textproduct: Chicago

KEY MESSAGES

- Showers and thunderstorms chances return Friday afternoon and evening, some may be severe, especially west of the Fox Valley in Illinois.

- Dangerous swim conditions expected at Porter County Lake Michigan beaches Saturday, then all Illinois and Indiana Lake Michigan beaches late Saturday night and Sunday.

DISCUSSION

Issued at 211 PM CDT Thu Aug 20 2026

Some much deserved sunshine and gorgeous summer weather today with comfortable humidity and mild temperatures. High pressure that's responsible for today's weather will move east into New England tonight and make way for the next system that will bring the a threat of showers and storms to the area Friday afternoon and evening.

Fairly potent looking shortwave trough and closed mid-upper level low is evident on water vapor imagery this afternoon straddling the international border with Minnesota. This trough is progged to continue moving rather sluggishly southeastward into northern WI by Friday morning. The associated surface low will move into the upper Great Lakes by Friday afternoon with a cold front trailing southwest from the low into northwest IL by early Friday evening.

Low level moisture in advance of the this front will be somewhat limited, consisting largely a recycled moist continental air mass and lacking a direct Gulf connection. Despite the absence of a feed of moisture from the Gulf, evapotranspiration and pooling of moisture ahead of the front may allow for a reservoir near 70 degree dewpoints ahead of the front Friday afternoon. Heating of that boundary layer air mass juxtaposed beneath some modestly cooling mid level temps is likely to result in an axis of 2000+ J/kg of MLCAPE along and ahead of the cold front.

A northwesterly mid-upper level jet extending from the northern Plains into the mid-Mississippi Valley will result in a corridor of seasonably strong deep layer shear. The strongest shear looks to remain southwest of, and mostly divorced from the stronger forcing over the Great Lakes. There is spread in guidance regarding strength of the shear across our CWA, with some models bringing the more favorably long hodographs into our western CWA Friday afternoon, while other models keep favorable shear farther west and southwest.

If the the stronger shear does materialize into our area, then a corridor of conditions quite favorable for northwest flow supercells would likely materialize over eastern Iowa into western portions of northern IL. Weak low level flow should greatly limit the tornado threat, but large hail and damaging winds would be in play with this more strongly sheared set-up. If more favorable shear remains farther southwest, then the severe threat would be more marginal, limited to mainly a potential for localized microbursts.

Instability is expected to wane after sunset and that should result in a weakening trend in the convection as it spreads east across our CWA Friday evening. How quickly that diminishing trend takes place and how far south and east a severe threat may materialize is something that we hope to have a better handle on by tomorrow morning. Showers and storms should end from the northwest Friday night with northwest winds ushering in a less humid and slightly cooler air mass for Saturday.

The upcoming weekend largely looks dry, though a secondary push of cooler air Saturday night could result in just enough lake induced instability to support a slight chance of some lake effect showers into Porter County late Saturday night into early Sunday morning.

The only other concern weather wise this weekend will be with the likelihood of hazardous swim conditions developing. Behind the initial cold front, northwest winds are likely to increase Saturday morning and could lead to hazardous swim conditions for mainly Porter County beaches and points east Saturday. Saturday afternoon, land-lake breeze circulation should result in a diurnal relaxing of the winds over southern Lake Michigan in the afternoon, which will allow waves to ease temporarily late Saturday afternoon into Saturday evening. A secondary cold front will down the lake Saturday night, with that push of cooler air steepening up the marine layer lapse rates and turning winds more briskly from the north down the full fetch of the lake. This will likely result in hazardous swim conditions at all IL and IN Lake Michigan beaches later Saturday night through Sunday.

Otherwise, look for dry and pleasant weather to continue into the start of next week.

- Izzi

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/

Issued at 1222 PM CDT Thu Aug 20 2026

Key messages for the 18Z TAF period:

* Chance for scattered showers and thunderstorms near the end of the 30-hr TAF period. Chances increase just beyond into Friday evening

NE winds up to 10 to 12 kt over Chicago (lighter inland) will subside early this evening before flopping over to SW late in the evening. Expect SW winds tomorrow increasing to around 10 kt for the afternoon.

A slight chance for a brief light shower exists during the early morning but the low probability kept a formal mention out of TAFs. A system of showers and thunderstorms will move in from the northwest late in the day on Friday. Precip may move into Chicago as early as the mid-late afternoon, but chances will build into the evening hours. Periods of MVFR vsbys will be possible beneath the rain, but especially during the evening.

Doom

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...None.