textproduct: Chicago

KEY MESSAGES

- A threat for isolated to widely scattered thunderstorms through this evening.

- Mainly dry and increasingly warm weather this weekend onward, though it will be cooler lakeside until later next week.

DISCUSSION

Issued at 237 PM CDT Thu Jul 9 2026

This Afternoon:

A weak remnant MCV (mesoscale convective vortex) has slowly pushed across the area through early this afternoon. It appears that the timing of the disturbance was early enough to curtail stronger destabilization from a combination of thicker cloud cover and already marginal mid-level lapse rates of <6C/km. Forcing on the backside of the MCV, subtle confluence, and a bit better destabilization south of I-80 recently enabled an uptick in coverage of smaller footprint pulse type convection. As the forcing from the MCV pushes farther east, a somewhat subsident regime may result in the current activity struggling for a bit. Farther north, the aforementioned more subsident regime as well as meager low-level convergence from close to due westerly boundary winds will probably keep convective coverage at a minimum for a few hours.

Late Today through Tonight:

Based on upstream radar presentation as of this writing, a weak approaching cold front should provide enough lift for a modest uptick in showers and isolated to widely scattered thunderstorms. Winds shifting onshore near the lake from an early evening lake breeze could plausibly provide modestly a modestly enhanced focus. However, the weakly forced, weak deep layer shear, and modest mid-level lapse rate environment does not appear that it will be conducive to much in the way of storm organization. Earlier concerns of localized flash flooding appear to be trending downward even if a few storms materialize into this evening, as they will likely continue to be smaller footprint and pulse-type (shorter-lived). Favorable moisture parameters and high freezing levels would thus be offset by limited areal coverage and duration of any heavy rainfall. If a more focused corridor of showers and embedded storms does not materialize by this evening, with a likely diurnal component to the activity, the flood threat for the rest of the night should be limited.

Overnight into early Friday as the cold front sags south, light winds and low dew point spreads may support fog development and localized visibility below 1 mile. Maintained areas of fog mention closer to the lake in northeast Illinois and far northwest Indiana, though confidence isn't all that high.

Friday Onward:

The sagging front will force the instability axis southward Friday and Saturday, and with it the threat for any showers and storms. Have confined chance PoPs to near and south of the Kankakee River Friday afternoon and evening and then primarily south of US-24 on Saturday afternoon (which may be too generous). For the rest of the area, a couple pleasant days with more comfortable humidity levels, and seasonably warm temperatures in the lower to mid 80s are in store. Synoptic onshore flow from surface high pressure settling over the Great Lakes will keep highs near the lake in the mid 70s to around 80F.

For Sunday into early to mid next workweek, an expansive and anomalously strong/warm mid-level ridge will establish itself from the central and northern Plains to the Midwest. Increasingly warm 850 to 925 mb temps underneath the influence of the ridge will result in a warming trend away from the lake. Meanwhile, the surface high pressure will remain in place until midweek or thereabouts, resulting in daily lake breezes and associated lake cooling. In addition, dew points and thus humidity levels will be held in check vs. our previous very warm to hot stretch. The ridge may become sufficiently flattened sometime later next week for a return of convective chances. Until then, a several day stretch of continued drying will help river and streams to return back to more typical flows.

Castro

AVIATION /00Z TAFS THROUGH 06Z SATURDAY/

Issued at 644 PM CDT Thu Jul 9 2026

Forecast concerns include...

Chance of thunderstorms through late this evening. Wind shift to northeast this evening. Low mvfr, possible ifr, cigs/vis overnight/early Friday morning.

A cold front will move across northern IL this evening, shifting winds to the northeast, with west/southwest winds ahead of the front. Once winds shift to the northeast, they are expected to remain northeast for the rest of the period, possibly increasing into the 10-15kt range Friday afternoon.

Showers and scattered thunderstorms have developed along this front and are expected to slowly increase in coverage as they move southeast across the terminals this evening, though overall coverage remains uncertain. Have maintained prob thunder mention and adjusted timing later based on current guidance trends but additional refinement may be needed this evening. Showers may lingering into the early overnight hours. Scattered showers and a few thunderstorms will be possible Friday afternoon, but should remain south of the terminals.

With the frontal boundary nearby and moist low levels, fog will be possible overnight into early Friday morning, especially west of the Chicago terminals across northwest IL. Low mvfr cigs and possibly ifr cigs will also be possible. Overall confidence is only medium for fog/low cloud trends and only tweaked timing, though added ifr cigs for RFD. Trends will need to be monitored overnight. Whatever fog does form should dissipate quickly after sunrise and cigs will slowly lift through mvfr and into low vfr by early Friday afternoon. cms

LOT WATCHES/WARNINGS/ADVISORIES

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