FXUS63 KARX 111955 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 155 PM CST Wed Dec 11 2024 .KEY MESSAGES... - Snow showers continues this afternoon - Temperatures fall quickly this afternoon and tonight. Wind chills tonight fall to-15 to -30F, coldest north of I-94 where a Cold Weather Advisory is in effect. - Our next round of winter weather for late Friday night into Saturday morning presents a complex and dynamic forecast situation. Snow, freezing rain, and rain are all on the table for potential weather types, but confidence is not high in what precipitation type will fall at any given location. - Another round of rain or snow looms on the horizon for Monday night into Tuesday. && .DISCUSSION... Issued at 150 PM CST Wed Dec 11 2024 This Afternoon: Snow Continues Various bands of low level frontogenesis coinciding with a saturated DGZ and lift from a subtle shortwave trough at 700hPa have allowed snow showers to continue into this afternoon. Snow showers will move east to southeastward with time as the aforementioned forcing moves east, ending by 6pm for our eastern counties. Slick spots on the roads and reduced visibility in snow showers will create challenges for those on the roads during the evening commute. Use caution if driving! Tonight: Rapidly Dropping Temperatures In the wake of the upper trough and northwest flow set-up across the region, arctic air will continue to be advected into the region this afternoon into the overnight hours, dropping temperatures below zero. Given ample CAA and partly cloudy to mostly clear skies expected, have opted to trend overnight lows towards the 25th percentile of guidance although there is some uncertainty in exact low temperatures given the potential for more low level cloud coverage overnight than currently expected. However, these cold temperatures combined with the gusty winds assocaited with the steep lapse rates from the CAA, apparent temperatures drop into the -20s overnight. Taylor and Clark counties in Wisconsin will feel the coldest apparent temperatures, dropping below -25F for several hours overnight. As such, a Cold Weather Advisory has been issued for these two counties. If temperatures over-perform tonight, additional counties that are right on the edge of advisory criteria may need to be added. Late Friday Night - Saturday: A Messy Wintry Mix Our well-advertised end of week weather system remains on track to move through the region late Friday night into Saturday. While confidence is high (90-100%) that we will see precipitation during this period, the devil is truly in the details when trying to nail down the potential precipitation types, amounts, and impacts. Medium range clusters are coming into fast agreement with the general synoptic pattern of a meridional shortwave/closed upper level low slicing eastward through the mean longwave ridge, with modest return flow off the Gulf lifting northward in advance of the system. In fact, PWAT values within this warm sector reach into the 90-95th percentile of the ENS/NAEFS model climatologies for this time of year. Strong 280-300K isentropic and kinematic ascent will be present along the elevated warm front lifting northward during the morning hours. If anything, there has been a slowing of the system from earlier forecasts and the main impacts look to be focusing more in the Saturday morning period. While moisture and lift do not look to be the problem, the gorilla on the back of this forecast is the thermal profiles of the lower troposphere/surface. Surface temperatures will be below freezing and only slowly warm through the morning under a cool ESE flow. Warm nose temperatures aloft will start to push above freezing about 6-12 hours before surface temperatures can do the same, setting the stage for a potential freezing rain event. A change of only a few degrees in either direction will mean the difference between freezing rain, snow, or rain. In such scenarios, especially 2-3 days in the future, it is more beneficial to assess trends in the model outputs versus latching onto specific values/solutions. Over the last 24-36 hours, LREF model warm nose temperatures have been inching warmer, with the 100-member mean 850-mb temperatures increasing by 2-3 degrees C and now pushing above 0 C south of I-90. The NBM freezing rain probabilities have jumped by 20% to 30% over the last 24 hours, with the highest values of 30-40% along and west of the Mississippi River. A run to run comparison of the extended SREF/WPC super ensemble members shows a trend of more members allocating their QPF to ice. In fact, the explicit ice accumulation outputs from the WPC ensemble has around 0.10" of ice west of the Mississippi River, with the most extreme outliers exceeding 0.25" in these areas. Freezing rain isn't the only concern we have to worry about. If the warm nose cools (especially north of I-90), the precipitation type may swing back to snow. While the WPC ensemble mean snowfall for areas north of I-90 is around 1-2", there is a modest skew in the data towards higher values with the outliers pushing 3-5 inches. Trends in the guidance with respect to snowfall have been more complex with the higher probabilities generally staying north of I- 90. Surface temperatures/wet bulbs are progged to warm above freezing for most locales by the afternoon, but how soon this occurs will play a role in snow/freezing rain amounts/impacts. This is a forecast that will likely contain a respectable degree of uncertainty right up until the event, so please check back for further updates over the next few days. Monday into Tuesday: Next Round of Rain/Snow Our progressive weather pattern ushers yet another weather system to our doorstep for the start of next week. While confidence is high in the presence of this next shortwave, the medium range guidance increasingly on the larger synoptic details of the upper wave orientation/position and resultant surface features. The 500-mb guidance clusters are fairly evenly distributed between a more progressive and amplified pattern, the latter of which could result in appreciable precipitation for the region. It remains too far to nail down much more for specifics on the system (precip types, timing, etc.), but the messaging remains the same that folks should keep tabs on the forecast as we head into the weekend. && .AVIATION /18Z TAFS THROUGH 18Z THURSDAY/... Issued at 1203 PM CST Wed Dec 11 2024 Snow continues along and north of I-90 east of the Mississippi River this afternoon, decreasing visibilities to 3SM or less. Another band of snow is currently moving across southeast Minnesota and northeast Iowa briefly, but rapidly, decreasing visibilities to 3SM or less. Winds also increase behind the band with a peak wind of 29KT noted at KRST. Expecting this band to be at the Mississippi River by 20z and have accounted for this with a TEMPO group at KLSE. Snow exits the area into eastern Wisconsin this afternoon and evening allowing for clearing skies, the exception being Clark and Taylor counties, where ceilings around 1-2kft may persist into the overnight hours. RAP soundings do suggest lower clouds around 1.5-2kft may linger across the rest of the forecast area this afternoon and into the overnight although confidence remains low on if these become ceilings. Otherwise, gusty northwest winds this afternoon continue into Thursday morning, diminishing throughout the day. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...Cold Weather Advisory from 10 PM this evening to 10 AM CST Thursday for WIZ017-029. MN...None. IA...None. && $$ DISCUSSION...Falkinham/Skow AVIATION...Falkinham