Chicago Weather Forecast (O'Hare Official) 3:19 pm CDT Oct 6, 2017 with Forecast Discussion

This Afternoon: Showers and possibly a thunderstorm. Patchy fog. High near 69. Southeast wind around 5 mph. Chance of precipitation is 80%. New rainfall amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.

Tonight: Showers and thunderstorms before 2am, then showers likely and possibly a thunderstorm between 2am and 4am, then a chance of showers and thunderstorms after 4am. Patchy fog. Low around 66. South wind 5 to 15 mph, with gusts as high as 20 mph. Chance of precipitation is 80%. New rainfall amounts between a tenth and quarter of an inch, except higher amounts possible in thunderstorms.

Saturday: A chance of showers, then showers and possibly a thunderstorm after 1pm. High near 78. Breezy, with a south southwest wind 15 to 20 mph, with gusts as high as 35 mph. Chance of precipitation is 80%. New rainfall amounts between a tenth and quarter of an inch, except higher amounts possible in thunderstorms.

Saturday Night: A 50 percent chance of showers before 1am. Mostly cloudy, then gradually becoming mostly clear, with a low around 55. West southwest wind 5 to 15 mph, with gusts as high as 25 mph.

Sunday: Sunny, with a high near 77. South wind 5 to 10 mph.

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

Columbus Day: Sunny, with a high near 75.

Monday Night: Partly cloudy, with a low around 55.

Tuesday: A slight chance of showers before 1pm. Partly sunny, with a high near 67.

Tuesday Night: A chance of showers after 1am. Partly cloudy, with a low around 51.

Wednesday: A slight chance of showers. Mostly sunny, with a high near 66.

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

Thursday: A slight chance of showers. Mostly sunny, with a high near 67.

Thursday Night: Partly cloudy, with a low around 52.

Friday: 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


305 PM CDT Fri Oct 6 2017


Through Tonight...

Main forecast concerns/challenges are with precip trends this afternoon into the evening, gusty winds Saturday, and thunderstorm possibility Saturday afternoon.

Latest radar imagery showing upstream area of showers pushing into the west/southwest CWA at this time, ahead of approaching wave. Despite slightly higher coverage of showers expected to move through northern Illinois this afternoon, am seeing showers extending further to the east into northwest Indiana and expect this trend to continue with most locations observing scattered showers. While the afternoon will remain on the wet side, do see somewhat of a back edge to this development and expect by early evening that a trend back towards drier conditions will occur. Still not overly concerned with thunder this afternoon but with some instability overhead and increasing ascent in place, can`t rule out some embedded thunder. Once again, there may be a few hour dry window through mid evening. However, isentropic ascent ramps up across eastern IA and northwest IL by mid/late evening. This should allow for precip to blossom in this location, and then shift east into northern IL during this time. Focused highest pops in northern IL, as don`t have as high confidence for additional shower development over east central IL and northwest IN during this time. If precip does occur in these areas, expect more widely scattered to isolated development. Similar thoughts with thunder continue, but have maintained slight chance thunder given the previously mentioned thoughts on instability and forcing. This precip will then shift north of the area after the midnight time frame, and although expect drier conditions to be in place for the remainder of the night, do have lower confidence of this. The warm front will be well north of the area with the LLJ also focused to the north, however, some weak ascent will likely remain and could support scattered light showers. Highest chances of this occurring look to be across north central IL.

Warm front will lift through the area this afternoon through early evening with mild conditions expected tonight. Maintained similar lows around the mid 60s tonight, with this warm air mass in place and while a steady southerly wind increases through the night.




Saturday through Friday...

A highly dynamic/strong system consisting of a deep surface trough/low, potent vorticity maxima, and strong mid/upper level flow will push through the region on Saturday. This will provide a time frame to keep an eye on, as gusty winds are likely and with the possibility of thunderstorms across the CWA Saturday afternoon. Guidance still varying with regards to the evolution of the surface trough/low and extent of the speeds aloft. Nonetheless, the pattern would suggest windy/gusty conditions on Saturday. Have continued similar thoughts with gusts as high as 30 to 40 MPH. This is sub advisory, but will need to still monitor later trends as higher gusts are still a possibility. At this time, speeds really ramp up by late morning and then continue through mid afternoon, before diminishing into the late afternoon and evening.

