Chicago Weather Forecast (O'Hare Official) 3:18 pm CDT Sep 16, 2017 with Forecast Discussion

Tonight: Partly cloudy, with a low around 68. South wind around 10 mph.

Sunday: A 40 percent chance of showers and thunderstorms, mainly between 10am and 5pm. Partly sunny, with a high near 82. South southwest wind 5 to 10 mph becoming northwest in the afternoon.

Sunday Night: A 10 percent chance of showers and thunderstorms before 7pm. Partly cloudy, with a low around 60. North wind around 5 mph.

Monday: A 20 percent chance of showers and thunderstorms after 1pm. Mostly sunny, with a high near 78. East northeast wind around 5 mph.

Monday Night: A chance of showers and thunderstorms. Mostly cloudy, with a low around 63. East northeast wind around 5 mph becoming southeast after midnight. Chance of precipitation is 40%.

Tuesday: A 40 percent chance of showers. Partly sunny, with a high near 79.

Tuesday Night: A 30 percent chance of showers after 1am. Mostly cloudy, with a low around 65.

Wednesday: A slight chance of showers and thunderstorms after 1pm. Mostly sunny, with a high near 82.

Wednesday Night: A chance of showers and thunderstorms before 1am, then a slight chance of showers. Partly cloudy, with a low around 65.

Thursday: Mostly sunny, with a high near 82.

Thursday Night: A slight chance of showers and thunderstorms. Partly cloudy, with a low around 64.

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

Friday Night: Mostly cloudy, with a low around 65.

Saturday: Partly 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


258 PM CDT Sat Sep 16 2017


Through Sunday...

Early afternoon satellite imagery shows the area on the western flank of an upper ridge with a large upper trough rotating across the Rockies and High Plains. At the surface, low pressure over western Lake Superior has a cold front stretching across western Iowa into Kansas. South to southwest winds are in place across the local area with a mix of diurnal cumulus and sun overhead. The rest of the afternoon and evening will remain dry with rather mild temperatures. Winds will ease somewhat this evening as clouds dissipate.

Showers and thunderstorms will continue to develop across central and eastern Iowa and make their way eastward into tonight. The main upper trough will begin to lift northeast taking the surface low with it which will stretch the front and weaken its convergence with time through Sunday. The evening convection will likely weaken as it moves into northern Illinois but expect that some showers will linger into the Rockford/I-39 corridor late tonight with some thunder possible if better organization can continue. Some of this activity could linger through mid/late morning into the Chicago area but prospects are not great for this to occur. The next question becomes how much development occurs along/ahead of the front Sunday? Low end instability is expected ahead of the front roughly from Chicago to Pontiac and points east by early afternoon. Residual cloud cover may limit development through early afternoon. Chances improve through the afternoon to the east and it may end up being that any new development is across northwest Indiana back into Ford/Iroquois Counties. For now favor some development closer to the Chicago-Pontiac I-55 corridor with increased coverage further east. Severe potential is on the low end and heavy downpours and lightning would be the main threats. Winds shift northwesterly behind the front from northwest to southeast into early evening.




Sunday night through Saturday...

Little change to the longer term forecast as the guidance for Sunday night through mid week remains consistent with previous trends for decreasing amplitude and faster progression to the upper level pattern, with a broad band of fast wly flow aloft, with a series of weak southern stream shortwaves bringing periodic chances for some scattered showers or thunderstorms. By mid week into next weekend, the pattern will trend back toward a more highly amplified pattern aloft, with broad troughing over the cntrl plains and wrn CONUS with ridging over the ern CONUS, which would the local area under swly flow aloft. There are significant differences, however, in the trends of shortwave energy rippling through the larger scale pattern, so, confidence is moderate in the temperature trends driven by the longwave pattern, but confidence is low in the PoP trends driven by the shortwave trends. In general, expect temperatures on Monday and Tuesday to be near seasonal normal levels with highs in the mid to upper 70s north of the I-80 corridor and low 80s to the south. The exception will be the lakefront where onshore flow will keep temps a bit lower, in the low to middle 70s. By midweek and into next weekend, temperatures should trend back up into the middle 80s as upper level ridging builds over the midwest. Periodic low chances for some showers or thunderstorms will continue through the week, though confidence is low in timing/coverage.


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

South to southwest winds continue early this afternoon with mixing bringing occasional gusts to near 20 kt. Scattered VFR cumulus will continue through the afternoon with bases rising with time. Wind speeds will ease slightly this evening with the direction turning south to southeast before turning more south to southwest overnight. Showers and thunderstorms are expected to move into northwest Illinois ahead of a cold front overnight but be on a decreasing trend. Have added shower mention at RFD late tonight as some activity does look to hold together. Cannot rule out some embedded thunderstorms. Less confident in precipitation holding together toward the Chicago area but did include a mention mid morning.

The cold front will cross the terminals into the late afternoon turning winds northwesterly. New storm development is possible along the front from roughly ORD to PNT and points east early to mid afternoon but it is possible that development holds off a little longer pushing the threat east of the Illinois terminals. GYY may have the best threat for thunder of any site by late afternoon. Expect a mix of clouds and sun and some MVFR stratocu may build in behind the front during the afternoon.




High pressure remaining parked across the Appalachians and New England as a trough and associated cold front push across the central and northern plains will keep brisk sly-slwy winds over the lake through tonight and into tomorrow. A cold front will push across the lake on Sunday. As high pressure builds in behind the front, winds will turn through nnwly to nely by for Monday and Monday night and easterly by Tuesday. Another trough and cold front are expected to push across the plains and to the Mississippi Valley by midweek, allowing winds to continue to veer to sely-sly.



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Hazardous Weather Outlook National Weather Service Chicago/Romeoville IL 323 PM CDT Sat Sep 16 2017

ILZ003>006-008-010>014-019>023-032-033-039-INZ001-002-010-011-019- 172030- 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 PM CDT Sat Sep 16 2017 /423 PM EDT Sat Sep 16 2017/


.DAY ONE...Tonight.

WEATHER HAZARDS EXPECTED:   Limited Thunderstorm Risk.


A few decaying thunderstorms may reach the I-39 corridor late tonight in advance of a cold front crossing the Mississippi Valley.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.

Sunday through Friday...   Limited Thunderstorm Risk.


Spotter activation will not be necessary through tonight.


Moving east at 20 MPH.