Chicago Weather Forecast (O'Hare Official) 11:18 pm CDT Aug 10, 2017 with Forecast Discussion

Tonight: A 20 percent chance of showers and thunderstorms before midnight. Mostly cloudy, with a low around 65. West southwest wind around 5 mph.

Friday: Partly sunny, with a high near 77. Northwest wind 5 to 10 mph, with gusts as high as 15 mph.

Friday Night: Partly cloudy, with a low around 60. North northwest wind around 5 mph.

Saturday: Mostly sunny, with a high near 77. North wind 5 to 10 mph.

Saturday Night: Mostly clear, with a low around 59. North northeast wind around 5 mph becoming calm in the evening.

Sunday: Mostly sunny, with a high near 77.

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 82.

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

Wednesday: Sunny, with a high near 82.

Wednesday Night: Mostly clear, with a low around 65.

Thursday: A chance of showers. Mostly sunny, with a high near 83.

Amf Ohare IL
 41.98°N 87.9°W

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


708 PM CDT Thu Aug 10 2017


653 PM CDT

Weak low level winds/convergence and diminishing instability due to extensive cirrus anvil shield (and approaching sunset) resulting a trend to only isolated to widely scattered thunderstorm coverage early this evening. While additional scattered storm development is possible with the cold front still off to the west of the cwa, it appears the coverage and intensity of storms will continue to diminish. Based on these trends have removed the northern tier or so of IL counties from severe thunderstorm watch 445.

West-central IL likely will maintain the greater potential for severe thunderstorms through mid-evening, with the strongest cells currently southeast of the Quad Cities. Some of these may move into portions of La Salle county after 7 pm this evening, though a general weakening trend is expected. Will continue to monitor activity east of the Quad Cities for a bit, though may be able to shave more counties from WW445 prior to the 9 pm watch expiration.




Through Friday...

Convective initiation is underway over southern Wisconsin into eastern Iowa and anticipate coverage and intensity of this activity to ramp up this afternoon and it moves into northern Illinois. Reference the mesoscale discussion section above for further details on convective forecast through this evening.

Showers and t-storms expected to end over southern CWA overnight with dry wx expected Friday. Guidance remains somewhat bullish with lower cloudiness Friday and would anticipate some "cold" air stratus/stratocumulus to rotate south into northern IL Friday morning. The high August sun angle will likely result in stratocumulus deck mixing up and probably scattering out during the afternoon. If cloudiness is more extensive Friday afternoon than currently expected, then our forecast high temps tomorrow may need to be inched down a couple degrees.

- Izzi



Friday night through Thursday...

An unseasonably cool/dry period is setting up for much of the longer term forecast period..

The longer range guidance is in decent agreement on the idea of prevailing high pressure at the surface through early next week. The upper level pattern will start out Friday night with broad upper troughing over the ern 2/3 of the CONUS with one wave tracking just north of the Great Lakes and another dropping into the northern plains.  The subtropical high will cover the south high plains through the Gulf of Mexico.  Through the weekend and into early next week, the northern stream shortwave is expected to drop slowly to the upper Mississippi Valley and wrn Great Lakes while long wave ridging builds over the nrn/cntrl plains and the subtropical high remains in place.  Any impact from weak shortwave energy should be suppressed south of the CWA by sfc high pressure centered over the Great Lakes, though the far srn portions of the CWA may see a slight chance for some showers.  Otherwise, the general trend for the extended forecast period will be for dry conditions with slightly below normal temperatures.  Highs through the weekend should only be in the upper 70s, expect for the lakefront, which should remain a bit cooler due to daily lake breeze development in a weak pressure gradient regime.  Temperatures should inch up a degree or 2 each day through midweek as the upper ridge builds across the cntrl plains to the western Great Lakes.  The next significant chance for pcpn may not be until late next week when the upper level pattern trends to more zonal and shortwave energy will be able to track across the northern plains to the Great Lakes region and sfc high pressure builds east and allowing a return flow of moister air back into the upper midwest.


.AVIATION... For the 00Z TAFs...

708 pm...An outflow boundary is moving south across the Chicago area terminals shifting winds north/northeast. Once this outflow boundary moves south...winds are expected to turn back southerly or southwesterly...though how long it takes for that to occur is somewhat uncertain. Winds will become more westerly overnight and then northwest Friday morning as a cold front moves across the area. Winds will continue to turn more to the north/northwest Friday afternoon with speeds 10-15kts. Its likely winds will shift to the north/northeast late Friday afternoon or early Friday evening at ord/mdw/gyy as a boundary moves in from the lake...but confidence from this distance is low.

Isolated to perhaps scattered thunderstorms are still possible through mid evening...south of a rpj/dpa/ord line but current activity southeast of the Quad Cities is expected to remain south of the terminals this evening.

Generally vfr conditions are expected through the early morning hours...outside any thunderstorms...but by morning some light fog may develop in addition to mvfr cigs. Confidence is only medium so only made timing changes to previous forecast but maintained mvfr cigs/trends. Cigs should lift to vfr Friday afternoon and likely scatter by mid/late afternoon. cms



Low pressure tracking across the upper Great Lakes this evening will drag a cold front across the lake. The front should push south and east of the lake by tomorrow morning, with southwest winds up to 20-25 kt shifting to nwly. As high pressure builds across the nrn plains and upper Mississippi Valley and the low lifts slowly enewd, winds will shift to nwly to 25 kt Friday night into Saturday. High pressure will overspread Lake Michigan Saturday evening and very slowly build ewd, setting up a period of relatively light and variable winds through the remainder of the weekend and into early next week. A very weak sfc trough will track through the upper Great Lakes Sunday night into Monday while another area of high pressure spreads across the lakes, keeping the generally light and variable winds over the lake through the middle of next week.



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Hazardous Weather Outlook...CORRECTED National Weather Service Chicago/Romeoville IL 356 PM CDT Thu Aug 10 2017

ILZ003>006-008-010>014-019>023-032-033-039-INZ001-002-010-011-019- 112100- 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- 356 PM CDT Thu Aug 10 2017 /456 PM EDT Thu Aug 10 2017/


.DAY ONE...Tonight.

WEATHER HAZARDS EXPECTED:   Elevated Thunderstorm Risk...with an associated:     Limited Tornado Risk. Limited Hail Risk...up to golf ball size.     Elevated Thunderstorm Wind Damage Risk...up to 60 MPH.


Scattered thunderstorms are expected along and ahead of a cold front through this evening. Some of these storms may become severe, with the greatest severe risk mainly northwest of Interstate 55 until 8 pm. Large hail and damaging winds are expected to be the primary severe hazards. The severe risk should diminish by the mid evening.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Saturday...   High Swim Risk.


Spotters will likely be needed into this evening.


East at 30 mph.