Chicago Weather Forecast (O'Hare Official) 9:17 am CDT Aug 20, 2017 with Forecast Discussion

Today: Mostly sunny, with a high near 88. South southwest wind 5 to 10 mph, with gusts as high as 15 mph.

Tonight: A 20 percent chance of showers after 4am. Partly cloudy, with a low around 70. South wind around 5 mph.

Monday: A 30 percent chance of showers and thunderstorms, mainly after 7am. Partly sunny, with a high near 88. South southwest wind 5 to 10 mph.

Monday Night: Showers and thunderstorms likely, mainly after 1am. Some of the storms could produce heavy rain. Mostly cloudy, with a low around 71. South southwest wind 5 to 10 mph, with gusts as high as 15 mph. Chance of precipitation is 70%.

Tuesday: Showers likely and possibly a thunderstorm before 1pm, then a chance of showers. Mostly cloudy, with a high near 79. Southwest wind 10 to 15 mph becoming northwest in the afternoon. Winds could gust as high as 25 mph. Chance of precipitation is 60%.

Tuesday Night: Mostly clear, with a low around 59.

Wednesday: Sunny, with a high near 76.

Wednesday Night: Mostly clear, with a low around 57.

Thursday: A slight chance of showers before 1pm. Mostly sunny, with a high near 74.

Thursday Night: Mostly clear, with a low around 57.

Friday: Sunny, with a high near 76.

Friday Night: Mostly clear, with a low around 58.

Saturday: Sunny, with a high near 76.

 Amf Ohare IL  41.98°N 87.9°W

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


640 AM CDT Sun Aug 20 2017


Through tonight...

Quiet weather is on tap for the short term forecast period, with the primary concern being timing of lake breeze development and the impact on temperatures along the lakefront.

The center of a large area of sfc high pressure is slowly sliding to the east though weak ridging will still extend west across the region.  Also, low pressure moving across central Canada is dragging a cold front across the upper Mississippi while another low is developing over the central plains.  The high will limit the eastward progression of the frontal system, with the northern portion of the front pushing east across the upper Great Lakes, and then extending southwest across northern Wisconsin and then into the Upper Missouri Valley.  More broad scale warm advection will set u Pacross the midwest, with 925mb temps of 24-25C, supporting temps approaching 90 F.  A couple factors may limit warming a little today, cirrus blow-off from the MCS over Iowa and nrn MO, and a mid level shortwave crossing nrn/cntrl IL this afternoon may generate some cu or a few showers.  However, have maintained a dry forecast since the warm advection aloft should keep the area capped and there is no sfc forcing to help break the cap.  So, have gone with temps in the upper 80s over much of the CWA, through an isolated 90 F is not out of the question if the cirrus is thin enough.  The exception will be the Illinois lakefront.  A lake breeze should develop, but the orientation of the boundary should be oriented more northwest to southeast, reflective of the southerly synoptic flow and with wind speeds arnd 10 mph this afternoon, the boundary should not penetrate inland very far.  Locations north of downtown Chicago to the Wisconsin border should see highs only in the middle 70s.  The cold front should see some sewd progression overnight tonight as the sfc high center moves to the mid Atlantic coast.  Cloud cover and pcpn chances will be in the increase overnight as the front progresses to the southeast.



Monday through Saturday...

Multiple concerns are shaping up for the longer term forecast period, most of which will be confined to a very unsettled period from Monday through Tuesday afternoon.  Upper level flow is trending to more zonal aloft with fast westerly mid and upper level flow setting up across the region.  As this happens, the sfc cold front will begin to lay out into a more west-east orientation.  A series of shortwaves rippling through the fast wly flow aloft will hel Pkeep tsra/shra chances going through the period.  While the daytime hours on Monday look to be somewhat unsettled with some chances for shra/tsra, most guidance suggests that activity should be scattered, and periods of broken sky cover may be limited.  Sky cover will likely not be very conducive to direct observation of the much anticipated eclipse.  With little progression to the larger scale pattern, temperatures on Monday should be very similar to Sunday, with highs in the upper 80s likely for much of the area.  Increasing cloud cover and slightly higher pcpn chances could keep ncntrl IL a little cooler.  Some isolated 90 F reading may be possible again in areas that experience some breaks in the cloud cover in the afternoon.

