textproduct: Chicago

KEY MESSAGES

- Showers and some thunderstorms likely areawide today. There is a threat of a few severe thunderstorms capable of damaging winds, mainly south of I-80 and east of I-55.

- A wind advisory is in effect due to the expected powerful winds, gusting over 50 mph at times, tonight into Monday morning areawide.

- Over northern Porter County, the winds advisory will likely be upgraded to a high wind warning today as damaging wind gusts of 50 to 65 mph are expected at times tonight and Monday morning near Lake Michigan.

- Wind whipped snow showers and flurries are expected late tonight into Monday morning, especially north of I-88/I-290 where a coating to locally an inch of accumulation could result in hazardous travel.

- Temperatures roughly south of I-80 and east of I-55 will briefly reach near record warm levels this afternoon, before a powerful cold front sends temps crashing tonight. Wind chills close to zero are expected Monday morning.

DISCUSSION

Issued at 341 AM CST Sun Dec 28 2025

Through Monday:

No big changes to the forecast this morning as things are generally on track with very active weather over the next 24 hours as a cyclone explosively deepens and moves across the region today and tonight.

Fog:

Dense fog has been growing more widespread north of the warm front lifting across the southern portions of the CWA early this morning. Once rain begins, visibilities should improve some, but until then felt it was prudent to issue a dense fog advisory until 6 AM. Closer to 6 AM we will reassess whether some portions of the advisory may need to be extended.

Showers/Thunderstorms:

Showers and thunderstorms will overspread a large portion of the area early this morning. Objective analysis shows a reservoir of MUCAPE over 1000 J/kg over northern MO with good agreement in near term guidance in 500-1000 J/kg of MUCAPE spreading east- northeastward across northern/central IL early this morning. Given the low freezing levels and strong effective shear, storms this morning will be capable of producing hail. Certainly wouldn't rule out some marginally severe hail and perhaps the need for a severe thunderstorm warning or two this morning as far north of the I-80 corridor or perhaps a bit north of there.

While storm motion will be fast, record breaking high PWATs and unseasonably strong instability means that thunderstorms this morning will be capable of producing heavy rainfall. A WSW to ENE oriented axis of heavy rainfall, possibly 1-2" could result in mostly minor flooding of low lying and flood prone areas. If the heavier rainfall rates occur in more urbanized areas, there is a threat of some flash flooding developing, but confidence is too low to justify a flood watch.

There will be fairly widespread showers and thunderstorms continuing north of the slowly northward advancing warm front and ahead of the surface low through early afternoon. To the south of the warm front, there will be much less coverage of precipitation and perhaps even some mixed filtered sunshine in the warm sector of the cyclone, including into our southern CWA. Guidance is in good agreement on advecting the low 60s dewpoints north into about the SE 1/3 or so of our CWA ahead of the advancing cold front. There are some slightly steeper mid level lapse rates (6.5-7C/km) upstream over the Ozarks, which should advect overtop the record warm and humid air mass in the warm sector. There is fairly strong agreement among model guidance that an axis of 500-1000 J/kg of MLCAPE will develop along and ahead of the cold front, including into our southern CWA.

Assuming this magnitude of instability materializes as progged, then there could be a corridor of locally higher chances of severe convection. The shear profiles are progged to be quite extreme, unsurprisingly given the explosively deepening cyclone, which could pose a problem either in or near our CWA this afternoon. By this afternoon across the northern portions of the warm sector, guidance has >50kt winds below 1km, which could easily result in even modest convection mixing this higher momentum to the surface and producing damaging winds. The 0-1km and 0-0.5km shear is progged to be extreme, so if any low topped/mini supercells were to develop, they would pose a tornado threat. The more likely scenario would seem to be a developing QLCS near the cold front which could produce damaging winds and some brief tornadoes associated with line embedded mesovortices. The convective threat should push east of our CWA by around or just before 00z.

Cool season, high shear-low CAPE severe events are notoriously difficult to forecast and seemingly favorable set-ups can easily fail to produce. Given the extreme shear, these type of set-ups warrant close watching.

