Chicago Weather Forecast (O'Hare Official) 1:57 pm CDT Sep 19, 2017 with Forecast Discussion

This Afternoon: Partly sunny, with a high near 76. East wind around 5 mph.

Tonight: Patchy fog after 4am. Otherwise, partly cloudy, with a low around 65. East wind around 5 mph becoming calm in the evening.

Wednesday: Patchy fog before 7am. Otherwise, mostly sunny, with a high near 88. South southeast wind 5 to 15 mph, with gusts as high as 20 mph.

Wednesday Night: A 30 percent chance of showers and thunderstorms, mainly after 1am. Partly cloudy, with a low around 69. Southwest wind 5 to 10 mph.

Thursday: Mostly sunny, with a high near 87. West southwest wind around 5 mph becoming east in the morning.

Thursday Night: Partly cloudy, with a low around 69.

Friday: Sunny, with a high near 87.

Friday Night: Mostly clear, with a low around 68.

Saturday: Sunny, with a high near 87.

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

Sunday: Sunny, with a high near 85.

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

Monday: Mostly sunny, with a high near 82.

 Amf Ohare IL  41.98°N 87.9°W

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


1247 PM CDT Tue Sep 19 2017

.UPDATE... 1105 AM CDT

The latest GOES-16 Water vapor imagery channels indicate that the main mid-level disturbance driving the showers this morning over northeastern Illinois is now beginning to shift eastward over southern Lake Michigan. As a result, the shower threat should be coming to an end for much of the area by midday as the better focus sets up east of the area. We still cant rule out a few isolated showers this afternoon, especially over northwestern Indiana, but most areas are likely to remain dry this afternoon.

In spite of the low chances for additional showers this afternoon, it does appear that lower level cloud cover is likely to remain with us for much of the afternoon, as any clearing areas are likely to experience a quick uptick in diurnal Strato-CU development. Because of this, high temperatures are likely to be held down into the 70s for much of the area. So, highs this afternoon have been lower a bit to reflect this thinking.




Through tonight...

Early this morning, a shortwave trough is lifting across the Midwest while a weak surface low is located over west central Illinois. Showers and a few thunderstorms will continue this morning out ahead of these features. HRRR has had a decent handle on conditions overnight and have leaned heavily on the latest runs for timing PoPs through the remainder of the morning. Expect most of the area to be dry by mid to late this morning with perhaps a few showers hanging on over northwest Indiana by early afternoon, if any at all. Mid/upper level height rises overspread the region behind the departing trough helping to erode the cloud cover through the afternoon. Expect afternoon highs in the mid 70s east to around 80 west. Temperatures along the lake front will be held in the low 70s with onshore flow through the day.




Wednesday through Monday...

Transient upper ridge will traverse the region midweek with a more blocky and amplified pattern developing late in the week and continuing through early next week resulting in a warm pattern throughout much of the extended time frame.

On Wednesday, closed upper low will be in place over the Pacific Northwest with a lead shortwave ejecting east across the northern Plains. Out ahead of the this feature, deep southerly flow will hel Ptransport high theta-e air all the way north across the Great Plains and into the Canadian Prairies. The thermal ridge axis with 850mb temperatures of 19-20C will advect east across the local area Wednesday contributing to very warm surface temperatures accompanied by high dewpoints. Afternoon highs on Wednesday are expected to be in the mid to upper 80s and a few spots may tag 90. Meanwhile, dewpoints are expected to rise to around 70 resulting in muggy conditions.

Wednesday night into Thursday looks to bring the best chance for any precipitation over the area as the aforementioned upper level disturbance lifts from the Upper Midwest into Canada while an weak surface cold front pushes east across the Mississippi Valley. Models are a bit mixed on how far east and south to bring precipitation with this front, and for now would err on the drier end of the spectrum given how far displaced we are from the upper wave and forcing along the front may struggle to overcome warm mid level air and capping. Moderately strong instability will be in place to support thunderstorm development, though, if forcing ends up being sufficient.

Meanwhile, the upper low over the Pacific Northwest is progged to dig into the Intermountain West Wednesday through Friday with further downstream amplification of the ridge expected to occur. Friday through the weekend the pattern becomes very blocky with the local area falling on the western periphery of a strong upper ridge. While widespread convection is possible off to our west, it looks like northern Illinois and northwest Indiana will remain close enough to the ridge axis to stay primarily dry with continued unseasonably warm temperatures.



.AVIATION... For the 18Z TAFs...

The main weather concern during the period is the potential for another period of fog and any associated very low CIGS late tonight into Wednesday morning.

The low clouds and precipitation have shifted out of the area. However, expect BKN low end VFR CIGS at times this afternoon as diurnal Strato-CU develops over the area. Otherwise, expect light easterly winds through the afternoon.

The winds should become very light across the area tonight, and this combined with amble low-level moisture could set the stage for the redevelopment of fog over the area overnight. Confidence is not the highest with the fog extent for the main Chicago terminals, as it is typically hard to get good fog to develop. However, given the extent of the fog that occurred to our west this morning, and the fact that this same airmass will be overhead tonight, I think there is a decent possibility to have at least some MVFR to IFR visibilities and possibly even some IFR to LIFR CIGS at the Chicago terminals overnight. Even lower conditions, with possible dense fog will be possible at KGYY and possibly even KDPA. These low conditions may linger through mid-morning Wednesday, but conditions are likely to improve thereafter following a surface warm frontal passage. Expect southerly winds to set up over the area by midday, with some gusts developing by the afternoon.




High pressure over Quebec will continue northeast. East winds become southeast tonight, and speeds increase to 15- 25 kt as the pressure gradient tightens over the lake Wednesday. High pressure passes over Lake Superior Thursday, and winds will become north briefly as the high passes by.  Southerly winds return across the lake Friday and remain through early next week due to the blocky pattern. A high pressure ridge remains stationary over the eastern half of the U.S. while a low pressure trough extends from the northern Mississippi Valley to the southern Plains through early next week.




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Hazardous Weather Outlook National Weather Service Chicago/Romeoville IL 323 AM CDT Tue Sep 19 2017

ILZ003>006-008-010>014-019>023-032-033-039-INZ001-002-010-011-019- 200830- 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- 323 AM CDT Tue Sep 19 2017 /423 AM EDT Tue Sep 19 2017/


.DAY ONE...Today and Tonight.

WEATHER HAZARDS EXPECTED:   Elevated Thunderstorm Risk.   Elevated Fog Risk late tonight.


Scattered thunderstorms will continue this morning. Severe thunderstorms are not expected.

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

Wednesday...   Limited Severe Thunderstorm Risk.


Spotter activation will not be necessary through tonight.


Moving east at 30 MPH.