textproduct: Chicago

KEY MESSAGES

- Areas of dense fog possible this morning, mainly near/west of the I-39 corridor.

- Showers and some thunderstorms likely areawide Sunday into early Sunday evening.

- Thunderstorms may pose a locally damaging wind threat Sunday afternoon/early evening, mainly southeast of I-55.

- Very strong winds will usher in drastically colder air Sunday night into Monday, with some snow showers possible.

DISCUSSION

Issued at 332 AM CST Sat Dec 27 2025

Ceilings and visibilities have been very slow to come down so far this morning, leading to some growing doubts about the extent of the dense fog threat this morning. Ceilings have fallen below 500ft far western and southwestern CWA and these areas stand the best chances at seeing gradually stratus build down to dense fog this morning. Interestingly, visibilities have trended upward over the past couple of hours near the Mississippi River and with light easterly low level flow developing on the backside of the departing sfc ridge. I am having some doubts about the dense fog threat into our area this morning. Opted to maintain areas of dense fog in the grids after 11z or 12z, mainly near/west of the I-39 corridor as well as near and south of highway 24.

GOES imagery early this morning depicts a massive stratus deck blanketing the western Great Lakes and much of the Midwest. Given the sharp inversion that this stratus is trapped beneath, it seems likely that today will remain overcast. Given the expected cloud cover have lowered high temps a couple more degrees with low to mid 40s probably the best we'll be able to do today. Wouldn't even be surprising to see some interior portions of northern IL struggle to even reach 40F.

Tonight, a strong upper trough will emerge out onto the northern and central High Plains. In response, surface cyclogenesis will take place over Kansas with warm frontogenesis taking place east from this developing cyclone across northern Missouri into downstate IL and IN. As height falls spread well east of the upper trough, it should encourage the development of a 40kt 925mb southerly low level jet across the Mississippi River Valley. Strengthening isentropic ascent north of the developing warm front and near the terminus of the low level jet should result in blossoming area showers overnight tonight across our CWA.

Strong low level theta-e advection and isentropic ascent is likely to continue into Sunday morning north of the slowly northward advancing warm front. Ascent will further be enhanced by the increasingly diffluent/divergent flow in advance of the approaching 140kt+ upper level jet. The strong and deep ascent, couple with ample slant-wise and perhaps some weak upright instability could allow for some embedded thunderstorms Sunday morning. This strong forcing and convection will have essentially record high PWATs leading to the threat of some locally heavy rainfall Sunday morning into the early afternoon. Particularly with any training heavier rainfall, which is a bit of a concern given the low level baroclinic zone becoming oriented parallel to the strengthening mid-upper level flow in advance of the mid-upper level trough. Antecedent fairly low stream flow and still largely unfrozen soils should help mitigate the flooding risk, but infiltration rates tend to be lower this time of year and with the potential for 0.75-1.50" rainfall totals in 3 hours or less, hard to rule out some nuisance type flooding of low lying and flood prone areas.

This morning rain and convection will probably slow the northward advance of the warm front and associated unseasonably warm and humid air mass. Eventually, the strong dynamics associated with the deepening cyclone should drag the front northward into at least our southern most CWA. Sfc dewpoints south of the warm front near or just over 60F should at the very least weaken the stability of the boundary layer if not result in a weakly convectively unstable boundary layer conditions. Wind profiles will only grow increasingly impressive with time Sunday afternoon and evening as the cyclone enters a period of rapid intensification. A strongly forced band of low topped convection is likely to develop along the quickly advancing cold front Sunday afternoon and evening. Given the strong forcing and very impressive wind fields, it wouldn't take much, even weak convection's downdrafts could potentially transport some of the stronger flow just off the deck to the sfc and result in strong to locally damaging wind gusts.

Weak low and mid level lapse rates will likely limit destabilization, leaving the question of whether 60F dewpoints in and of themselves will be sufficient for some weak boundary layer destabilization. Just a few hundred J/kg of SB CAPE would be sufficient for a more meaningful severe weather threat given the strong forcing and wind fields. The difference between 300 J/kg of SBCAPE and no SBCAPE is really in the noise level of model guidance at this distance and very difficult to predict. Unfortunately, that very small difference in instability could mean the difference between a gusty line of heavier showers and a more intense squall line with locally damaging winds and perhaps even a QLCS tornado threat given the extremely strong low level shear. Interestingly, this time of year it is FAR easier to get lightning with elevated convection in WAA regimes than it is along cold fronts. So our highest chances of lightning look to be earlier in the day Sunday, with a much lower lightning threat with cold frontal convection, even if we end up with a few hundred J/kg of SB CAPE. In the end, if a locally damaging wind threat does materialize with the squall line it is unlikely to be accompanied by lightning.

