Peru, IL 61354 Weather

Key Messages

- Dense fog may develop late tonight into early Friday morning across portions of northeast IL, especially the central and northern portions of the Chicago metropolitan area.

- Dry and unseasonably mild weather will continue through the upcoming weekend, though it will be slightly cooler near the lake this afternoon and again Friday afternoon.

Discussion

Issued at 150 AM CDT Thu Oct 22 2026

No significant change to our weather is expected through the weekend. One subtle difference today is that the surface high pressure over the Ohio Valley that was responsible for our very light southwesterly gradient Wednesday has moved east and has been replaced with a new high over the upper Great Lakes. This will result in a very light east or northeasterly gradient today, which should facilitate a slightly more formidable lake breeze that will spread farther inland this afternoon. This should result in temps falling a few degrees into the 60s this afternoon near the lake, however the more potentially meaningful impact from the lake breeze will be the advection of higher dewpoint marine layer inland into northeast Illinois.

Given the ideal radiational cooling set up tonight, the higher dewpoints could set the stage for areas of dense fog to develop over northeast Illinois. The area most susceptible appears to be the central and northern portions of the of Chicago metro area. Fog forecasting is always tricky, so really only medium confidence in dense fog tonight, but opted to introduce some areas of dense fog into the grids late tonight into early Friday morning.

Temperatures today will once again climb into the low to mid 70s, except in the 60s behind the lake breeze. Friday looks to be a near carbon copy of today both in terms of lake breeze and temps. 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 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 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 pattern, it isn't surprising that there is considerable spread in how that plays out and when/if it will rain here next week.

- Izzi

Aviation

Issued at 1154 PM CDT Wed Oct 21 2026

- Brief period of some shallow ground fog possible early this morning, primarily at RFD and GYY - There's around a 30% chance of dense fog adjacent to the lake into mainly northeast IL tonight.

Surface dewpoint depressions are generally running several degrees larger than last night at ORD, DPA, and MDW, so potential for VSBY restrictions at those terminals this morning is more more limited. Maintained TEMPOs for IFR/low end MVFR VSBY at GYY and RFD where dewpoint depressions are smaller and on pace with last night. If and where any VSBY restrictions occur early this morning, conditions would likely return to VFR by around 14z.

Calm to very light northeast winds will be the rule today, which should allow for a lake breeze to move inland. Behind this lake breeze, northeast winds of 5-9kt are likely at ORD, MDW, and GYY. An increase in moisture behind this lake breeze opens the door to a potential dense fog problem over primarily northeast IL tonight. Confidence is low, so held off on introducing anything into the TAFs now, but there is probably around 30% chance of LIFR/VLIFR conditions into ORD, MDW, and possibly GYY and DPA.

- Izzi

Lot Watches

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