Expires:No;;140790 FXUS63 KARX 040505 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 1105 PM CST Tue Dec 3 2024 .KEY MESSAGES... - VERY WINDY Wed afternoon-evening. Wind Advisory for most locations west of the Mississippi river (could be expanded east). Wind gusts 35 to 45 mph for entire area with sustained from 20 to 30 mph. Winds WILL impact travel and blow lightweight objects around (holiday decorations). - BITTER COLD start to Thu with combination of wind and cold air producing 5 to 15 below wind chills. Bundle up for your commute to work/school. - SCATTERED SNOW SHOWERS Wed (20-50%). Accumulations look minimal at this time, but could provide further travel impacts with sudden reductions in visibility, some slick spots on roads. - Cold is short-lived with above normal air moving in for the weekend - back in the 40s for some! && .DISCUSSION... Issued at 215 PM CST Tue Dec 3 2024 - WED/THU MORNING: windy, colder and low end snow shower chances (20- 50%) OVERVIEW: upper level shortwave trough still on track to drop southeast out of Canada tonight, digging across the great lakes Wed night, developing a negative tilt as it drives over the new england states by thu evening. Although the associated sfc low will hold across southern Canada, a couple related sfc fronts will move across the region. Very strong cold air advection, some low level instability, and tightening pressure gradient accompany the system, promising strong/gusty winds, much colder air, and some snow shower chances. The bulk of its impacts will be shifting east with the storm Thu morning. WINDS: a lot of elements progged to come together and foster a very windy period for the region with Wed afternoon-evening likely harboring the strongest gusts for the local area. 1) Tightening sfc pressure gradient 2) Unidirectional winds (northwest) through nearly the entire column Wed aft/eve. 3) Mixing upwards of 825 mb with RAP/HRRR/NAM/GFS bufkit soundings suggesting upper 30s to around 40 kts will be brought to the sfc for the afternoon/early evening Wed. 4) Trop fold could bring even stronger winds surfaceward, with the 1.5 PV sfc reaching 750-800 mb in the NAM/GFS, but as low as 850 mb in the EC. The EC has been the most robust with this. All this has been a very consistent signal in the models the past several days. The EC (as already alluded to) has been the most bullish on strong winds. Essentially 100% of its ensemble members paint 40 mph gusts for the entire forecast area for Wed aft/eve. In its 03.12z run, 20 of the EPS members suggest 50+ mph could be attained Wed afternoon at KRST. In addition, 03.00z EFIs are 0.8+...an increase over already high values from previous runs (0.7+). The EC is "all in" for a high wind event, perhaps too much as it is running at the top end of the guidance. That said, ample support from the other short/medium range guidance that 45 mph gusts could be reached, along with periods of sustained 30 mph. Per coordination with surrounding offices, will issue a Wind Advisory for most of SE MN/NE IA for 18z Wed through 03z Fri. May need to expand this eastward - and will let further model updates, realtime observations, steer that need. Eitherway, it will be a very windy day Wed/Wed night. In addition, with how cold it is expected to get, any potential loss of power due to strong winds will have added impacts on heating. Prepare accordingly. TEMPS: don't expect much drop in temps from the early evening through the overnight as the southerly fetch/low level warming persists. Much colder air starts to return after 12z Wed, in earnest post the second front. Highs for most will be by noon, with steady or falling through the afternoon. 850 mb temp anomalies via the NAEFS and ECMWF are around -2.5 for Wed night while determinist runs of the EC/GFS 850 mb temps fall from near 0 at 12z Wed to -15 C by 12z Thu. The Grand Ensemble places a 80-100% for single digit lows west of the Mississippi River Thu morning with less chances north of I-94 where clouds could be more prevalent. Add in the strong northwest winds and wind chills from -5 to -15 should be planned for. Bundle up early morning commuters to work/school Thu. PCPN CHANCES: a variety of lifting mechanisms will provide multiple means to force snow showers Wed into Wed evening. A loft, the upper level trough drops southeast across WI with mostly mid/upper level QG convergence with it. At the sfc, while the system's associated low will be tracking west-east just north of Great Lakes, 2 cold fronts will stretch south/southwest of the low. The first is progged to slide southeast across the region in the early morning hours (12- 15). The second takes a similar track during the afternoon. Strong cold air advection in the low levels will promote 1000:850 mb lapse rates of +9 C/km (mostly behind the second front). Meanwhile RAP/HRRR Bufkit soundings depict weak instability through the day. Same soundings point to enough saturation to form clouds/showers. While there is initially weak, disjointed low level Fgen along both sfc fronts, CAMS modeled reflectivity depicts mostly cloud streets/convection parallel to the mean wind (not in relation to the frontal lift). RAP does become more organized and increases the low level Fgen as the second front shifts southeast of the local area, so perhaps frontal orientated convection would increase then. Short term guidance is low with its snow chances - perhaps more reflective of their low QPF output. The forcing, however, says chances should be higher. Will increase above the model blend with this in mind. Where snow falls, amounts do look relatively minor. That said, if parallel bands do form, very localized snow could drop for a longer period of time - and thus, higher amounts could be realized. A lot of "ifs" here, but its a scenario to monitor. - WEEKEND: warming up! Change in the upper level pattern moving into the weekend with the zonal/broad ridging taking over. Latest deterministic runs have 850 mb temps climbing from the teens below zero at 12z Thu to the single digits above for Sat-Mon. High temps in EPS suite of ensemble members have placed 75% of its members in the low/mid 40s for highs Sun, just a few degrees cooler either side of that day. A very consistent signal in the EPS. The GEFS has been cooler than the EPS, but has shown some increase over its past few runs. Either way, all trends point to at/above normal temps moving in for the weekend and the start of the new work week. The change in the pattern could allow for bits of shortwave energy to spin near/across the region. Current outlay of the ensembles suggest the higher pcpn probabilities would lie north/south of the forecast area. && .AVIATION /06Z TAFS THROUGH 06Z THURSDAY/... Issued at 1103 PM CST Tue Dec 3 2024 We are looking at VFR conditions with southerly winds between 10 and 20 knots for tonight. In the wake of a strong cold front on Wednesday morning, winds will shift to the northwest. Sustained winds will be 20 to 30 knots with gusts of 30 to 45 knots. In addition to the strong winds, there will be MVFR ceilings in the wake of this front. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...Wind Advisory from noon to 9 PM CST Wednesday for MNZ079-086- 087-094-095. IA...Wind Advisory from noon to 9 PM CST Wednesday for IAZ008>010- 018-019-029. && $$ DISCUSSION...Rieck AVIATION...Boyne