Chicago Weather Forecast (O'Hare Official) 12:18 pm CDT Oct 3, 2017 with Forecast Discussion

This Afternoon: Mostly cloudy, with a high near 79. South southwest wind around 15 mph, with gusts as high as 25 mph.

Tonight: Showers likely and possibly a thunderstorm after 1am. Mostly cloudy, with a low around 66. South southwest wind 10 to 15 mph, with gusts as high as 20 mph. Chance of precipitation is 70%. New rainfall amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.

Wednesday: Showers likely and possibly a thunderstorm before 1pm, then a chance of showers. Cloudy, with a high near 73. West southwest wind around 5 mph becoming north northwest in the afternoon. Chance of precipitation is 70%. New rainfall amounts between a tenth and quarter of an inch, except higher amounts possible in thunderstorms.

Wednesday Night: A 20 percent chance of showers. Partly cloudy, with a low around 55. West northwest wind around 5 mph becoming calm in the evening.

Thursday: Showers likely, mainly after 1pm. Partly sunny, with a high near 70. North northwest wind around 5 mph becoming east northeast in the afternoon. Chance of precipitation is 60%.

Thursday Night: Showers. Low around 57. Chance of precipitation is 80%.

Friday: Showers likely, mainly after 1pm. Mostly cloudy, with a high near 69. Chance of precipitation is 60%.

Friday Night: Showers, mainly after 1am. Mostly cloudy, with a low around 58.

Saturday: A chance of showers and thunderstorms before 1pm. Partly sunny, with a high near 70.

Saturday Night: Mostly clear, with a low around 54.

Sunday: Sunny, with a high near 73.

Sunday Night: A chance of showers. Partly cloudy, with a low around 53.

Columbus Day: A chance of showers. Partly sunny, with a high near 68.

Amf Ohare IL
 41.98°N 87.9°W

Amf Ohare IL
 41.98°N 87.9°W

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


644 AM CDT Tue Oct 3 2017


Today and Tonight...

Near term forecast concerns include cloud cover, temperatures and the timing of shower/thunderstorm chances.

Early morning surface analysis depicts a broad region of southerly flow across the Plains and Mississippi Valley, along the western periphery of strong surface high pressure centered along the east coast. Gulf moisture was spreading into the region as indicated by surface dew points rising into the lower 60`s across southern and western IL, and low cloud development across southern IL per GOES IR imagery. Various model forecasts indicate that low clouds will continue to spread north into northern IL and northwest IN today, as moisture continues to stream north ahead of a slowly advancing cold frontal trough to our west. In addition to cloud cover, many of the high-res model forecasts indicate the potential for some spotty light precip to develop mainly across the southeastern parts of the cwa by late morning, primarily along/east of the Interstate 55 corridor from east central IL into northwest IN.

With current temps in the mid-upper 60`s in most spots (near our normal daytime highs for early October), we`ll have a warm start to the day. However, the expanding cloud cover will likely limit our high temps somewhat despite breezy south winds, and have held forecast a category below the mid 80`s supported by warm 925 mb temps.

Surface cold front slowly approaches tonight, and pushes into northern IL by morning. Impressive moisture plume (surface dew points in the mid-60`s, PW`s 1.80+ inches) will spread into the area ahead/with the front, though primary large scale forcing associated with short wave energy propagating across the upper Midwest will be pretty far to the north. Anomalously moist air mass should still support showers and a few thunderstorms overnight as the cold front moves in.




Surface cold front will push slowly southeast across the forecast area Wednesday, before eventually becoming nearly stationary across central IL/IN Wednesday night. Showers and a few thunderstorms are likely to persist along/south of the frontal zone, with the greatest chances of rain settling south of I-80 with the front Wednesday night. Weak high pressure spreading across MN and WI will likely keep far northern counties of the forecast area dry overnight.

