Chicago Weather Forecast (O'Hare Official) 1:18 pm CDT Aug 14, 2017 with Forecast Discussion

This Afternoon: Partly sunny, with a high near 83. South southwest wind around 10 mph.

Tonight: A 40 percent chance of showers and thunderstorms, mainly between 10pm and 4am. Mostly cloudy, with a low around 65. South southwest wind around 5 mph.

Tuesday: A slight chance of showers before 7am, then a slight chance of showers and thunderstorms after 1pm. Partly sunny, with a high near 82. West northwest wind 5 to 10 mph becoming north northeast in the afternoon. Chance of precipitation is 20%.

Tuesday Night: A 30 percent chance of showers and thunderstorms after 1am. Mostly cloudy, with a low around 67. Northeast wind 5 to 10 mph.

Wednesday: A 40 percent chance of showers and thunderstorms. Mostly cloudy, with a high near 84. South southeast wind 5 to 10 mph.

Wednesday Night: Showers and thunderstorms likely. Mostly cloudy, with a low around 71. Chance of precipitation is 60%.

Thursday: A 30 percent chance of showers and thunderstorms, mainly before 1pm. Mostly cloudy, with a high near 83.

Thursday Night: Partly cloudy, with a low around 66.

Friday: Mostly sunny, with a high near 81.

Friday Night: Partly cloudy, with a low around 65.

Saturday: A slight chance of showers and thunderstorms. Mostly sunny, with a high near 82.

Saturday Night: A slight chance of showers and thunderstorms. Partly cloudy, with a low around 64.

Sunday: Mostly sunny, with a high near 82.

Amf Ohare IL  41.98°N 87.9°W

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


1254 PM CDT Mon Aug 14 2017


Through tonight...

The main concerns during the period will be the potential for a period of showers and thunderstorms over northern Illinois this evening and into the overnight hours.

Currently, a line of showers and thunderstorms are shifting eastward across north central Iowa. This activity is expected to weaken over the next few hours as it moves into a progressively more stable airmass. So it is unlikely that this activity would poss much of risk for thunderstorms over north central IL this morning. However, a stray remnant light shower cannot be totally ruled early this morning. Otherwise, it appears most, if not all of the day will be dry across the region.

Winds are expected to become south-southwesterly during the day as a weak surface frontal trough of low pressure sets up over Iowa and into southeastern Wisconsin this afternoon. This will allow a warmer and more moist airmass to begin shifting back over the area. As a result, expect temperatures today to warm a few degrees over Sundays highs, which should put most areas into the low 80s this afternoon. Slightly cooler conditions can be expected near Lake Michigan in northeastern IL, however, as a lake breeze develops this afternoon. It appears unlikely that this lake breeze will move inland as far as yesterdays lake breeze, due to the southwesterly winds, so any cooling would be confined to the lakeshore areas.

New scattered thunderstorms are likely to develop in the vicinity of the surface frontal trough to our west later this afternoon. This activity should be aided by the approach of a decent mid/upper disturbance currently over the Upper Midwest. These thunderstorms then look to shift eastward across far northern IL (north of I80) through the evening as the surface frontal trough begins to approach the area. There is some uncertainty on the exact timing of these storms, but it does appear that far northern Illinois will have the best potential this evening. Overall this does not look like a big severe weather producer, but given the amount of downdraft CAPE, some of these storms could produce some gusty winds.

The surface frontal trough is expected to gradually sag southward into northern Illinois later tonight. Some showers and storms may continue to fester along this boundary overnight, but overall weakening instability and forcing for ascent should result in much less storm coverage in Illinois overnight.

Some fog may also try to develop over portions of north central Illinois late tonight as the surface flow weakens in the vicinity of the surface frontal trough. This combined with ambient low- level moisture could result in some fog into early Tuesday morning.




Tuesday through Sunday...

The main forecast concern is shower and thunderstorm potential, particularly Wednesday through Wednesday night. The short-wave tonight will have a weak surface low reflection moving across the central and eastern Great Lakes, which will drag a weak cold front southward on Tuesday. Mid and upper forcing will be weak in the presence of height rises. The progress of the frontal boundary likely slow south of I-80, with some moisture pooling, weak convergence and perhaps a weak wave possibly enough to kick off isolated to widely scattered showers and storms. It appears the greater chance for this to occur would be south of I-80 closer to the frontal boundary, so PoPs will likely need refinement/tightening up should these trends continue in the guidance today. Severe weather is not expected given light flow aloft resulting in weak deep layer shear. Progged thermal profiles support highs in the lower-mid 80s, except upper 70s in far northeast Illinois and along the lake shore.

A trough over the high Plains will cause surface cyclogenesis by Tuesday evening over eastern CO/western KS vicinity. Warm advection over the stalled frontal zone could yield showers and elevated thunderstorms overnight into early Wednesday, but it`s possible more appreciable activity holds off until during Wednesday morning (perhaps in the form of a decaying MCS). There is plenty of uncertainty in the evolution, track, timing and strength of the mid week system in the latest guidance, along with mesoscale convective details, thus confidence is low in trends with this forecast.

A warmer and more humid air mass will be drawn northward as the previously stalled frontal boundary lifts northward as a warm front. Stronger forcing from the trough will result in a higher likelihood of showers and storms by late Wednesday into the evening, but probable to be well ahead of the cold front. Magnitude and any risk for strong to severe storms will be driven by how much instability can be realized, pending cloud cover concerns, as well as diurnal timing possibly being somewhat unfavorable.

While the exact track of the surface low is highly uncertain, cold frontal passage does appear it will occur during Thursday, with the higher shower and thunderstorm chances possibly focused along/southeast of I-55. Strong wind fields for mid August and resulting deep layer shear could also yield a risk for strong to severe storms with the cold front if sufficient instability can be realized, particularly if fro-pa can be slowed a bit. Friday should be mainly dry and slightly cooler behind the front, with low shower and thunder chances indicated for now over the weekend, though there is plenty of uncertainty/low confidence based off large differences in the guidance.



.AVIATION... For the 18Z TAFs...

South-southwest winds continue this afternoon, and the winds should be strong enough to keep the lake breeze east of ORD and MDW. Scattered thunderstorms are still forecast to form this evening near RFD and then shift east through the mid to late evening. I have medium-high confidence in showers and storms forming near RFD, but I have medium-low confidence in timing and coverage. As such, I kept TSRA as is in the TAFs.

Light winds tonight will probably result in fog mainly at RFD. A cold front sinks south early Tuesday morning and winds become north to northeast. Low cigs are also expected with the front. IFR cigs are most likely at RFD, while other sites will likely see MVFR cigs. Clouds scatter out in the mid to late morning, and thunderstorm activity Tuesday should be well south of the terminals.




Fairly light and generally southerly winds can be expected over the lake today. A weak cold front will drop southward tonight, turning winds northeast and then north, earliest on the northern portion of the lake. Weak low pressure moving into the central lakes will usher the front south across the remainder of the lake Tuesday morning. The northerly winds on Tuesday will likely to Pout at up to 20 kt.

A stronger system will likely take shape by mid week, with strengthening south and southeast winds ahead of it, particularly on Wednesday night/early Thursday. There is plenty of uncertainty with the track and strength of the surface low into Friday, which will impact wind directions and speeds Thursday. In the wake of the system Thursday night into Friday morning, brisk northwest /north-northwest flow could certainly be stronger than the 15-25 kt currently indicated in open lakes forecast with a favorable surface low track. Speeds should diminish fairly quickly later Friday as high pressure spreads in from the west. Periods of thunderstorms are also likely with this system.




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