Chicago Weather Forecast (O'Hare Official) 1:16 pm CDT Aug 9, 2017 with Forecast Discussion

This Afternoon: 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. Southwest wind 5 to 10 mph, with gusts as high as 15 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.

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


116 PM 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 18Z TAFs...

Surface high pressure remains the dominant feature impacting the weather across the region today. he center of the high has shifted east, over the eastern Great Lakes. A trough of low pressure is deepening over the northern plains, with an associated cold front extending south to western Kansas. A weak sswly gradient has set up across the region at the sfc. However, with a weak pressure gradient in place and sfc winds generally light, a lake breeze boundary has developed. latest radar imagery indicates that the lake breeze is steadily pushing east, and have timed the boundary to push through ORD/MDW btwn 22-23z. Winds behind the boundary should shift to enely with speeds arnd 10-12kt for a short period as the lake breeze pushes through ORD/MDW. Wind speeds should only exceed 10 kt for a short time, likely arnd 30 min before settling back to 6-8 kt through 01z. Overnight, expect that winds should becm sly/vrbl at less than 5 kt.

For Thursday, concern will shift to increasing chances for pcpn through the day. The main focus for shra/tsra development will be the passage of a cold front. Latest guidance suggests that should reach the middle Mississippi Valley by mid day and that any convective development through the morning hours will likely be along the boundary. Into the afternoon, thunderstorm activity is expected to increase in coverage. For the 24-30 hr forecast period for ORD, have introduced a prob30 for ts after 21z. Some shra/tsra may be possible at RFD before 18z tomorrow, but confidence in timing is too low to include a pcpn mention in the RFD forecast at this time as the front will be moving into far nwrn IL at a diurnally unfavorable time.



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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