textproduct: La Crosse

KEY MESSAGES

- Light snow moves into north-central Wisconsin this evening, continuing through the overnight hours. Accumulations around 1 inch expected.

- Impactful winter weather is possible Tuesday afternoon into Wednesday. Accumulating snowfall is possible north of I-94 where a Winter Storm Watch remains in effect for the potential for upwards of 6 inches of snow (30-50%). Snow accumulations are lower south of I-94 where a wintry mix is favored.

- Very cold temperatures for the end of the week. Highs will struggle to climb out of the single digits Friday and Saturday.

DISCUSSION

Issued at 229 PM CST Mon Dec 8 2025

Today

For the rest of the day, some isolated to scattered flurries are possible within the saturated low levels combined with 850hPa warm air advection moving through the region.

This evening and overnight, a shortwave trough currently over southern Saskatchewan and Manitoba per water vapor imagery will shift southeastward within the northwest flow regime in place. With some modest moisture advection under currently southerly winds, light snow is expected to develop overnight into Tuesday across much of the region. However, amounts will be light with the greatest accumulations expected for areas north of I-94 where accumulations around 1 inch are possible.

Following the snowfall overnight into Wednesday morning, RAP/HRRR soundings indicate dry air beginning to work its way into the region above -10C, resulting in a decrease of cloud ice. This will coincide with forcing via continued warm air advection and isentropic upglide on the 280K surface ahead of the next system, which may be enough to result in periods of light freezing drizzle as we head into Tuesday morning. This could result in some slick spots for the morning commute before deeper saturation moves east an hour or two after sunrise.

Impactful Winter Weather Tuesday Afternoon - Wednesday

A messy winter scenario sets up Tuesday afternoon into Wednesday as a dynamic system develops across southern Canada and the northern Great Plains before moving through the Upper Midwest. A strong 500hPa shortwave trough deepens Tuesday in association with the left exit region of a stout jet streak, promoting upward vertical motion throughout the atmosphere resulting in a deepening mid-latitude cyclone. The exact location of this feature still remains somewhat in question, but a slight southward shift has been noted in the recent GEFS/EPS solutions.

As this winter system moves through the region, a plethora of precipitation types are possible across the region. The rain/snow delineation line sets up somewhere over the Upper Mississippi River Valley, generally along a line from the Twin Cities to far southwest Wisconsin. North of this line, confidence is high that all snow will be realized and south of this line all rain. However, it's along the line and on the leading edge of the snow that is the most uncertain owing to 850hPa warm air advection fluctuating thermal profiles. RAP/HRRR soundings suggest a warm nose develops at 850hPa in response to the warm advection, nudging above freezing in some locations. This warm nose should be deep enough to melt hydrometeors falling into it before then falling into a shallow cold layer near the surface. This leads to the potential for freezing rain, primarily for portions of northeast Iowa and southwest Wisconsin where the 08.12z HREF suggests this potential is highest. However, there soundings also indicate that periods of nearly isothermal profiles along the 0C isotherm are possible which would suggest snow is favored over freezing rain leading to plenty of uncertainty in the ultimate precipitation type. Overall, thinking that the majority of the region will see either rain, snow, or a rain/snow mix, but there is the potential for a few hours of freezing rain primarily over northeast Iowa and southwest Wisconsin Tuesday afternoon and evening before a transition to rain occurs across these regions. Regarding the snowfall, areas along and north of I-94 have the highest probability to see snowfall accumulations greater than 6 inches, 30-50% per the 08.13z NBM.

Apart from the messy precipitation type situation, a strong low-level synoptic wind field sets up on the western and southern sides of the low in response to the tightening pressure gradient. Winds upwards of 50kts at 850hPa with a unidirectional profile to the surface combined with steepening low level lapse rates of 7-8C/km will allow efficient momentum transfer of these winds to the surface across the region. Probabilities for gusts greater than 40mph across southeast Minnesota, northeast Iowa, and far southwest Wisconsin are currently situated at 20-40% in the 08.00z LREF, although are noticeable higher in the 08.12z HREF which highlights 70-90%. The latitudinal extent of the low will likely play into how windy our region gets. More southerly and winds will skirt south, more northerly and winds will likely increase.

Low level moisture lingers on Wednesday with saturation and lapse rates reaching well into the DGZ. Some weak surface based instability also develops, generally 40 J/kg or less. These factors within cyclonic flow lingering over the region should lead to some additional snow showers developing on Wednesday.

Very Cold Temperatures Late This Week

When all is said and done with the Tuesday/Wednesday system, strong cold air advection takes hold, ushering colder air into the region through the rest of the week. Temperatures progressively drop Wednesday onward, currently expected to bottom out Saturday. 925hPa temps fall to -18C to -22C overnight Friday climbing only to -17C to -19C during the day Saturday across the region in the 08.00z LREF mean, suggesting lows fall into the teens below zero with highs struggling to climb out of the single digits. Along with the very cold temperatures, probabilities for wind chills of -25F or below sit at 40-60%.

AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/

Issued at 555 PM CST Mon Dec 8 2025

A round of light snow, possibly mixed at times with freezing drizzle (20-30% chance), moves through the region overnight and departs before sunrise. IFR conditions are most likely with the passage of this snow, but some localized LIFR restrictions are possible at a few sites. Conditions look to improve briefly around midday Tuesday before another round of rain/snow arrives in the afternoon--possibly mixed with freezing rain before sunset--and lasts through the overnight. The heaviest snow is forecast north of I-94 just after 00Z through the early morning where IFR to LIFR conditions are expected.

Southerly winds this evening veer overnight to the southwest at 10-15G20kts, lessing and veering more to the west for the morning on Tuesday before backing back to the south in the afternoon. These winds quickly increase for Tuesday night, though by how much is uncertain with a 30-50% chance of WNW gusts in excess of 40-45 kts west of the Mississippi River overnight into Wednesday morning.

An active weather pattern continues through the week with the risk for aviation impacts for a time nearly every day into the weekend.

ARX WATCHES/WARNINGS/ADVISORIES

WI...Winter Weather Advisory from 3 PM Tuesday to 6 AM CST Wednesday for WIZ032>034-042>044. Winter Storm Watch from Tuesday afternoon through late Tuesday night for WIZ017-029. MN...Winter Weather Advisory from 3 PM Tuesday to 6 AM CST Wednesday for MNZ079. IA...None.