Berwyn, IL Weather

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.

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.

Aviation

Issued at 1237 PM CDT Thu Oct 22 2026

Key aviation forecast messages:

- Potential for VSBY reductions in shallow fog early Friday AM.

Primary concern through the TAF period is the potential for fog development near Lake Michigan again late tonight into early Friday morning. Model guidance has trended toward slightly lower coverage of fog, particularly any dense fog, which may end up focusing primarily into far northeast Illinois (Lake County) and into portions of southeast Wisconsin. Have converted inherited prevailing VSBY reductions (mainly MVFR) to TEMPO groups during the 9-13Z timeframe given decreasing confidence. Shallow fog (MIFG) still appears likely over low-lying and grassy areas which could drift over some terminal sites at times early Friday AM outside of Chicago.

Light and variable to near calm winds early this afternoon will turn NE behind a weak lake breeze (under 10 kt), which become light and variable again overnight. Winds then settle into a prevailing SSE to SE direction Friday morning and turn E to ESE behind a lake breeze in the afternoon near the lake.

Petr

Lot Watches

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