textproduct: La Crosse

KEY MESSAGES

- Light snow moves into north-central Wisconsin tonight with accumulations of around an inch or less possible north of I-94.

- Impactful accumulating snow possible north of I-94 where a Winter Storm Watch is in effect for the potential of accumulations of up to 6 inches Tuesday afternoon through Tuesday night. Snow accumulations quickly diminish south of I-94 with a rain/snow mix favored.

- Very cold temperatures to end the week with highs struggling to get out of the single digits Friday and Saturday. Wind chills on Saturday morning may be 20 degrees below zero or colder.

DISCUSSION

Issued at 339 AM CST Mon Dec 8 2025

Today - Tonight: Light Snow Chances in North-Central Wisconsin

A very subtle and weak shortwave can be noted on GOES-19 water vapor imagery and RAP 500mb heights across southern MN this morning which will slow push its way into the region for the morning and early afternoon. While moisture is not very plentiful with this wave, the very weak forcing may be just enough to develop a few flurries or snow showers for the morning and early afternoon. Otherwise, a more distinct wave will move through northern WI later this evening and into tonight bringing a quick round of light snow, an appetizer if you will, for north-central Wisconsin with accumulations of around 1 inch or less generally expected as probabilities in the recent HREF have medium values (30-60%) for amounts of an inch or greater through Tuesday morning in Taylor County.

Tuesday - Wednesday Morning: Rain/Snow South of I-94, Accumulating Snow North of I-94

Tuesday and into Wednesday morning will be the main focus of this forecast cycle as deterministic global models (GFS/EC/Canadian/NAM) all agree on bringing a rather dynamic clipper system through northern Wisconsin during the evening Tuesday and into the overnight. The key question with this will be the precise location of the attending surface low as any locations relative position to the low will determine precipitation types and snow accumulation potential. Overall, the frontogenetic forcing in the NAM appears more disjointed and less defined in the 600-800mb layer than in previous runs which in turn provides less confidence on the exact duration of higher snowfall rates in the 1"+/hr range during the evening on Tuesday. Much of forcing for this setup is more related to isentropic upglide on the northeast sector of the low in combination with some decent QG convergence in combination with the very favorable synoptics. As mentioned earlier, still some differences in the exact placement of this low with the operational GFS attempting to bring accumulating snow all the way down to I-90 which is in contrast with the hi-res RAP/HRRR which pushes the rain/snow line all the way into Taylor County. Overall though there has been a general slight shift across much of the global ensemble members.

In any case, currently have I-94 as the approximate delineation for rain/snow and accumulating snow. Currently, probabilities in the grand ensemble (GEFS/EC/Canadian ensemble) are fairly high (60-80% chance) for amounts greater than 3" across Clark and Taylor counties. The probabilities in this data for 5" or greater are lower (20-40%), however these might be underdispersive given that the GEFS has a much further south solution than the EC in their most recent runs. As a result, given the synoptics with a fairly deep surface low and fairly abundant moisture feeding into this system, have opted to include Clark and Taylor counties in a Winter Storm Watch in collaboration with surrounding offices as confidence is increasing for impactful snow north of I-94. Further south, there will be a sharp cutoff in snow amounts as more robust surface warming will keep rain being the predominant precipitation type south of I-94, assuming that model guidance does not continue to shift south. Cannot fully rule out a brief period of freezing rain on Tuesday afternoon south of I-94 before surface temperatures warm up into the evening. However, confidence was not high enough (under 15% chance) at this time to include any mention in the forecast at this point.

As we head into Tuesday night and early Wednesday morning, cold air advection along with an increasing surface pressure gradient on the western edge of the system will allow winds to increase across portions of southeast MN and northeast IA. Consequently, probabilities in the EC ensemble for wind gusts over 35 mph are high (40-80%). Additionally, with steep low-level lapse rates, some instability and low-level frontogenesis with the cold advection early Wednesday morning, could see a period of snow showers with the gusty winds as the cold advection pushes through. Overall, accumulations south of I-94 with these snow showers would be fairly minimal, on the order of a few tenths.

Wednesday - Saturday: Getting Progressively Colder

As we head into the second half of the work week, northwesterly flow combined with the cold advection in the wake of our Tuesday system will establish a persistent fetch of colder air into the region with progressively falling temperatures through Saturday. As a result, NBM generally progs highs on Wednesday from the 20s falling into the single digits and low teens by Friday and Saturday. Overall Friday and into Saturday looks like a fairly cold period overall as 850mb temperatures depicted in the GFS/NAM reaching to around -20C. Consequently, the grand ensemble (GEFS/EC/Canadian ensemble) have already fairly high probabilities (50-70% chance) for wind chills under -25F for Saturday morning with even a small chance (10-20%) for wind chills colder than -35F in southeast MN. This will be combined with likely (60-80% chance in the NBM) highs only in the single digits above zero on Saturday. Needless to say, this will probably be the coldest conditions we have experienced so far this season.

As far as snow chances are concerned, it is difficult to pinpoint any exact features with periodic bouts of surface high pressure building into the region as well. However, given the northwesterly flow and occasional weak perturbations that attempt to sneak through the flow, would not rule out some snow chances late week, especially on days where low-level lapse rates are steeper. As a result, the grand ensemble has broad brushed medium to high probabilities (50- 70% chance) for measurable snowfall throughout this period across the entire local area. Therefore have elected to hold onto the precipitation chances in the NBM for Thursday and beyond.

AVIATION /12Z TAFS THROUGH 12Z TUESDAY/

Issued at 519 AM CST Mon Dec 8 2025

Low ceilings spread east through the day with conditions eventually becoming MVFR across much of the area by 00z and IFR by the end of the period. For areas west of the Mississippi, IFR will arrive sooner, by 03z. Some flurries may occur today, particularly around RST, but occurrence and impact to visibility is expected to be brief. More consistent light snow will likely overspread areas roughly north/east of an EAU-LNR line but may extend to the Mississippi River. Have therefore retained PROB30 snow mention with this update.

ARX WATCHES/WARNINGS/ADVISORIES

WI...Winter Storm Watch from Tuesday afternoon through late Tuesday night for WIZ017-029. MN...None. IA...None.