Chicago Weather Forecast (O'Hare Official) 4:16 pm CDT Aug 9, 2017 with Forecast Discussion

Tonight: Partly cloudy, with a low around 65. East southeast wind 5 to 10 mph becoming south after midnight.

Thursday: A 40 percent chance of showers and thunderstorms, mainly after 4pm. Partly sunny, with a high near 83. Southwest wind 5 to 10 mph, with gusts as high as 15 mph.

Thursday Night: Showers and thunderstorms likely, mainly between 7pm and 1am. Mostly cloudy, with a low around 65. South southwest wind 5 to 10 mph. Chance of precipitation is 60%. New rainfall amounts between a tenth and quarter of an inch, except higher amounts possible in thunderstorms.

Friday: Partly sunny, with a high near 78. West northwest wind 5 to 10 mph.

Friday Night: Partly cloudy, with a low around 61. North wind around 5 mph.

Saturday: Mostly sunny, with a high near 77.

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

Sunday: Mostly sunny, with a high near 78.

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

Monday: Mostly sunny, with a high near 79.

Monday Night: Mostly clear, with a low around 61.

Tuesday: Sunny, with a high near 81.

Tuesday Night: Mostly clear, with a low around 63.

Wednesday: Mostly sunny, with a high near 81.

Area Forecast Discussion below ad ...

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


339 PM CDT Wed Aug 9 2017


Through tonight...

The center of a broad area of high pressure has shift east, centered over the eastern Great lakes while low pressure moves through the eastern Dakotas with an associated cold front extending south through western Kansas. The air mass over nrn IL/nwrn IN is still largely influenced by the high pressure, with temps this afternoon topping out around 80 F. A lake breeze boundary formed earlier this afternoon and is steadily pushing inland. This caused lakefront temps to drop back to the middle 70s early this afternoon and expect that as the boundary pushes inland, temps will drop into the middle 70s as it moves inland through the Chicago Metro area. Under general high pressure and modest dewpoints, only in the upper 50s, conditions should remain dry through the remainder of the afternoon and overnight tonight. A majority of the cloud cover this afternoon is diurnally driven, so expect skies with become mostly clear after sunset.

However, the cold front over the nrn Plains is expected to steadily push east overnight. As the frontal trough progresses eastward, expect increasing layered cloudiness overnight with skies becoming mostly cloudy by daybreak. Also, as the high moves slowly to the east and the low over the Dakotas moves to nrn Wisconsin be tomorrow morning and the cold front reaches the middle Mississippi Valley, expect that low level moisture will slowly increase into tomorrow morning. Expect that any pcpn associated with the front will remain west of the I-39 corridor into early tomorrow morning, with any shra/tsra closely tied to the location of the front. With increasing cloud cover and low level moisture, overnight lows tonight will be a bit higher than recent days, bottoming out around 60 F, except for the urbanized areas of the Chicago metro area, which will only drop to the middle to upper 60s.



Thursday through Wednesday...

The main concern is with thunderstorm potential on Thursday afternoon and evening and associated risk for strong to severe thunderstorms. On Thursday morning, a weakening area or cluster of showers and thunderstorms may try to sneak into portions of north central and far northern Illinois. However, this activity will be outrunning the instability axis and may completely dissipate before arriving. Thus, capped PoPs in slight chance range.

For Thursday afternoon and evening, a cold front trailing from a surface low over northern Wisconsin will be approaching. This low will be associated with a fairly stout mid-upper short-wave trough and upper level jet streak providing forcing for shower and thunderstorm development focused in the frontal trough. Considering the dry air mass that will be in place through tonight and only low-mid 60s dew points in source region to our south and southwest for pre-frontal air mass, it appears that NAM/GFS dew points approaching or just above 70 right ahead of front will be overdone. With only modest mid-level lapse rates progged (6-6.5 C/KM), expectation of lower dew points and some convective debris cloudiness will likely be a limiting factor for instability, possibly topping out at 1000-1500 j/kg as temperatures warm into the lower 80s.

Furthermore, low level convergence will be limited somewhat by expected veering of surface winds to southwesterly, which could prevent higher convective coverage. With this being said, mid and upper forcing and limited frontal convergence should be enough for at least scattered showers and storms developing to our west and north and then spreading southeast. Presence of strengthening wind fields due to short-wave trough will result in favorable west- northwesterly 0-6 km shear of 40-50 kt. Aforementioned limiting factors are expected to prevent a higher severe risk, however the very favorable deep shear could still support isolated to widely scattered strong to severe storms, with supercellular mode possible. Main threats would likely be hail and damaging winds, as lacking low level moisture/higher LCL and comparitively modest low level shear with veered boundary layer winds and weaker 850 mb flow will likely limit tornado threat.

The loss of instability diurnally as the front progresses farther across the area overnight Thursday night is then expected to erode the coverage of showers and storms. Fairly slow progress of the front clearing southern areas on Friday could provide an opportunity for showers and isolated storms mainly southeast of I-55, with instability again a limiting factor. Can`t rule out a very isolated shower/storm in far northern Illinois during the afternoon closer to the trough axis and a secondary cold front. Friday`s highs look to top out in the mid 70s to around 80, warmest south of I-80.

Beyond Friday, a return to primarily quiet weather appears likely through mid next week with surface high pressure over the region. The minor exception could be late Saturday night into Sunday as a short-wave may spread low shower potential into areas mainly well south of I-80. Temperatures look to average a bit below normal for mid August, topping out in the 70s to around 80s, with daily onshore flow keeping lakeside areas slightly cooler. Coupled with dry dew points in the 50s to low 60s, this stretch will be quite pleasant versus "dog days of August" weather that commonly occurs in our area.



.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.



On Thursday, weak low pressure will move across northern Wisconsin, with southerly winds increasing to up to 15-25 kt ahead of it, highest on the north half of the lake. The cold front trailing from the low will move across the lake Thursday night, with increasing northwest winds Friday, shifting northerly Friday evening. There is uncertainty as to how quick stronger northerly winds will spread south down the lake on Friday evening/night, which will impact wave heights into Saturday on the southern end of the lake, including the nearshore waters. Stronger northerly winds than currently forecast over the south half Friday night into Saturday would result in larger waves and the potential for conditions hazardous to small craft in the nearshore. Lighter winds can then be expected on Sunday as high pressure moves over the lake.




+ + + + + +

Important links below ad ...
NWS Forecast Chicago (O'Hare)
(NEXRAD Radar Chicago)
(Chicagoland Weather Radar)
(Satellite Cloud Cover East United States)
(US Weather Forecast Chart)
Ultraviolet Index/UV Index
Chicagoland Sunrise, Sunset and Solar Information for Chicago area
Chicagoland Moonrise, Moonset and Lunar Phase information for Chicago area

Full climate/weather details
(Desktop Mac/PC links)

Chicago record high temperatures

Chicago record low temperatures

Chicago extreme records

Chicago normal temperatures

Chicago Weather Radar

Chicago Weather Forecast

See also ...
Arlington Heights Weather Forecast
Chicago allergy weather
Chicago arthritis weather
Chicago respiratory weather

Much of the information in, Chicago Weather Stations and Chicago Weather Station consists of material from the National Weather Service, which is in the public domain (See ABOUT section).