274 FXUS63 KLOT 310503 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1103 PM CST Tue Dec 30 2025 .KEY MESSAGES... - Accumulating snowfall will be ending through about midnight tonight, briefly as freezing drizzle in spots, but this is not expected to be overly impactful. - Another period of accumulating snow again Wednesday afternoon leading to slowed and hazardous travel. && .UPDATE... Issued at 945 PM CST Tue Dec 30 2025 The main batch of accumulating snow is ending north of I-80, and will be ending south of I-80/east of I-57 through about midnight or so. As snow ends, a final period of light freezing drizzle may occur as deeper saturation is lost (IR cloud top temperatures have been warming recently across northern Illinois towards -10 C). However, based on upstream radar trends, the coverage of lingering drizzle/mist continues to diminish, and a general transition to low-level descent is forecast through the rest of the evening. This should help limit the coverage and intensity of any trailing FZDZ. Not seeing any terribly significant changes in the handling of tomorrow's system with the incoming evening guidance. The main difference seems to be a continued warming trend through late- morning/midday as precipitation initially breaks out, with above-freezing temperatures advertised south of about I-80. While there is some potential this results in some rain south of I-80 as precip starts, this should be short-lived as the column cools, so have not introduced a rain/snow mix at this point. Given the presence of stronger mid and upper level flow and associated fgen forcing, the potential exists for some banding to facilitate some locally increased precip/snow amounts, although forcing still doesn't look "off the charts". SLRs will probably end up a bit lower than the current system, but could envision some totals pushing 2" depending on banding development and persistence. Am also somewhat concerned about how quickly a sharp temperature drop will follow on the heels of snowfall, with some potential for any melt/wet spots to freeze through the evening. Altogether, no significant changes to Wednesday's forecast are needed at this time. Carlaw && .DISCUSSION... Issued at 233 PM CST Tue Dec 30 2025 An initial batch of warm advection-driven light snow continues to press east across the region early this afternoon. Overall weak ascent continues to support mainly small flakes. This first round of light snowfall will wind down through mid-late afternoon, with a brief gap in activity before snowfall re- develops this evening. There hasn't been much change in the guidance depiction of the main wave for later this evening with water vapor loops suggesting this feature is just now pressing south of the Minnesota Arrowhead Region. Light snow looks like it'll fill in from northwest to southeast after about 6 PM or so, with the highest coverage expected north of a roughly Rockford, IL to Rensselaer, IN line. Have commensurately increased PoPs this evening across the northeast half of the forecast area, and also smeared lower values farther southwest given the latest model guidance. Forcing overall is pretty lackluster, but there will be a brief 2-4 hour window this evening where at least modest mid-level fgen looks to provide a bit of a boost to UVVs. No major changes to the going snowfall forecast, with a general coating to 1 inch looking reasonable, with the highest values along and NE of I-90/290 and into parts of Lake and Porter Counties. Air temperatures in the 20s will result in slippery travel conditions even within the limited snowfall amounts. The bulk of the accumulating snowfall will be ending through 10 pm to midnight with a rapid loss of deeper moisture as the main pocket of ascent swings east of the region. Have noted some periodic freezing drizzle reports across parts of the Twin Cities metro area today. Forecast soundings in our region overall look a bit less supportive of a transition to FZDZ with a very quick loss of moisture above 800 mb and transition to general downward motions within the main stratus layer. Right now, this looks more supportive of things ending as a little snizzle as opposed to all FZDZ which tends to be less impactful, but will obviously keep a close eye on trends this evening. The second clipper arrives on Wednesday, with light snow expected to develop towards late morning/midday. Frontogenesis looks like it'll be a bit more of a player with this second system with an increase in upper jet support and slightly more robust thermal gradients. As a result, the higher precipitation amounts might end up a bit streakier in nature, and it's a bit difficult at this range to say where the highest amounts will fall as a result. With this current wave sort of setting up the main baroclinic zone, it seems reasonable to expect the main swath of snowfall (amounts near or locally a bit over an inch) near or perhaps a bit SW of a Rockford to Rensselaer line. In this vicinity, will need to watch for a potential for a little given the stronger fgen signal, but currently not seeing anything too concerning in the guidance. Temperatures also look to be warmer with this system, and there's some potential these push into the upper 30s south of the Kankakee River during the afternoon which could end up cutting into ratios and amounts leading to a snowfall gradient somewhere in the vicinity. Quick shot of seasonably cold air is expected Wednesday night into Thursday. As winds veer to more northerly direction and prior to column really drying out, there could be a short window of some lake effect snow into northeast Porter County Wed evening, but latest forecast soundings don't look too impressive, with a rapidly building subsidence inversion overnight. There are indications that we could temporarily transition to more of a zonal flow pattern over the weekend. This would allow for some moderation in temps and with dry conditions. Carlaw && .AVIATION /06Z TAFS THROUGH 12Z THURSDAY/... Issued at 1103 PM CST Tue Dec 30 2025 Forecast concerns include... A period of snow Wednesday afternoon. Low mvfr, possible ifr cigs through early Wednesday evening. Gusty westerly winds turning north/northwest Wed afternoon. The current light snow will be ending from north to south over the next few hours. There may be a short period of patchy freezing drizzle as the precipitation ends, then the overnight hours are expected to be dry. There will be a chance of flurries after daybreak that will continue into the early afternoon when the next system arrives. Light snow is expected to develop over the terminals in the early afternoon and move south of the terminals by early evening. During this time period, there will likely be a narrow band of moderate snow with too much uncertainty to determine where exactly that will form. Prevailing ifr vis in the 1-2sm is expected with vis under 1sm wherever this band develops. There may be some lingering flurries into the early evening and then the rest of Wednesday night is expected to be dry. Low mvfr cigs are expected through early Wednesday evening, when cigs are expected to scatter out. Ifr cigs are possible overnight and again during the period of snow Wednesday afternoon but confidence is low for prevailing ifr cigs and for now, have included only scattered mention. Westerly winds are expected through early Wednesday afternoon with gusts into the 15-20kt range possible by mid/late morning. Directions will turn northwest during the period of snow and then shift to the north/northwest by early Wednesday evening. Gusts are expected to increase into the mid 20kt range during the afternoon and continue into the early evening. Its possible there may be some higher gusts into the 30kt range as well. Speeds/gusts will slowly diminish Wednesday night. Blowing snow is also possible Wednesday afternoon, but with temperatures expected to be near freezing during the falling snow, the snow may be a bit wetter. Thus, confidence is low for how much blowing snow may materialize. Its possible the snow may become drier/fluffy prior to ending. cms && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory from 9 AM Wednesday to 3 AM CST Thursday for the IL nearshore waters. Small Craft Advisory from 9 AM Wednesday to 10 AM CST Thursday for Calumet Harbor IL to Burns Harbor IN. Small Craft Advisory until 3 PM CST Thursday for Burns Harbor to Michigan City IN. && $$ Visit us at weather.gov/chicago