textproduct: Chicago

KEY MESSAGES

- Cloudy conditions persist, with nearly steady temperatures tonight.

- Another period of drizzle and light rain showers likely (70 to 80% chance) tonight into early Friday morning. A period of dense fog may occur, as well.

- Widespread moderate rain with embedded thunderstorms and heavy rain possible on Sunday, especially near and southeast of I-55.

- A return to more winter-like conditions is on track to arrive next week. Wind chills falling toward zero may return to the region as early as Monday morning, followed by a return of chances for snow by midweek.

DISCUSSION

Issued at 238 PM CST Thu Dec 25 2025

Through Friday Night:

It is an unseasonably mild Christmas afternoon across the region, albeit cloudy. Clouds will hang tight with us tonight into Friday, as our next weather maker over the Plains quickly shifts eastward into the lower Great Lakes. Extensive cloud cover will keep temperatures fairly steady (in the lower 40s north of I-80 and mid to upper 40s south of I-80) this evening. Thereafter, readings look likely to climb a couple of degrees overnight (particularly south of I-80) as the lower-level flow briefly shifts to the south-southwest in advance of the approaching weak surface low.

Precipitation chances (in the form of drizzle and light showers) will ramp-up from west to east across the area later this evening and overnight as low-level moistening tied to increasing isentropic upglide fosters lowering cloud heights and increasing cloud depths. Overall, we are not expecting a lot of rain from this overnight activity, but some areas across far northern IL could end up with a couple tenths of an inch before it shifts east of the area by mid morning on Friday. However, of more concern is the threat for a period of fog very late tonight into Friday morning as the surface low tracks directly overhead. At the very least, very low clouds (less than a couple hundred feet AGL) will occur with the low overnight into early Friday morning. However, such environments are also favorable for dense fog, and given that there is plenty of fog already noted upstream of our area, I think there is a better than 50/50 chance that some dense fog will materialize in at least parts of the area into Friday morning.

The main uncertainty at this point lies with the extent of the fog into northeastern IL, where ongoing rain showers overnight could keep visibilities from tanking for several hours. Accordingly, my western areas (generally interior sections of IL) look to have the best chance for this period of dense fog Friday morning. With that said, I cannot rule out some of this fog working its way into portions of the Chicago metro area for a few hours around or shortly after daybreak Friday morning. The current plan is to pass our concerns on to the evening shift and have them monitor observation trends through the evening to determine if a Dense Fog Advisory may be needed for parts of the area.

Cloud cover may be slow to clear on Friday, with much (if not all of) of the day potentially remaining cloudy again. However, any dense fog should gradually abate into Friday afternoon. In spite of a turn towards northwesterly winds following eastward departure of the surface low on Friday, temperatures will remain mild for the time of year, with readings generally holding in the mid to upper 40s north, to the mid and upper 50s far south.

A weak surface ridge axis is progged to shift across our area Friday night. Light winds under this ridge could set the stage from some good radiational cooling Friday night, assuming we are able to scatter out our lower cloud deck by Friday evening. If this occurs, an additional period of fog could materialize late Friday night into early Saturday morning.

KJB

Saturday through Thursday:

The forecast in the long term picks up Friday evening when conditions should be quiet around the area. Following tonight's/Friday morning's rain potential, relative high pressure will settle in Friday night into Saturday. Forecast soundings depict enough moisture will remain trapped beneath the subsidence inversion to provide a setup for areas of fog Friday night into Saturday morning, especially outside of the interior Chicago metro. Probs for dense fog from the HREF get pretty high (widespread >50%) west of the Fox Valley.

During the day on Saturday, a baroclinic zone will gradually meander northward across southern and central IL east of a deepening low pressure system. We'll remain north of the boundary during the daytime and afternoon highs are progged to range from the mid 40s in our north to middle to upper 50s in our south. Before the low tracks across the CWA early Sunday, the warm front will surge northward into our CWA late Saturday night. This will bring steady, if not slightly rising, temperatures during the overnight. Guidance has recently slowed the progression of the low across the CWA to later in the day on Sunday, putting a delay on the strong cold advection expected in its wake. There's still a fair deal of uncertainty, but latest guidance suggests this should allow Sunday afternoon temperatures to warm into the upper 40s and 50s around the area before they come crashing Sunday night.