Synoptic Winds:

Sfc low is progged to deepen 20+ mb in just 18 hours and should result in a very large and powerful wind field. The intense cold air advection and resultant steep low level lapse rates combined with the very strong pressures rises should result in frequent gusts between 45-55 mph developing this evening and continuing into at least early Monday morning. Not uncommon for winds to overperform in these type of set-ups with bomb cyclones, so there may well be some measured gusts up to 60 mph. Not confident enough that those gusts will be widespread enough to justify an upgrade to a high winds warning. This will likely be a quickly developing situation this evening and it is possible that a short fused upgrade to a high wind warning may be needed for portions of our CWA.

The other concern is that northern Porter County, downwind of Lake Michigan will likely experience fairly regular gusts of 50-65 mph tonight into Monday morning. Porter County will likely need to be upgraded to a high wind warning.

Snow:

Wrap around snow is expected to make it into far northern IL overnight into early Monday morning, mainly north of the I-88/I-290 corridors. The snow will be wind whipped and given the rapidly falling temperatures, will likely immediately stick to untreated roads. Amounts don't look to be very much, probably less than an inch for most areas, but sometimes the biggest travel problems/accidents can occur with seemingly innocuous minimal snows like where impacts of such small snowfall amounts can be easily underestimated by drivers. Also, worth noting that rain will have long since washed away any prior road treatment.

Flash Freeze:

Temperatures will rapidly fall tonight, 20 or more degrees in just a couple/few hours. Very strong winds very often dry out pavement quickly enough to prevent widespread "black ice" problems, however these type events are notoriously difficult to predict. In addition, areas that pick up heavy rainfall today could see areas of standing water in low spots or poor drainage areas on roads freeze tonight creating a hazard.

- Izzi

Monday night through Sunday:

The upper level low that is driving this weekend's weather will be over Quebec by Monday night. Another upper level low is expected to drop southward from Hudson Bay Monday night through Tuesday. While these two upper level lows phase with each other, impacting the weather out east, it will send a series weak disturbances extending back over Lake Michigan that could provide a couple additional rounds of wintry precipitation. Unfortunately, models are showing little agreement providing lower confidence in the details.

The first weak disturbance drops southward out of Canada on Tuesday. The Euro ensemble has a surface low in south-central Canada dropping into northern Minnesota before weakening as it moves from the Minnesota arrowhead to Michigan's western mitten. The GEFS meanwhile has a weaker wave and a more eastern track. For now, kept snow out of the forecast. Maybe some flurries or light snow is possible for areas closer to Lake Michigan, but unless models shift more westward, dry conditions should prevail.

Models have shown some decent agreement that as the previously mentioned upper lows phase around each other Tuesday night into Wednesday, they will slowly retrograde westward into Ontario with a strengthening 300 mb 100+ knot jet and a reflected deepening surface cyclone. With a weak mid level wave dropping southward, a surface cold front is expected to pass over the area on Wednesday with much more robust cold air advection behind the front. No changes were made to the PoPs as latest NBM run keeps chances for snow for much the area. There is still plenty of model disagreement on exact timing and snow amounts. And while current indicators are that it does not have a signature of a major snow storm, it could provide a coating on the ground. Holiday revelers may expect some snow falling down as 2025 runs out of time.

Temperatures will feel much more winter like through the week. As northwest winds persist through Monday night into Tuesday morning, single digits to low teens are expected in the morning with wind chills at or just under zero degrees F. High temperatures are expected in the 20s to just below freezing Tuesday and Wednesday. New Years Day looks to be a chilly one as temps drop once again behind the front. The NBM is suggesting solidly in the 20s for highs, but that might be biased by the GFS/GEFS having no snow, whereas the Euro suite is advertising highs in the teens (a likely result "if" there is a coating of snow on the ground).

Temperatures are expected to climb slowly back into the upper 20s and low 30s for the end of the week/weekend. There is a chance for another winter system that could bring some more precip to the area, potentially on Sunday, but models are not showing a lot of agreement at this distance to have great confidence.

