Chicago Weather Forecast (O'Hare Official) 11:17 am CDT Aug 9, 2017 with Forecast Discussion

Today: Mostly sunny, with a high near 83. South wind 5 to 10 mph.

Tonight: Partly cloudy, with a low around 64. South wind 5 to 10 mph.

Thursday: A 40 percent chance of showers and thunderstorms. Partly sunny, with a high near 82. South southwest wind 5 to 10 mph.

Thursday Night: A 40 percent chance of showers and thunderstorms, mainly before 1am. Mostly cloudy, with a low around 65. Southwest wind 5 to 10 mph.

Friday: Mostly sunny, with a high near 79. West wind 5 to 10 mph.

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

Saturday: Mostly sunny, with a high near 77.

Saturday Night: Partly cloudy, with a low around 60.

Sunday: Mostly sunny, with a high near 77.

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

Monday: Mostly sunny, with a high near 80.

Monday Night: Mostly clear, with a low around 62.

Tuesday: A slight chance of showers. Mostly sunny, with a high near 81.

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


637 AM CDT Wed Aug 9 2017


Through Thursday night...

Broad surface high pressure is once again in place across the midwest. Conditions are somewhat similar to last night, but we have some occasional cloud cover and slightly higher dewpoints holding temps up. Therefore temperatures are correspondingly some 5-7 degrees warmer than to this point yesterday. Patchy ground fog (which could thicken briefly at times to 1SM vsby or so) in river valleys is again possible. The high will only slowly shift east today, and thus for today the influence of a series of shortwaves ahead of an upper low across the Canadian prairies will be limited to high cloud cover. With the cool air aloft forecast soundings again some afternoon cumulus development. Look for highs again in the upper 70s to lower 80s.

The upstream upper low will slowly dig southeast into the upper midwest toward daybreak Thursday, then shift across central Lake Michigan Thursday night. It appears there will be several windows for shower and thunderstorm development through the afternoon, and with chances lingering in the evening ahead of the surface low/cold front. There are several lead shortwaves that could trigger some showers even Thursday morning across north central IL, and then some thunderstorms in the afternoon. The leading activity is most aggressive on the GFS with other sources still weakening any initial precip as initial forcing will be along and north of the WI border. Moisture pooling ahead of the front suggest some downpours are possible with the afternoon/early evening showers or storms.




Friday through Tuesday...

The front will shift south into central Illinois, leading to continued cooler than normal conditions. The 00.EC, which is still similar to its previous few runs, is the only medium range guidance source that clips our southern counties with a shower/storm chance late Saturday night into Sunday. This is ahead of another slow moving shortwave on the leading side of a huge omega block across central Canada. With the front south, northeast flow will maintain cooler than normal temperatures, with highs in the 70s.

Low confidence beyond this point as surface high pressure will remain across the region, but some weak ridging suggests more dry weather. This is certainly not a given with the fact that there will be a lingering frontal zone in the region. Temperatures look to recover to around 80 which is getting closer to the normal high as we press on into the month of August.



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

Southwest winds develop this morning with VFR clouds. A lake breeze will develop this afternoon and impact MDW and GYY. I have low confidence in the lake breeze reaching ORD, but kept a wind shift at 23Z. Winds back to south to southeast tonight, and clouds thicken early Thursday morning as low pressure moves over the upper Midwest. Winds veer to south-southwest Thursday morning. Precip will be pushing east Thursday morning into northern IL, but I think precip will stay west of ORD through 18Z. There is a small chance that precip will be quicker than I think, but I did not have enough confidence to put precip in the ORD TAF.




High pressure over the Midwest shifts east today with winds becoming south behind it tonight.  Low pressure currently over the Dakotas will move over the upper Midwest Thursday, and south winds will increase to 15-25 kt.  The low will move over the lake early Friday morning, and winds will become northwest behind it.  A baggy pressure field and high pressure dominate the pattern through early next week. Winds will vary in direction across the lake with wind speeds of 10-20 kt or less.





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