The 00z GFS and ECMWF both depict explosive deepening of the surface cyclone Sunday afternoon and night, with both deepening the surface low between 23-28mb in just 18 hours. This magnitude of deepening is rare for this part country and in response, this projected bomb cyclone will likely result in a large and very powerful wind field Sunday night into Monday. Very strong cold air advection will keep boundary layer well mixed overnight Sunday and Monday, this should aid in transporting some powerful gusts to the surface. Based on current model output, it is possible that we could see winds flirting with high wind warning criteria (sustained winds 40+ mph or gusts 58+ mph) for a short time period late Sunday night/early Monday morning. It isn't uncommon for guidance to trend a bit weaker with high winds as the event nears, so I bumped up winds and gusts solidly into advisory criteria, but didn't go as high as Bufkit momentum transfer would suggest (at least not yet).

GFS and ECMWF both still suggest sufficient moisture wrapping around the occluding cyclone could result in some wind driven snow showers late Sunday night into Monday morning. Perhaps enough to cover the ground, but confidence isn't terribly high in just how much QPF will be flung this far around the cyclone into our CWA, so didn't make any changes to the modest (30-60%) POPs late Sunday night into Monday morning.

Temperatures behind this storm will only be slightly below average, but the strong winds and dramatic contrast from the unseasonably mild air preceding it, which will make the cold all that much more biting. Winds will ease Monday night and begin to back southwesterly Tuesday in advance of low pressure progged to pass well to our north as it wraps around the massive departing cyclone tracking north across Quebec. That will be just one of what looks to be several vort lobes pivoting around the huge, slow moving, closed mid-upper level low that becomes established near Hudson Bay.

GFS and ECMWF both suggest a more potent/westward extending shortwave could pivot around the upper low and dislodge a more formidable shot of cold air New Year's Eve night into New Year's Day. Still some variability in ensemble guidance with the details on just how far west the coldest air gets and also whether it will be accompanied by any snow. For now, made no changes to NBM output, but this will be a time period we'll need to watch in coming days, particularly once we get past the Sunday-Monday bomb cyclone.

- Izzi

AVIATION /12Z TAFS THROUGH 18Z SUNDAY/

Issued at 542 AM CST Sat Dec 27 2025

Key Aviation Messages for the current TAF period:

- IFR conditions expected through the entire TAF window

- Drizzle/Showers move in tonight with steadier rainfall expected tomorrow morning with potential LIFR conditions during heavier precip after 12Z Sunday

Winds will be light and out of the southeast through the TAF period. However, the main story today will be the low ceilings and visibilities once again. Vis for areas away from the Chicago Metro are currently between 3 to 4 SM, with a chance to lower after day break down to around a mile with patchy fog possible. Meanwhile, closer to the lake vis and cigs are slightly better, but still with a chance to degrade in the next couple hours. Dense fog is not expected at the Chicago Terminals with a less than 20 percent chance for dense fog near KRFD.

There is moderate confidence in cigs and vis slightly lifting back to higher end IFR after 16Z this morning. There are a few models that show the slight chance for a brief window of low end MVFR cigs this afternoon, but confidence in that transpiring is low. There is another chance for lowering vis/fog after 00Z tonight (with better chances near KRFD), not enough confidence to mention it in the TAF presently. After 03Z tonight, a low level jet will increase to around 40 knots out of the southwest at or just above 2000 feet AGL. With light southeasterly winds at the surface, there is a non-zero chance for low level wind shear tonight, but seems both marginal and for a short enough window to not need a formal mention of it in the TAF.

The next system will approach the airspace overnight tonight and into tomorrow morning. Southerly flow will increase moisture for more prevailing lower cigs and vis. Spotty light showers/drizzle are expected around and after 06Z. Steadier rain can be expected after 12Z wither moderate rain rates that can lower conditions to LIFR levels. Lastly, the best chances for thunder with this system looks to be far southwest of the Chicago Metro. While no mention of thunder is needed at this window of the TAF, it will be reviewed for later TAF packages.

DK

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Storm Watch from Sunday evening through Monday morning for Winthrop Harbor IL to Gary IN.

Storm Watch from Sunday evening through Monday afternoon for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.