McHenry, IL Weather

Key messages

- Some fog is possible tonight, especially south of I-80, with some threat for locally dense fog.

- Above to much above average temperatures expected to continue through the upcoming weekend.

- While not a wash out, there will be some periodic chances for showers and perhaps isolated thunderstorms at times late Wednesday night through the upcoming weekend.

Discussion

Issued at 112 PM CDT Mon Oct 26 2026

Weak and somewhat ill-defined frontal boundary has slowed and becoming nearly stationary across central Illinois early this afternoon. East to northeast winds in the wake of the front have ushered in somewhat cooler air into much of the CWA, though still above the average highs which are in the upper 50s. Surface analysis early this afternoon shows an axis of low-mid 50 degree dewpoints pooled near and just north of the front. Satellite imagery shows a large area of clearing upstream, which should overspread the local area this evening. The clearing skies, pool of high dewpoints, and lighter winds near the weak boundary could allow for fog to develop tonight. Interestingly, guidance isn't very bullish on the fog threat, however guidance is generally too low with dewpoints and too aggressive with high cloudiness, so that could explain the relative lack of fog in guidance. For now, introduced a large area of patchy fog across much of the western and southern CWA tonight. Certainly plausible that dense fog could become a problem, but given the lower confidence opted to let evening shift to take another look at observational trends to reassess the dense fog threat tonight.

Shortwave trough over Texas is progged to move northeast into the mid-upper Mississippi Valley Tuesday. Dry low level air mass should preclude much threat of any rain, however extensive mid- upper level cloudiness should result in mostly cloudy skies tomorrow. Associated surface low is expected to track north into the upper Mississippi Valley tomorrow, which should cause the weak frontal boundary over central Illinois to return north as a warm front. Southeast winds ahead of the boundary should allow a lake breeze to push into northeast IL Tuesday afternoon, slowing the warm front progress and keeping it a cooler near the lake. Elsewhere, low level warm air advection should push temps into the upper 60s to lower 70s. If we see a bit more sunshine than currently expected, then some mid 70s are certainly possible.

Warm front should push north of the area Tuesday night with Wednesday shaping up to be yet another unseasonably warm day. Highs in the low to mid 70s are likely, though if high cloudiness isn't as extensive as some guidance suggests, then some upper 70s would be possible.

Another shortwave will pass north of the area Thursday with associated surface low track east into Ontario. A cold front trailing west-southwest from this low is expected to sag south into northern Illinois on Thursday. Moisture pooling ahead of this front could allow for dewpoints to climb into upper 50s to lower 60s. Low level forcing along the front looks pretty weak and with the best mid-upper level forcing north of the area, confidence is pretty low in potential for rain with the front. Having said that, even with weak mid-level lapse rates, the unseasonably moist air mass will support from weak destabilization. Opted not to introduce a thunder threat into the grids Thursday at this time, but will be something to consider in later forecasts.

Quasi-stationary boundary is likely to waffle around the region through the upcoming weekend. Air mass to the north of the boundary isn't particularly cool, the front really just separates slightly above average temps to the north from much above average temps to the south. Additional low amplitude shortwaves are likely to ripple east across the northern tier of the country through the weekend, which should help drive the front north and southward. There will be some periodic chances of showers and perhaps some storms through the weekend with the passing shortwaves. Timing of these rather subtle waves this far out could certainly change, so a bit early to lock in timing, but occasional rain chances look reasonable through the weekend. Also, worth noting that the dry (rain-free) hours this weekend should far out number the hours it is raining.

There continue to be some signal in medium range guidance in a more substantial shortwave moving bodily out into the central Plains and then Mississippi Valley early next week. There is considerable spread in the ensemble guidance in regards to timing, track, and intensity of this system, but the idea of additional chances of showers and thunderstorms early next week looks reasonable. Temperatures early next week look to remain above to much above average as warm air advection would likely ramp up in advance of a more formidable shortwave.

- Izzi

Aviation

Issued at 1228 AM CDT Tue Oct 27 2026

There are no key concerns to highlight for the 06Z TAF period.

NE winds below 10 kt will veer to SE by morning. Expect SE to SSE at near or below 10 kt during the day on Tuesday, subsiding and veering to S for the evening and overnight. VFR throughout the period.

Doom

Watches, warnings and advisories

IL: None. IN: None. LM: Small Craft Advisory until 10 AM CDT this morning for Winthrop Harbor to Wilmette Harbor IL.