not be necessary through tonight.


Moving to the northeast at 35 MPH.