Still monitoring convective trends on Saturday, once again with this anticipated strong system moving through the region. Cloud cover and showers may already be in place Saturday morning, and likely be a limiting factor to the extent of the intensity of any storms on Saturday. With this in mind, at this time, moisture plume and higher instability look to be just to the east of the CWA along with the threat of more organized storms. If this were not to be the case, and instability were able to increase across the area, then could be monitoring for at least an isolated strong to severe storm Saturday afternoon. If this scenario did occur, locations across east central IL and northwest Indiana could see these possible storms. Once again, will continue to monitor trends and adjust forecast/messaging as necessary. Outside of these storms, showers will push east through the area throughout the day on Saturday with the highest chances occurring in the afternoon. Given the strong winds aloft, it would not be out of the question for a stronger shower or isolated storm to mix down some stronger winds. So, the threat of convective gusts is also a possibility on Saturday. Any showers/storms should quickly shift to the east by early Saturday evening, with dry conditions expected for the remainder of the night and into Sunday. Sunday looks dry and sunny, with temperatures rising to the mid/upper 70s by midday.



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

Concerns with the Chicago area TAFs are:

- Low clouds/visibility early afternoon improving mid afternoon

- Showers mid-late afternoon and likely again late this evening

- Winds gusting to over 30 kt from the due south Saturday morning

- Showers, possibly storms, moving through during Saturday   afternoon

Murky conditions continue over the Chicago airports, though improvement in prevailing ceilings and visibility is creeping northward with a warm front. This will spread over the Chicago sites though confidence on the exact timing is low, as pockets of drizzle are continuing to reduce visibility and ceiling per webcams/observations.

Showers are increasing in coverage on upstream regional radar and that will continue through the afternoon. While some thunder is possible, especially toward northwest/north central Illinois, the coverage is expected to be limited enough over Chicago to not include in the TAFs at this time.

A wind shift to south will occur with the frontal passage late this afternoon. Light speeds will see an increase after the shift to southerly and then a gradual increase through tonight. Off the deck, southwest winds around 40-45 kt at 1500 ft are expected late this evening into overnight though true LLWS is not anticipated given the surface wind increase.

Saturday morning winds will be around due south with high confidence. Gusts in excess of 30 kt have medium-high confidence, though confidence is low on how frequent they will be. The winds will shift to southwest and then west-southwest with a cold front during the mid-late afternoon. This front will be accompanied by showers, some of which could be gusty, and even possibly thunder.




A warm front will gradually advance northward across the southern part of the lake this evening and the northern part of the lake overnight. Winds will shift south with an increase in speed after the passage of this boundary, which will help dissipate any pockets of dense marine fog.

As low pressure lifts from the Central Plains to the Great Lakes a ramp up in southerly wind speeds will occur by late tonight into Saturday morning. Gales of 35-40 kt are expected across the open water during the day Saturday and sporadic gale force gusts are probable along the Illinois and Indiana nearshores. The system cold front will push across the lake early in the evening, turning winds more westerly. Speeds will likely show variability ahead of and behind the front, with possibly some gusty showers at frontal passage time.

West winds will turn back to the south during Sunday and then fli Pto northerly during Sunday night as a backdoor cold front swings down the lake. The northerly winds with gusts toward 25 kt could last through Tuesday.



.LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Gale Warning...LMZ777-LMZ779...3 AM Saturday to 7 PM Saturday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-      LMZ745...3 AM Saturday to 1 AM Sunday.

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Hazardous Weather Outlook National Weather Service Chicago/Romeoville IL 335 PM CDT Fri Oct 6 2017

ILZ003>006-008-010>014-019>023-032-033-039-INZ001-002-010-011-019- 072045- 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- 335 PM CDT Fri Oct 6 2017 /435 PM EDT Fri Oct 6 2017/


.DAY ONE...Tonight.

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


There is a slight chance of thunderstorms mainly this evening. Severe weather is not anticipated.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday.

Saturday...   Limited Thunderstorm Risk.   Limited Non Thunderstorm Wind Risk.


Strong southerly winds are expected Saturday morning which will shift westerly Saturday afternoon. Wind gusts as high as 40 mph are possible Saturday morning through early Saturday evening.


Spotter activation will not be necessary through tonight.


Moving to the northeast at 40 mph.