Chicago Weather Forecast (O'Hare Official) 8:17 am CDT Aug 29, 2017 with Forecast Discussion

Today: Scattered showers and thunderstorms, mainly between 10am and 4pm. Patchy fog before 10am. Otherwise, mostly cloudy, with a high near 76. Northeast wind 5 to 10 mph, with gusts as high as 15 mph. Chance of precipitation is 40%.

Tonight: Isolated showers and thunderstorms before 7pm. Patchy fog after 5am. Otherwise, partly cloudy, with a low around 61. Northeast wind 5 to 10 mph becoming light north in the evening. Chance of precipitation is 10%.

Wednesday: Patchy fog before 7am. Otherwise, mostly sunny, with a high near 80. Calm wind becoming north northwest around 5 mph in the morning.

Wednesday Night: Partly cloudy, with a low around 63. North northeast wind around 5 mph becoming calm.

Thursday: A 20 percent chance of showers before 1pm. Partly sunny, with a high near 74. Northeast wind 5 to 10 mph, with gusts as high as 15 mph.

Thursday Night: Mostly clear, with a low around 56.

Friday: Sunny, with a high near 73.

Friday Night: Mostly clear, with a low around 57.

Saturday: Sunny, with a high near 79.

Saturday Night: Partly cloudy, with a low around 61.

Sunday: Sunny, with a high near 81.

Sunday Night: Partly cloudy, with a low around 62.

Labor Day: 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


649 AM CDT Tue Aug 29 2017


Today through Tonight...

Short term weather concerns include dense fog this morning, and scattered shower and thunderstorm development again today as upper trough slowly moves off to the east.

Early morning GOES-16 water vapor imagery shows two distinct vorticity centers rotating within upper trough over the western Great Lakes. The northern vort is expected to wrap cyclonically southward across mainly the eastern parts of the WFO LOT forecast area today, while weak inverted north-south surface trough currently along the IL/IN border drifting east. In the near term, moist low levels in the vicinity of the trough have cooled beneath mostly clear to partly cloudy skies overnight, resulting in fog development across much of the IL portion of the forecast area. While cloud cover was more prevalent along and east of the IL/IN border, several locations under clear/partly cloudy skies to the west have seen visibility drop to 1/4-1/2SM and expect this trend to continue through sunrise. Thus have issued a dense fog advisory for areas west of the IL/IN state line (and with exception of Lake/Cook counties in IL) through 9 am this morning.

Aside from the fog this morning, a few showers continue to fester mainly over Lake Michigan at this hour. Various guidance generally increase shower and thunderstorm coverage across northeast IL and northwest IN during the morning and early afternoon hours, as diurnal instability develops beneath the southward wrapping mid-level vort center. Forecast soundings indicate instability will be weaker today than on Monday, as the core of the mid-level cold pool has moved east of the area. Should be a steady decrease in coverage from the west late this afternoon and quick dissipation by early evening as the upper trough finally pulls away to the east.

Light synoptic north-northeast winds will develop today as the surface trough moves off to the east, and this will support modest lake breeze/cooling. Temps should range from the lower 70`s along the lake to the mid-upper 70`s inland, with the warmest temps across the southwest/south counties this afternoon. May see some patchy fog again tonight as skies become partly cloudy and low levels remain relatively moist.




Wednesday through Monday...

Weak surface high pressure develops across the area early Wednesday, in the wake of the departing upper trough which impacted the area early in the week. Other than some diurnal stratocu, and an afternoon lake breeze in light north-northwest surface wind field, dry and mild weather is expected with temperatures near 80 degrees per low level thermal field progs.

Guidance remains in good agreement in depicting a cold front settling southward across the region late Wednesday night into Thursday, in response to a short wave digging southeast across Ontario and the far northern Great Lakes region. A period of stratus/stratocu is likely with and behind the front during the morning and midday hours, with forecast soundings depicting cooling and saturation below about 800 mb. Models continue to squeeze out some light qpf along/behind the front as well, as the front continues to push south across the cwa during the day. Drier air does spread in during the afternoon and evening however, as surface high pressure builds across the Lakes in the wake of the digging short wave. Cool advection and the tightening gradient associated with the building high should make for breezy northeast winds Thursday, with cooler air resulting in a large northeast to south temp range by late afternoon from around 70 far northeast near the lake to the upper 70`s far south.

