textproduct: Chicago

KEY MESSAGES

- After a see-saw in temperatures this weekend, a warming trend will take hold next week and peak on Christmas with high temperatures potentially in the 50s to locally lower 60s.

- The upcoming pattern will not be supportive of widespread soaking precipitation events in our region. With that said, there will be occasional opportunities for light rain and/or drizzle, particularly Wednesday PM into Thursday.

DISCUSSION

Issued at 216 PM CST Sat Dec 20 2025

Through Sunday:

A cold front will move across the area this afternoon and there remains a small chance for a few sprinkles or flurries through early evening, when the cloud cover clears out. Mostly clear skies are then expected overnight with mostly sunny skies Sunday.

After highs in the upper 30s north and lower 40s south, temps will slowly fall into the 20s this evening with lows in the lower teens north to around 20 south by Sunday morning. The air behind this front is not too cold and with sun expected Sunday, its possible high temps might rebound back to around 30 north to lower 30s south.

Winds have diminished some this afternoon but periodic gusts will remain possible. Winds are expected to increase again this evening when a short period of gusts into the 30-35 mph range will be possible, especially across northern IL. Winds will then diminish overnight with light winds expected on Sunday as high pressure moves across the area. cms

Sunday Night through Saturday:

On Sunday evening, surface high pressure just to the east, mostly clear skies, and temporarily calm winds will result in temperatures briefing to the upper teens to lower 20s (mid 20s in Chicago). This will be followed by rising temperatures overnight from increasing southerly winds as the surface high slides farther east, along with mid and high level overcast.

The advertised pattern change will commence on Monday as anomalous blocking south of Alaska generates aggregate troughing across the Pacific Coast and broad quasi-zonal flow along the US/Canadian border. With our area expected to be positioned south of the energetic jet stream, we should largely escape opportunities for widespread precipitation events and instead have the opportunity to enjoy a period of above-average temperatures. Confidence is high in well above normal temperature departures for the upcoming stretch, but much of it will likely be cloudy.

On Monday, strong low-level warm air advection on the backside of the departing high pressure system and ahead of an upper- level wave propagating along the US/Canadian border will facilitate 850mb temperatures rising some 8 to 10C/24 hours to +2C or so into the Lower Great Lakes. So, in spite of expected widespread mid to upper-level cloud cover, do expect temperatures Monday afternoon to warm into the upper 30s to low 40sF (locally mid 40s south). A wedge of dry low-level air left in the wake of the high pressure system should prevent any widespread precipitation falling from mid-level echoes that develop within the persistent the warm air advection/isentropic ascent regime. Can't completely rule out flurries or even brief ice pellets near the WI state line early to mid Monday morning.

A large bank of low stratus should be drawn northward later Monday-Monday night by continued southwesterly low-level warm and moist advection. If the low-level saturation depths are sufficiently deep, there's a play for patchy drizzle development, but there's not enough overall guidance support for explicit mention in the gridded forecast. The clouds and southerly winds Monday night will prevent temperatures from dropping much, if at all. A weak cold front will move across the area on Tuesday. Despite mid-level subsidence as well as dry air advection noted down to the 925-850 mb level in the wake of the frontal passage, there's reason for skepticism that an existing stratus deck will be readily eroded. Given the complete lack of cold air advection and the magnitude of warmth aloft (850 mb temps very mild at +10 to +12C), the low-level inversion will be very steep and moisture (ie. stratus based below 1.5-2kft AGL) may very well become trapped.

While there is still a decent signal for a ~24 hour scouring out of low clouds, the global guidance doesn't often handle these regimes well during the lowest sun angle time of year, so trended a bit more pessimistic with the sky cover forecast. There will be bust potential either way depending on how the cloud cover plays out, though either way, temperatures will be solidly above normal. The presence (or lack thereof) of low clouds will then also modulate Tuesday night's lows given light winds with surface high pressure overhead. With a lean toward the clouds hanging on, nudged Tuesday night temps a bit milder than previous forecasts.

A weak surface trough extending toward the mid-MS Valley will gradually draw a warm front northward on Wednesday. With this progged pattern and overcast skies, expect a non-diurnal temperature trend through Wednesday night, which will result in temps only slowly rising into the 40s prior to sunset on Christmas Eve. The true unseasonably mild and moist air mass will not be ushered north until after dark and there's also uncertainty on how far north the warm front is able to progress.

With increasing low-level saturation depths underneath lingering mid-level dry air, the isentropic ascent regime Wednesday afternoon and night will likely be supportive of periods of light rain/showers and/or drizzle (~40-60% PoPs). It's unlikely to be a washout, though. The drizzly regime may even be prolonged into Christmas Day as dew points rise well into the 40s and even 50s. There was a considerable increase in EPS (ECMWF ensemble) membership depicting measurable QPF into Thursday, and as such, added in a chance of light drizzle mention. If this comes to fruition, temperatures likely won't be quite as warm as they'd be without low clouds and drizzle, but dew point values should still push high enough to result in highs from the low 50s to locally lower 60s (warmest far south). This would give it a shot as ending up as a top-5 warmest Christmas Day at Chicago and Rockford.

Even in the wake of another weak cold front passage Thursday night into Friday, the positioning of the jet stream north of our area supports a continuation of above average temperatures into the weekend. There may be a brief cool-down back closer to seasonable in the Sunday-Monday timeframe (beyond current day 7/Sat 12/27), but mild conditions should again return heading towards New Year's Eve and New Year's Day 2026.

Castro/Borchardt

AVIATION /18Z TAFS THROUGH 00Z MONDAY/

Issued at 1122 AM CST Sat Dec 20 2025

Key messages for the 18Z TAF cycle:

* A marginal LLWS setup this evening

Westerly winds will gust to around 20 kt through the afternoon, becoming less frequent with time. As a low level jet builds down toward the surface, W gusts will open up for the evening to 25 to 30 kt, occasionally higher. 45 to 50 kt of flow will be found at 2kft AGL, but given the expected gustiness at the surface, decided to omit LLWS from the TAFs. Winds will gradually step down during the overnight. Beyond early Sunday morning, expect largely sub-10 kt flow out of the W, backing to SSW for the afternoon.

Doom

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 4 AM CST Sunday for Winthrop Harbor IL to Gary IN.

Small Craft Advisory until noon CST Sunday for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.