DK

AVIATION /06Z TAFS THROUGH 12Z MONDAY/

Issued at 1130 PM CST Sat Dec 27 2025

Aviation forecast concerns:

- Extensive LIFR conditions overnight into Sunday morning, with areas of VLIFR possible. Highest confidence in cigs, vis may fluctuate with precip.

- SHRA expected Sunday with embedded TS likely. Localized gusty winds possible within SHRA/TS.

- Sharp cold front early Sunday evening with strong westerly winds with gusts around 40 kt. Falling temps likely supports a period of snow/snow showers overnight.

Strengthening surface low over central KS late this evening will track northeast across northern IL Sunday. Low-level south-southeast flow is bringing low-level moisture into the region with a northward- moving warm front, helping to steadily lower cigs into LIFR since late this afternoon. Development of drizzle also leading to lowering vis, generally ranging from 3/4SM to 1 1/2SM currently with some up/down fluctuation noted. Expectation is things will continue to settle lower, with a areas of VLIFR possible overnight into Sunday morning. With increasing shower coverage predawn however, confidence in solid persistent VLIFR is somewhat low, though certainly can't be ruled out.

Surface low center is progged to track across the terminals midday into mid-afternoon, with the surface warm front lifting into the Chicago metro area. Unseasonably mild/moist low levels combined with steepening lapse rates aloft will likely support some embedded TS, and have included PROB30 mention for TSRA at all four Chicago metro TAF sites (trend has been slightly farther north than earlier guidance runs). Any convective SHRA/TSRA may be able to produce brief locally gusty winds, most likely from the southwest (though wind field will otherwise be fairly light and somewhat variable in the vicinity of the surface low center track). SHRA will likely help mix cigs a bit higher during the day, though low-end IFR likely to prevail.

As the low pulls away late in the afternoon, winds will shift sharply to the west-northwest, while becoming strong and gusty with gusts ramping up into the 35-40+ kt range during the evening. Advection of much colder air will also support a period of snow/snow showers from mid-evening into the overnight hours, with IFR vsbys possible and some accumulations possible.

Ratzer

MARINE

Issued at 1130 PM CST Sat Dec 27 2025

- Gale Warning in effect from 6 PM Sunday to 10 PM Monday from Winthrop Harbor IL to Gary IN.

- Storm Watch remains in effect from Gary IN to Michigan City IN, from 6 PM Sunday to 3 PM Monday.

Areas of dense fog are expected into Sunday morning across the nearshore, with a Marine Dense Fog Advisory in effect until 10 AM Sunday.

A powerful cold front will sweep across the lake late Sunday, resulting in a rapid increase to gale force west-southwest winds. There's some concern that these winds could start a bit earlier than the current 6 PM start time of the gale warning and storm watch. As low pressure continues to rapidly deepen near Lake Huron Sunday night into early Monday, westerly winds will further increase. Confidence in occurrence and duration of storm force speeds/gusts is highest for the southeast portion of the lake and hence held onto a Storm Watch there (Gary to MI City) in coordination with WFO Northern Indiana.

Freezing spray will also be possible early Monday morning and continue into Monday night as air temperatures are expected to be in the lower 20s and possibly into the teens.

Castro/CMS

LOT WATCHES/WARNINGS/ADVISORIES

IL...Wind Advisory from 6 PM this evening to 6 PM CST Monday for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012- ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039- ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Wind Advisory from 6 PM CST /7 PM EST/ this evening to 6 PM CST /7 PM EST/ Monday for INZ001-INZ002-INZ010-INZ011- INZ019.

LM...Dense Fog Advisory until 10 AM CST this morning for the IL and IN nearshore waters.

Gale Warning from 6 PM this evening to 10 PM CST Monday for the IL nearshore waters.

Storm Warning from 6 PM this evening to 3 PM CST Monday for the IN nearshore waters.

Gale Warning from 3 PM to 10 PM CST Monday for Calumet Harbor to Gary IN.