As far as precip expectations go with this system, isolated pockets of rain may be found well out ahead of the low as early as Saturday evening. Better rain chances will be found immediately ahead of the low center and especially during the afternoon. As the associated mid level shortwave works into the Midwest Saturday night, it will phase with an existing upper low straddling the US-Canadian border to the north. As it does, the storm's warm conveyor will strengthen rapidly into Saturday and the baroclinic zone along the cold front will sharpen up significantly; most camps even resolve a secondary low spinning up along the baroclinic zone upstream. A strong, deep jet will also develop atop the warm conveyor. As this whole process takes place, expectations are for precip coverage to expand significantly, mainly along the face of the sloped cold front and track of the secondary low. By the time this happens late Sunday morning and afternoon, the corridor of greatest precip potential will be focused away from our northwest and more toward areas near and southeast of I-55 and over into Indiana. Model camps agree on around or just over 1.5" of PWAT in our south and southeast by Sunday afternoon. Noteworthy dynamic forcing will also take place as the trough swings across. 500mb height falls are in excess of 20dam/12hr during the morning and afternoon. All signs lead to widespread moderate rain during the day on Sunday, primarily near and southeast of I-55 where coverage and intensity look to be generally lesser farther northwest. Most deterministic camps resolve a pretty sharp QPF gradient across our CWA with a few tens of miles possibly meaning a difference of several tenths of an inch of rainfall. Confidence is rather high that the highest precip totals locally will be found east of the I-55 corridor. Probs for greater than 1" of precip from the LREF (GEFS/EPS/GEPS conglomerate) are up to 25% in our southern and southeastern CWA and as high as 50% for greater than 0.5".

There's even a potential for a few thunderstorms in our south on Sunday. Models resolve as many as several hundred Joules of MUCAPE into our southern and eastern CWA during the late morning and afternoon owing to some marginally steeper mid level lapse rates, which could be ample for thunderstorm development. There will be plenty of layer moisture and forced ascent to make the most out of this instability. Any storms that do develop will also have access to upwards of 40kt of effective shear. This means a potential for pockets of heavy rainfall embedded in thunderstorms. While the ground is well-thawed by now, soil moisture remains rather high likely due to the recent snowmelt. This means some localized flooding or ponding may be possible beneath any heavy rainfall, but this doesn't appear to be a widespread concern. The NBM populated the forecast with slight chances for thunder near and south of US Rt. 24 which was maintained in this package.

In the cold advection behind the system, guidance favors a brief period of a wintry mix or light snow showers to close out the precip late Sunday. Outright snow showers appear more likely closer to the lakeshore. No meaningful accumulations, if any, are expected. One possible exception could be northern Jasper County in Indiana where some models want to bring in a couple of inches of lake-effect snow, but the greater potential is farther northeast.

Another thing of note regarding this system will be strong northwest winds developing during the latter half of Sunday and continuing through the day on Monday. Gusts could get up to the 25 to 30 mph range by early evening and possibly closer to, if not in excess of, 40 mph Monday morning when the strongest winds are expected. MSLP rises Monday morning will push 1mb/hr. Winds will gradually ease during the late afternoon and evening.

We can also expect much cooler conditions beginning on Monday in combo with the strong winds. Morning temperatures are forecast in the teens and lower 20s with wind chills in single digits on either side of 0F. Wind chills in the single digits above 0F are forecast during the warmest part of the day.

Upper 20s and lower 30s are forecast for Tuesday and Wednesday. Medium range guidance has been struggling with a clipper system that it wants to bring through the Midwest on New Years Eve (Wednesday). Most ensemble members favor the upper Midwest and leave the local area unscathed, but there are a handful of members, namely within the EPS, that track of swath of accumulating snow across the area. While the potential for this appears low at this time, given the holiday, we will keep a close eye on forecast trends for the possibility of any travel impacts.

Doom

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/

Issued at 1200 PM CST Thu Dec 25 2025

- Lingering MVFR stratus at times through early evening

- Showers and drizzle develop late evening and overnight with CIGs and VSBYs lowering to LIFR and potentially VLIFR into Friday AM.

MVFR stratus has persisted through the morning and may continue at times through early evening across the area. There are some breaks in the stratus near and east of the Chicago area terminals which may lead to occasional periods of VFR conditions before it fills back in or additional low clouds move in off the lake.

Widespread LIFR (and lower) conditions to our west and southwest will begin to slosh back northeast into the area later this evening and overnight paired with a period of showers and drizzle. Guidance continues to trend more pessimistic and have adjusted the TAFs accordingly. Lowest CIGs and VSBYs to low-end LIFR to possibly VLIFR develop by early Friday morning (~10-15Z). If trends continue, an introduction of VLIFR conditions may become warranted for some (if not all) sites. Expect CIGs and VSBYs to improve late Friday morning with MVFR stratus potentially lingering through the afternoon.

Winds remain steady out of the east northeast around 10-15kt. They will gradually turn east and then southeast through the afternoon. Confidence in directions decreases later tonight into early Friday morning as a surface low moves across the area. Winds may become light and variable during this time before prevailing northwesterly in its wake Friday morning.

Petr

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until noon CST Friday for Winthrop Harbor to Northerly Is. IL.

Small Craft Advisory until 6 AM CST Friday for Northerly Is. to Calumet Harbor IL.