With the front stalled west-east across central IL/WI, the baroclinic zone will remain the focus for periods of showers through the end of the week, as a series of minor amplitude short waves ripple through the flow aloft. Rain chances look to increase Thursday night into Friday night as a jet streak emerges from the lingering western CONUS trough, resulting in surface pressure development across the Plains and backing of low level winds into our region. This will set up strengthening forced ascent across the frontal zone, and an eventual northward push lifting the front back as a warm front. Model differences persist with the fine details, though it appears the best push would be late Friday and Friday night as the surface low deepens to our west. General agreement exists which depicts this low lifting northeast of the area Saturday, with a trailing Pacific cold front moving through the cwa and brining rain chances to an end by Saturday evening. Temperatures through much of the extended forecast are expected to be average to slightly above average, generally upper 60s to lower 70s.

Toward the end of the period, guidance is in general agreement in developing a long wave trough over the central CONUS Sunday into Monday, and a developing surface low moving through the region toward Monday. This would bring rain chances back to the area Sunday night or Monday.



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

Multiple concerns for the forecast period.

Winds...With high pressure parked over the New England and a trough of low pressure and associated cold front approaching from the west, winds will be sly-sswly through the day and into tonight. A strengthening gradient will bring increasing sustained winds as well as gustiness through the day, with gusts in the 20-25 kt range through late afternoon or early evening.

Ceilings...have been monitoring via satellite imagery the spread of low clouds newd through MO and into wrn IL in the warm advection ahead of the approaching cold front. MVFR cigs are spreading newd per latest obs, and guidance suggests that the MVFR cigs should reach the terminals by arnd 15z. The concern then becomes, how long will the mvfr cigs last? RH timesections would suggest that the mvfr cigs could persist through the remainder of the period until the passage of the approaching cold front. While there is some justification for this, there is still enough low level warming and mixing to suggest that the lower level clouds could sct under a higher bkn deck. Confidence in the persistence of the mvfr cigs is low, so have only gone with mvfr cigs into the afternoon hours and then improving to vfr with stronger mixing.

Pcpn/tsra associated with the fropa...latest guidance would suggest a chance for a fairly extended period of shra/tsra as the cold front slowly pushes through the region late tonight and early tomorrow morning. Given some lingering uncertainty in the timing of the front reaching the area and how widespread shra/tsra will be with the front, have trended up with a prob30 and will likely refine timing as the event approaches.



Strengthening southerly flow is expected over the lake today with the potential for some near gale force southerly winds, especially for the northern portions of the lake. Winds will then subside behind a cold front dropping down the lake Tuesday night into early Wednesday. A weak pattern behind the front should lead to a period of lighter winds over the lake midweek. Deepening low pressure developing over the plains later in the week will result in increasing easterly flow on Friday. By the weekend, the potential exists for another round of strong south to southwest winds, potentially reaching gale force, over the lake. However, differences in model solutions keeps confidence low and will not mention gales in the forecast just yet.


.LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory...IL nearshore waters until 4 PM Tuesday.

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Hazardous Weather Outlook National Weather Service Chicago/Romeoville IL 409 AM CDT Tue Oct 3 2017

ILZ003>006-008-010>014-019>023-032-033-039-INZ001-002-010-011-019- 040915- 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- 409 AM CDT Tue Oct 3 2017 /509 AM EDT Tue Oct 3 2017/


.DAY ONE...Today and Tonight.

WEATHER HAZARDS EXPECTED:   Limited Thunderstorm Risk Tonight.

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

Wednesday...   Limited Thunderstorm Risk. Thursday...   Limited Flooding Risk.   Limited Thunderstorm Risk. Friday and Saturday...   Limited Thunderstorm Risk.


A nearly stationary frontal boundary will lay out west to east across the region. Periods of showers and a few thunderstorms will be possible into Saturday. Locally heavy rainfall may be possible Late Thursday...especially across west central Illinois.


Spotter activation will not be necessary through tonight.


moving east to northeast at 30 MPH...