textproduct: Chicago

KEY MESSAGES

- Patches of drizzle and fog may continue to be observed through tonight beneath widespread stratus.

- A period of rain is possible on Friday in areas near and especially south of the Illinois and Kankakee rivers.

UPDATE

Issued at 818 PM CST Thu Nov 20 2025

Have issued a Dense Fog Advisory until 10 AM CST Friday for southern parts of the forecast area.

Fog has developed this evening across parts of central IL and IN, with visibilities dropping below a mile (and as low as 1/4SM) in spots. In the WFO LOT cwa, this is most largely across the southern tier of counties, where both surface temps and dew points are in the mid-upper 40s. Slightly weaker flow aloft and a later arrival of a weak cold front into Friday morning suggest these southern areas will remain at greatest risk for dense fog development/expansion. Farther north, temp-dew point spreads are slightly larger, with slightly more flow atop the boundary layer supporting more of a persistent low-stratus situation rather than widespread denser fog, though patchy fog is certainly possible there as well. A weak surface cold front currently stretching northeast to southwest across WI and northeast IA will push into far northern IL overnight, and will bring some weak dry advection which should further limit fog impacts across the north.

Early evening AMDAR aircraft soundings from PIA, RFD and MDW generally depict a shallower stratus layer than last night, roughly 1500-2000' thick. While still maintaining the patchy drizzle mention ahead of the front, suspect it will remain less widespread than we saw 24 hours ago.

Other than expanding area and mentioning dense fog across the southern cwa, no changes made to going forecast.

Ratzer

DISCUSSION

Issued at 333 PM CST Thu Nov 20 2025

This afternoon, the region remains socked beneath a 4000-5000 ft deep stratus deck trapped beneath a temperature inversion at the 850 mb level. Still seeing some occasional drizzle reports in regional METARs, and with flow in the cloud layer expected to increase modestly over the next few hours while an upper-level shortwave trough passing to our north deals a glancing blow of ascent, would not be surprised to see these drizzle observations become more common going into this evening.

Low-level dry air advection behind a weak cold front dropping in from the north late tonight should cause the stratus deck to become patchier and eventually scatter out in our northern counties sometime tomorrow, while the stratus status quo is preserved through the daytime hours in our southern counties. Areas that do manage to shed the stratus will still, however, see increasingly thicker upper-level cloud cover move overhead as a sheared-out upper-level shortwave pivots into the region from the southwest. Thus, one way or another, the magnitude of solar insolation that we'll see will be blunted once again, and tomorrow's temperatures will likely end up being similar to today's as a result with highs in the mid 40s to low 50s.

The northeastward approach of a weak surface low associated with the aforementioned shortwave clashing with the southward push of cooler, drier air behind the aforementioned cold front will yield a sharpening baroclinic zone and strengthening frontogenesis across central Illinois/Indiana. An expansive precipitation shield with a sharp cut-off on its northern periphery (due to the impinging drier air from the north) will be found in this region of stronger forcing for ascent Friday morning through Friday evening. Some uncertainty still remains regarding how far north the rain will get. The past several runs of the deterministic GFS have actually been outputting QPF as far north as the I-88 corridor. However, these runs appear to be outliers relative to most other guidance, and more often than not, the dry air coming in from the north tends to win out in these kinds of setups. Therefore, suspect that the bulk of the incoming precipitation should stay confined to the south of the Illinois and Kankakee rivers, and possibly even south of our forecast area altogether. Have kept PoPs capped in the chance range in our southern counties to reflect this thinking.

After this late week weather system clears the area, dry conditions with moderating temperatures are expected through the weekend. An upper-level low ejecting northeastward out of the Desert Southwest should then bring the possibility of widespread rain back into the region in the Monday-Tuesday time frame. A northern stream trough will follow closely on the heels of this initial disturbance and should bring some colder air into the region just in time for Thanksgiving. Depending on how these two disturbances interact, it's possible that some precipitation could also be seen here on Wednesday and/or Thursday. However, only a minority of global ensemble members depict such a scenario at this time, and the latest NBM keeps PoPs in our CWA largely below 15% as a result.

Ogorek

AVIATION /06Z TAFS THROUGH 12Z SATURDAY/

Issued at 1058 PM CST Thu Nov 20 2025

Forecast concerns include...

Ifr/lifr cigs overnight with patchy drizzle. Mvfr cigs lifting late Friday morning/early afternoon. North/northeast winds possible Friday afternoon. Chance of showers Friday afternoon/early evening.

A weak frontal boundary will move across the area overnight and cigs are lowering into ifr with some lifr cigs across northwest IL. Expect prevailing ifr cigs through the predawn hours and then cigs should lift into low mvfr around daybreak, then slowly scatter out from north to south late Friday morning into early Friday afternoon. Patchy drizzle will also be possible overnight that may reduce visibilities into the 1-2sm range. Areas of dense fog will be possible well south of the terminals overnight into Friday morning.

Light westerly winds will shift north/northwest overnight and may turn to the north/northeast for the Chicago terminals Friday afternoon, though only medium confidence. Wind directions will likely favor north/northwest Friday evening. Prevailing speeds are expected to be less than 10kt through the period.

There remains a chance for showers mainly south of the terminals Friday afternoon into Friday evening with the latest guidance showing these showers possibly reaching as far north as MDW/GYY. Opted to include vicinity mention at GYY and trends will need to be monitored. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...Dense Fog Advisory until 9 AM CST Friday for ILZ032-ILZ033- ILZ039.

IN...Dense Fog Advisory until 9 AM CST /10 AM EST/ Friday for INZ010-INZ011-INZ019.

LM...None.