Chicago Weather Forecast (O'Hare Official) 2:21 am CDT Aug 22, 2017 with Forecast Discussion

Overnight: A 30 percent chance of showers, mainly after 5am. Cloudy, with a low around 71. West southwest wind around 10 mph.

Tuesday: A 40 percent chance of showers, mainly before 9am. Cloudy through mid morning, then gradual clearing, with a high near 79. West southwest wind 10 to 15 mph becoming northwest in the afternoon. Winds could gust as high as 20 mph.

Tuesday Night: Clear, with a low around 58. Northwest wind around 10 mph, with gusts as high as 15 mph.

Wednesday: Sunny, with a high near 75. Northwest wind 5 to 10 mph, with gusts as high as 15 mph.

Wednesday Night: Partly cloudy, with a low around 57. North wind around 5 mph becoming calm in the evening.

Thursday: Partly sunny, with a high near 72.

Thursday Night: Mostly clear, with a low around 55.

Friday: Mostly sunny, with a high near 74.

Friday Night: Mostly clear, with a low around 56.

Saturday: Mostly sunny, with a high near 77.

Saturday Night: Mostly cloudy, with a low around 59.

Sunday: Partly sunny, with a high near 77.

Sunday Night: A slight chance of showers. Mostly cloudy, with a low around 60.

Monday: A slight chance of showers. Partly sunny, with a high near 78.

 Amf Ohare IL  41.98°N 87.9°W

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


253 AM CDT Tue Aug 22 2017


Through Tonight...

A change in air mass will gradually occur through today with the tropical air mass ousted for one more seasonal of late August.

Early morning satellite and radar imagery indicate a broad area of convection, with a couple embedded MCVs, from northern Indiana to western Illinois, back through the Kansas/Missouri border region. One MCV will track across the southern CWA edge through daybreak, keeping thunderstorms confined to that area. Some parts of far southern Ford/Iroquois/Benton Counties will likely finish by mid-morning with 2-3.50 inches of rain, but spread out over many hours which is limiting flash flooding concerns. To the north of this MCV, stratiform rain will occur for the early morning commute up to near the I-88 corridor.

Most of this activity, assisted by the long wave through progression and embedded cold pools, will shift southeast of the area by mid-morning. Isolated showers behind this may develop on the true cold front, which will clear the area by early afternoon. The morning clouds will be scoured out shortly after the frontal passage as well. Northwest winds will gust to around 20 mph behind the front, gradually advecting in drier air. Upstream temperatures yesterday peaked in the upper 70s and would expect similar for local highs today, with dependence on specific time of clearing.

The orientation and speed of the winds along the northwest Indiana shore will cause concerns for rip currents, particularly the Porter county shore. A Beach Hazards Statement for this goes into effect this afternoon through midday Wednesday.




Wednesday through Monday...

By Wednesday morning, a strong upper long wave trough for late summer will have taken shape over eastern Canada into the Great Lakes region. After a nice day Wednesday, the northwest flow aloft will drive a short wave impulse down across the western Great Lakes into early Thursday. This clipper feature has been indicated on several runs of global guidance, and will likely be enough to boost clouds Thursday morning with possibly some sprinkles or light showers. This wave also influences a secondary cold front southward and a cool enough air mass (850mb temperatures <8C) over the lake to support some lake effect clouds. The moisture depth is pretty marginal for any lake effect showers, but could see lakeside areas especially in northwest Indiana more cloudy on Thursday.

Highs look to be in the lower 70s Thursday leaning toward raw model guidance. With a 1025 mb high forecast to be centered over Wisconsin Thursday night after the cooler day, lows into Friday morning look cool. Some patchy mid 40s in outlying locations seem reasonable, with 50s in the metro area. For the start to high school football for many communities on Friday night, temperatures will likely still have that early autumn taste as the air mass only gradually modifies.

While some significant differences in global guidance exist heading into the latter half of the weekend into early next week, the chances for convection presently look limited, and only have small chances for showers mentioned at this time. With such a stout high over the region heading into the weekend, it may take a while to usher it out, and do not see any significant systems forecast from the west/northwest in the GFS/EC/GEM.