The high continues to strengthen across the central and eastern Great Lakes Thursday night into Friday, with dry, cool easterly low level flow across the forecast area. Model 925 mb temps again indicate afternoon temps to range from around 70 over northeast IL, to the low-mid 70`s in the far south/southwest counties of the cwa. By Friday evening, remnants of Hurricane Harvey are expected to be lifting into the lower Ohio Valley, with the current NHC forecast position over northeast Arkansas by 00Z Saturday. This appears to be a good compromise between the slightly faster ECMWF and slightly slower GFS solutions. While their timing is a bit different, both models do spread some very light rain into the far south/southeast parts of the cwa by late Saturday, though would think subsidence along the northern periphery of this system, and dry easterly low level flow off the Lakes would limit the northward spread of rain.

While Harvey`s remnants pass to our south into the Labor Day weekend, guidance continues to indicate a stout short wave digging across the Upper Midwest and northern Great Lakes. While the strongest forcing with this system looks to remain well north of the forecast area, a weak cold front does trail into the region late Saturday night. This is a little quicker progression than solutions from 24 hours ago. This front appears to wash out across the area Sunday however, as a stronger wave begins to dig into the Upper Midwest late Sunday or Sunday night. Guidance continues to differ with the timing/details of the cold front with this system, with the ECMWF a full day faster than the GFS and bringing it into the area late Sunday. While confidence is low at this distance with the details and timing of the Monday cold front, confidence is higher in a significant pattern change resulting in cooler weather next week as models depict development of a dee Plarge scale upper trough over eastern NOAM just beyond the end of this forecast period.



.AVIATION... For the 12Z TAFs...

649 am...Main forecast concern is the chance of thunderstorms later this morning into this afternoon. Other concern is fog this morning and possibly again tonight into Wednesday morning.

An upper level low will continue to slowly move east today. As it does...showers and few thunderstorms may develop over the area late this morning into this afternoon. Confidence on coverage is low and not expecting as much thunder as occurred on Monday. So maintained vcsh mention and precip should drift west of ord/mdw as a lake breeze moves inland today so there remains uncertainty with timing. Precip will eventually dissipate by early evening.

Areas of dense fog have developed generally west/southwest of the Chicago area terminals with light fog at ord/mdw. This fog will slowly lift/dissipate through mid morning. Additional fog is possible later tonight into Wednesday morning though confidence is low regarding how widespread and low visibilities drop.

Light winds will become north/northeast this morning and then are expected to increase into the 7-10 kt range with a lake breeze later this afternoon. Winds diminish to light and variable this evening. cms



157 am...A ridge of high pressure will remain across the lakes region into Wednesday. Weak low pressure will develop near James Bay today and quickly move into northern Quebec Wednesday. A trailing cold front from this low will move south across Lake Michigan Wednesday afternoon into Wednesday night shifting winds northerly. Prevailing winds should remain in the 15-25kt range but a period of 30kts is still possible with the frontal passage. These northeast winds will persist Thursday and then shift more easterly Thursday night. The gradient is expected to remain fairly tight as strong high pressure builds across the northern lakes and the remnants of Harvey begin to slowly move north late in the week. Thus...a prolonged period of northeast then easterly flow is possible on the south end of the lake which may result in small craft and beach hazard conditions. cms


.LOT WATCHES/WARNINGS/ADVISORIES... IL...Dense Fog Advisory...ILZ003-ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-      ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ022-ILZ023-ILZ032-      ILZ033-ILZ039 until 9 AM Tuesday.

IN...None. LM...None.

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Hazardous Weather Outlook National Weather Service Chicago/Romeoville IL 407 AM CDT Tue Aug 29 2017

ILZ003>006-008-010>014-019>023-032-033-039-INZ001-002-010-011-019- 300915- 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- 407 AM CDT Tue Aug 29 2017 /507 AM EDT Tue Aug 29 2017/


.DAY ONE...Today and Tonight.

WEATHER HAZARDS EXPECTED:   Limited Thunderstorm Risk.   Significant Fog Risk.


Areas of dense fog are expected early this morning, mainly west and south of Chicago. Scattered thunderstorms are likely from midday through the afternoon hours.

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

Wednesday...   Elevated Fog Risk. Thursday...   Limited Thunderstorm Risk.   High Swim Risk. Friday...   High Swim Risk. Sunday and Monday...   Limited Thunderstorm Risk.


Spotter activation will not be necessary through tonight.


moving southeast at 10 to 15 MPH.