Chicago Weather Forecast (O'Hare Official) 6:17 pm CDT Sep 21, 2017 with Forecast Discussion

Tonight: Mostly clear, with a low around 74. East northeast wind around 5 mph becoming south southwest after midnight.

Friday: Sunny, with a high near 93. South southwest wind 5 to 10 mph.

Friday Night: Mostly clear, with a low around 71. South wind around 5 mph.

Saturday: Sunny, with a high near 90. South southwest wind around 5 mph.

Saturday Night: Mostly clear, with a low around 68. Southeast wind around 5 mph.

Sunday: Sunny, with a high near 89.

Sunday Night: Mostly clear, with a low around 66.

Monday: Sunny, with a high near 88.

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

Tuesday: A slight chance of showers and thunderstorms after 2pm. Mostly sunny, with a high near 84.

Tuesday Night: A slight chance of showers and thunderstorms. Mostly cloudy, with a low around 60.

Wednesday: A chance of showers and thunderstorms. Partly sunny, with a high near 73.

Wednesday Night: A slight chance of showers. Mostly cloudy, with a low around 55.

Thursday: A slight chance of showers. Mostly sunny, with a high near 70.

 Amf Ohare IL  41.98°N 87.9°W

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


630 PM CDT Thu Sep 21 2017


Through Friday...

Late September heat remains the main focus of the forecast in the near term. New record high temp already set today at O`Hare with a 94 degree reading there.

Midwest remains along the periphery of highly amplified upper level ridge axis, downstream of a deep long wave trough across western NOAM. 12Z RAOB from DVN indicated 850 mb temp of +20C, and a 925 mb temp of +26C, resulting in low-mid 90 degree temps across the region. Residual high cloud cover from decaying MCS over southeast IA/northeast MO has kept temps slightly lower across western parts of the cwa, though a decrease in the denser cirrus has temps approaching 90 there as well this afternoon. Of course the weak surface pressure gradient in the vicinity of the stationary front just north of the cwa has allowed a lake breeze to push inland across far northeastern IL, keeping temps in the mid-upper 80`s for areas near the lake.

Recent runs of the HRRR indicate the potential for isolated thunderstorm development across mainly the southern parts of the cwa late this afternoon. Experimental ESRL HRRR also hints at this though with much lower coverage. It appears these models may be focusing on a remnant MCV from the IA/MO convection, though no distinct circulation is noted in water vapor imagery. While current RAP mesoanalysis depicts 3000 J/kg of MLCAPE and little cin across the southern tier of cwa counties, weak flow and fairly shallow cu field per GOES-16 vis imagery suggests these models may be quite overdone with thunderstorm potential. Maintained less than 20 percent pops through sunset, but may have to add an isolated mention if cu field becomes more impressive or something pops.

Another mild night is in store for the region with persistent light south winds and unseasonably high dew points. Overnight mins should remain in the 70`s in most locations, which will also give a warm head start for temps on Friday. Less cloud cover and similar low level thermal field should allow highs in the low 90`s in most spots again Friday afternoon, again at or near record levels for September 22nd. (records for tomorrow are 92 for Chicago, 93 for Rockford) Weak southeasterly lake breeze should again keep temps in the 80`s right along the lake.




Friday night through Thursday...

218 pm...The models continue to show slight thermal cooling this weekend but this will likely have little if any noticeable affects as overnight lows remain in the 60s...low 70s downtown...and highs in the upper 80s/near 90 Saturday/Sunday. Bumped temps up a few degrees to near current guidance values. Lake breezes are possible both days which will keep the immediate lakeshore areas cooler but confidence is low regarding how strong these lake breezes may be and how far inland they will move. Forecast highs are cooler near the lake but trends will need to be monitored as these time periods approach. Highs may still reach the upper 80s before temps cool and winds shift off the lake.

There is still some potential that the models are cooling the airmass too fast and if that were to occur...highs could still be a few degrees warmer than currently expected this weekend. However by early next week it does appear that the entire airmass will cool enough for highs to remain in the 80s. Additional cloud cover...perhaps a more broken afternoon cu field...may also hel Pput a lid on high temps...keeping them in the 80s.

A cold front will move across the area Tuesday night into Wednesday next week. While an isolated shower is possible any afternoon from Sunday onward next week...this front will be the next best chance of rain for the entire area. However...not confident on how widespread precip will become and its possible just scattered precip in a narrow time window moves across the area ahead of the front. Though a slower front...that arrives later into Wednesday could allow for more daytime heating and thus better precip coverage. This front will allow much cooler air to spread across the area with highs back into the 70s. A second push of colder air is possible late next week. cms


.AVIATION... For the 00Z TAFs...

Lake breeze came close to ORD, but has begun to retreat and no longer poses much of a threat of making it through this evening. Will probably be another lake breeze tomorrow, but opting to go with persistence for now and not include a wind shift at the terminals. Otherwise, other than possibly some patchy ground fog/MVFR VSBY at DPA early Friday morning, not anticipating any significant weather at the terminals through this TAF cycle.

- Izzi



218 pm...A weak cold front has stalled over south/central portions of Lake Michigan this afternoon. This front will lift back north as a warm front tonight...shifting light winds back southerly. A large trough of low pressure will remain nearly stationary over the plains through early next week maintaining southerly winds over the lake. This trough/front will move across the Great Lakes region Tuesday night into Wednesday shifting winds northwest with a period of 15-25 kts expected. cms



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Hazardous Weather Outlook National Weather Service Chicago/Romeoville IL 329 PM CDT Thu Sep 21 2017

ILZ003>006-008-010>014-019>023-032-033-039-INZ001-002-010-011-019- 222030- 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- 329 PM CDT Thu Sep 21 2017 /429 PM EDT Thu Sep 21 2017/


.DAY ONE...Tonight.


  Limited Fog Risk.

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

Friday Morning...   Limited Fog Risk. Tuesday and Wednesday...   Limited Thunderstorm Risk.


Spotter activation will not be necessary through tonight.