.AVIATION... For the 06Z TAFs...

Main forecast concern will be ts/pcpn trends, and associated cig/vis impacts, for the remainder of the overnight hours. The remainder of the day should be quiet.

Latest radar trends and short range guidance suggests now that the greatest ts potential is along the leading edge of an old cold pool/outflow from earlier thunderstorm complexes. The line of TS is now south and east of all terminals, from north-central Indiana through central Illinois. Latest obs invof the convective line indicate nwly outflow gusts, suggesting that the line will continue to sag south and east, and will be less of a threat to the terminals. So, have removed all mention of ts from the TAFs.

An area of more stratiform rain and showers associated with remnants of convection over srn Iowa and east-central IL is still spreading east under fast westerly flow aloft and the terminals could still see some rain or showers for the remainder of the overnight hours, but the potential for any additional ts is diminishing as the main forcing continues to push to the southeast. Cigs/vis could still be a concern with any additional rain in an already very moist environment. Cigs/vis reduction should generally be at mvfr levels.

There may be some lingering showers at daybreak, but conditions should quickly dry out with cigs/vis improving as high pressure building into the northern plains spreads east across the middle and upper Mississippi Valley. Winds will shift from sely early this morning to nwly by mid day with ocnl gusts to 18-20 kt. The nwly flow will usher in a cooler, drier air mass through the afternoon hours and expect no significant operation impacts for the remainder of the period as clear skies and light nwly winds set up by early Tuesday evening and continue Tuesday night.



Main marine forecast concerns continue to be on a period of northerly winds down the length of the lake Tuesday night into Wednesday. While not overly strong, the long fetch will likely result in small craft advisory conditions for the shores of southern Lake Michigan.

In the near term, a cold front has become nearly stationary near the north shore of the Lake in the Upper Peninsula. Low pressure is expected to develop along this front over Wisconsin tonight, moving northeast over far northern Lake Michigan early Tuesday morning and trailing a cold front which will push down the lake during the day. The low is expected to deepen later in the day as it pulls away into Quebec, setting the stage for breezy northwest winds across the lake. These north-northwest winds will continue into Wednesday before weakening as high pressure spreads into the region. The high eventually late Thursday, allowing winds to diminish and waves to subside. The strongest winds, 20-25 kts, are expected late Tuesday and Tuesday night, with small craft advisory conditions expected along the southern shores of the lake from Tuesday afternoon through Wednesday.



.LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...Beach Hazards Statement...INZ002...1 PM Tuesday to 4 PM      Wednesday.

LM...Small Craft Advisory...LMZ744-LMZ745...1 PM Tuesday to 4 PM      Wednesday.

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Hazardous Weather Outlook National Weather Service Chicago/Romeoville IL 346 PM CDT Mon Aug 21 2017

ILZ003>006-008-010>014-019>023-032-033-039-INZ001-002-010-011-019- 222100- 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- 346 PM CDT Mon Aug 21 2017 /446 PM EDT Mon Aug 21 2017/


.DAY ONE...Tonight.

WEATHER HAZARDS EXPECTED:   Significant Thunderstorm Risk...with an associated:       Limited Hail Risk...up to nickel to quarter size.       Limited Thunderstorm Wind Damage Risk...up to 50 to 60 MPH.   Limited Flooding Risk.


Scattered to numerous thunderstorms are expected across the outlook area this evening and overnight. With a very moist air mass in place, heavy downpours are likely with thunderstorms and localized flooding is possible. The strongest storms could also pose an isolated risk for gusty to damaging winds and hail.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

Tuesday...   Limited Thunderstorm Risk.   High Swim Risk for Porter County Indiana beaches. Sunday...   Limited Thunderstorm Risk.


Spotters are unlikely to be needed this evening and overnight.


Moving to the east at 20 mph through this evening, and moving to the east at 35 mph overnight.