Forecast concerns will shift to more significant weather Monday night into Tuesday morning. There should be significant moisture pooling along the front, with pwats reaching or slightly exceeding 2 inches.  With flow aloft still zonal into Monday night, the swd progression of the front should be relatively slow.  The combination of high levels of deep layer moisture, slow progression of the front and upper level flow generally parallel to the front all point to the potential for widespread heavy rain across the area.  Latest guidance suggest that the highest rainfall amounts should be over portions of the CWA north of the I-80 corridor where areal average QPF in excess of 1 inch is likely and locally higher amounts possible.  There may be flooding potential, especially for ncntrl/nwrn IL where the heaviest rainfall is expected.  There will also be a severe thunderstorm threat Monday night.  The latest guidance remains consistent on indicating that the environment will be especially moist and unstable, with steep mid level lapse rates, CAPE values arnd 2000 J/kg and mid-level flow of 30-40 kt.  So, thunderstorms will not only have the potential to produce very heavy rainfall, strong, damaging winds may be possible as well.  Given these factors, SPC has highlighted nrn IL in a Slight Risk Severe Wx area and WPC has highlighted nwrn IL with a Slight Risk for Excessive Rainfall.  The remainder of the CWA are highlighted with Marginal Risk for Severe and Excessive Rainfall.

By Tuesday morning, high pressure building out of Canada and spreading across the upper Mississippi Valley along with increasing amplitude to the upper level pattern will help accelerate the front, pushing it more quickly south and east of the CWA by mid-day Tuesday, bringing an end to both pcpn and the warm/humid conditions. Much quieter weather is expected for Tuesday afternoon through the end of the period as the upper level pattern transitions to high amplitude, slow progression with a deep upper low setting up over sern Canada and upper ridging building over the Rockies.  Northwest flow aloft and sfc high pressure will cover the area through Friday, with the longer range guidance suggesting that the pattern will break down and become more progressive by Friday or Saturday.  Until then, dry weather and below normal temperatures will prevail, with highs in the upper 60s to low 70s along the lakefront and middle to upper 70s inland.  The next chance for any pcpn will likely not be until next weekend.


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

For the Chicago TAF sites today and tonight, concerns are south- southwest winds approaching 10 kt during the afternoon, a chance for isolated showers late this afternoon and evening, and then a chance of storms late tonight into Monday.

Light south-southeast winds will become south-southwest by mid to late morning. Speeds will increase with 9-10 kt expected at ORD and MDW by 19Z-20Z. A few gusts in the mid teens are possible mid- late afternoon. Winds will drop in speed near sunset and back slightly to due south or a little east of south. Confidence in winds for this TAF is medium-high.

The convective complex across central Iowa early this morning will continue to move east-southeast with a gradual weakening likely to continue. The upper level disturbance supporting the activity will continue to move east and will drift over the area this afternoon/early evening. Isolated showers are possible with this, and cannot 100% rule out a storm or two. This window of opportunity for showers will be near RFD from 18Z-21Z and ORD/MDW from 21Z-02Z. Confidence on the showers occurring is low.

For overnight into early Monday morning, the next upper disturbance will likely trigger storms across Iowa and possibly further north/northeast. This convection should slowly progress southeast and may impact RFD as early as overnight and its possible the Chicago TAF sites see this during Monday morning. However, confidence on evolution of storms tonight into Monday morning is very low. It is possible the most robust Monday morning convective activity passes to the southwest of local area airports.




As high pressure moves east today, winds will turn southerly across the lake. The exception will be in the Illinois nearshore, where southeast to possibly due east onshore winds will be seen in the afternoon. The highest speeds over the lake today will be across the northern part, with the far north seeing gusts of 20-25 kt late today into this evening.

Low pressure will take shape across northern Lake Michigan by daybreak Tuesday, and accelerate and deepen as it tracks eastward during the day Tuesday. Given the pressure gradient in its wake, northwest winds will increase with 20-25 kt gusts likely. The northwest direction on Tuesday would favor Small Craft Advisory winds and waves across the Indiana nearshore and possibly the Illinois one due to winds.




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Hazardous Weather Outlook National Weather Service Chicago/Romeoville IL 502 AM CDT Sun Aug 20 2017

ILZ003>006-008-010>014-019>023-032-033-039-INZ001-002-010-011-019- 211015- 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- 502 AM CDT Sun Aug 20 2017 /602 AM EDT Sun Aug 20 2017/


.DAY ONE...Today and Tonight.

WEATHER HAZARDS EXPECTED:   Limited Thunderstorm Risk.


There will be a chance of some scattered thunderstorms along and ahead of a cold front late tonight, mainly over north-central Illinois.

.DAYS TWO THROUGH SEVEN...Monday through Saturday.

Monday...   Elevated Flooding Risk.   Elevated Severe Thunderstorm Risk.

Tuesday...   Limited Thunderstorm Risk.   High Swim Risk.

Wednesday...   High Swim Risk.


Deep layer moisture pooling along a cold front moving slowly through northern Illinois Monday night will generate strong thunderstorms. These storms will be capable of producing very heavy rainfall and strong, damaging winds. The heavy rainfall may lead to localized flooding.


Spotter activation will not be necessary through tonight.


Moving southeast at 10 MPH.