textproduct: Chicago

KEY MESSAGES

- Locally dense fog may develop late tonight into early Friday morning across portions of far northeast Illinois.

- Dry and unseasonably mild weather will continue through the upcoming weekend, though it will be a bit cooler again near Lake Michigan Friday afternoon.

UPDATE

Issued at 759 PM CDT Thu Oct 22 2026

Only forecast adjustment this evening was to increase fog and cloud coverage across far northeast IL later tonight into early Friday morning, mainly across Lake, eastern McHenry and northern Cook counties.

GOES IR imagery and surface obs continue to depict low stratus and fog spreading slowly south-southwest along the western shore of Lake Michigan into eastern/southeastern WI. Guidance continues to indicate that stratus/fog will eventually develop into far northeast IL later tonight and persist into early morning, with dense fog possible centered especially on Lake county, IL and nearby portions of McHenry and Cook counties. Bumped to areas of fog in these areas, and also increased sky cover there as well as this may develop as a low (100-200 foot) stratus deck. Some question as to how far south fog/stratus will extend, as light low-level winds become more east-southeasterly with time.

Will continue to monitor these trends into the overnight hours, and can't rule out the potential need for a Dense Fog Advisory over some portion of the northern Chicago metro area late. Some patchy, shallow fog will also be possible west and south of the main area of concern mentioned above. Fog/stratus should burn off after sunrise Friday morning.

Ratzer

DISCUSSION

Issued at 123 PM CDT Thu Oct 22 2026

Once again, no significant change to our weather is expected through the weekend. The fog setup tonight is a lower confidence element. The inland extent of modestly enhanced low level moisture behind the lake breeze passage this afternoon may continue to be offset by deep mixing and recent dryness. In addition, forecast soundings, even on guidance that has been simulating fog development over northeast Illinois, indicate 10-15 kt south-southeast winds just above a very shallow moist layer late tonight. These above elements may serve to somewhat counteract more appreciable fog development despite an otherwise favorable setup with ideal radiational cooling conditions.

Our official forecast indicates patchy fog initially in far northeast Illinois earlier in the overnight. Then into the early morning hours, we're advertising the potential for shallow patchy ground fog down into the Chicago metro (away from downtown) and far northwest Indiana. Overall, confidence is on the lower side, also considering the lack of stronger support from statistical (MOS) guidance for areas of dense fog. A relatively higher chance for patchy dense fog anywhere in our CWA still likely is confined to primarily within Lake County IL (roughly in the 3 AM to 8 AM Friday timeframe).

Friday looks to be a near carbon copy of today both in terms of the lake breeze along the Illinois shore and temps. Southeasterly synoptic winds on the back side of the surface high pressure should keep the lake breeze confined to close to the immediate northwest Indiana shoreline.

This weekend, a strong trough is progged to track east across central Canada with a trailing cold front pushing east across the northern Plains toward the upper Mississippi Valley. This should result in more of a southerly then southwesterly gradient, albeit still not that strong, to develop this weekend. While not strong, it will probably be enough to hamper lake breeze development and result in some slightly warmer temps advecting into the area. Temperatures over the weekend could climb into the mid to perhaps locally upper 70s on Sunday given enough sunshine.

As the strong upper ridge and omega block start breaking down, it will probably allow that cold front to move gradually southward across the area Monday as it lays out east-west. There is still spread in guidance on timing of the frontal passage and given it will be running into the weakening upper ridge, it shouldn't have much push or a significant cold punch behind it. It should be enough to start cooling our temps down closer to, but possibly still slightly above, average by Tuesday and Wednesday. At some point early-middle next week, shortwave energy from the western trough could meander east into the Midwest bringing the threat for some rain to the area. Given the tendency for medium range guidance to struggle with breaking down blocking patterns, it isn't surprising that there is considerable spread in how that plays out and when/if it will rain here next week.

Castro/Izzi

AVIATION /06Z TAFS THROUGH 12Z SATURDAY/

Issued at 105 AM CDT Fri Oct 23 2026

The only aviation weather concern continues to center the dense fog threat in and near the Chicago terminals early this morning.

A surface high pressure ridge axis over the western Great Lakes will continue to slide eastward today. As it does, variable to nearly calm surface winds early this morning will become light (4-7kt) from an east-southeasterly direction.

The main question that remains is how far south the area of ongoing dense fog is able to develop into northeastern IL prior to daybreak this morning. Through Thursday evening, an area of dense fog has persisted across the western shore of Lake Michigan, particularly across eastern/southeastern WI, with the southward extent reaching KUGN in far northeast IL. Interestingly, satellite trends over the past couple hours suggest very little southward push to this point, possibly as a result of the light low-level flow beginning to turn to a less favorable east-southeasterly direction. Accordingly, there remains uncertainty with the extent of fog at the main Chicago terminals early this morning. Nevertheless, we did not make any major changes for the 06Z TAFs as it remains possible for some patchy areas of shallow ground fog to develop for a couple hours around daybreak.

Fortunately, any fog that does develop will burn off rather quickly after sunrise. Accordingly, VFR conditions are expected to quickly return and persist through the remainder of the forecast period.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

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