Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 459 AM EST Mon Feb 13 2023 Valid 12Z Mon Feb 13 2023 - 12Z Thu Feb 16 2023 ...Pacific Northwest and Northern Rockies... Days 1-2... Lead shortwave in a strung-out swath of vorticity tied back to the Gulf of Alaska will move through the Pac NW today, with the cold front already onshore. Nose of a 100kt jet will push into OR this afternoon as upper divergence provides for broad lift over the area. Upslope enhancement into the Cascades and Coastal Ranges will compensate for only modest PW values (near normal due to the source region). With this front will be an initial intrusion of colder air, with snow levels dropping from around 2000ft down to near or just above sea level by early Tuesday in association with the second half of this 1-2 punch. Lowland snow is quite possible as the precipitation ends late D1 (early Tuesday), and some minor accumulation is possible around Portland. Otherwise, more than a foot of snow is likely for the Cascades with significant snow possible for the passes. Cold upper low (700mb temps near -20C or -2.5 sigma) will stream through the area D2 in tandem with a strong 150kt jet, squeezing out a few more inches over the OR Cascades and Klamath Mountains into NorCal before ending later in the day. Eastern part of the lead shortwave on D1 will move through ID and WY with surface cyclogenesis over far western SD. Northeasterly flow combined with height falls into the area will sustain a period of modest snow for south central Montana into northern Wyoming as a front dips southward from Canada. Tail-end of this front will favor additional snowfall back to the ID ranges and northwest MT where more than 6 inches is likely in the higher terrain. WPC probabilities >30% for at least 4 inches of snow encompasses a large area on D1.5 across much of central/southern MT southward through the Tetons, Absarokas, and Bighorns. ...Four Corners region & Upper Midwest... Days 1-2... Deep upper low over northwestern Mexico will weaken as it moves across AZ and NM today. The upper low is strong and anomalous with heights and temperatures in the 500-700mb layer within the upper low below the 10th climatological percentile over southern AZ according to both the NAEFS and Euro situational awareness tools (SAT). Modest moisture and flux will overspread the Four Corners region today as a strong 150 knot jet streak over northern Mexico/southern NM places its diffluent left exit region over the southern Rockies. Mean 850-300mb winds are also oriented out of the S-SW, which favors beneficial upslope flow into mountain ranges such as the Mogollon Rim, San Juan, and Sangre De Cristo. The higher elevations of the San Juans and Sangre de Cristos will see higher snowfall D1 as the upper low heads toward the Raton Mesa, likely over 6 inches. On D2, the upper low exits the Four Corners region, the trough will begin to take on a negative tilt and re-strengthen as it tracks northeastward towards the Corn Belt. Surface low will lift out of southeastern CO into IA and southeast MN, drawing a moisture plume from the Gulf northward up and over the warm front and around the surface low. There, temperatures will support mostly snow, especially as colder air is entrained on increasing northerly winds. Recent trends have been toward a stronger system, and accumulating snow is likely in a stripe from northeastern SD to northern MN (WPC probabilities of at least 4 inches are >30% here). Combination of strong winds and even modest snow may create hazardous travel conditions Tuesday night. ...Four Corners into the Central Plains... Days 2-3... Second part of the PacNW 1-2 punch will dive through the Great Basin D2 then eastward to the Central Plains by the end of D3. Snow levels will eventually drop to all valley floors outside the lower deserts as the precipitation ends and the core of the cold arrives late Tue into Wed (700mb temps near -3 sigma or below the 1st percentile). WSW flow ahead of the upper low coinciding with a modest moisture plum from the northeast Pacific will favor the Mogollon Rim (again) and also into the San Juans and Sangre de Cristos, but with more snow than on D1. WPC probabilities for at least 8 inches of snow D2 are high over these areas. By D3, as the upper low move eastward along the CO/NM border, surface cyclogenesis over NM will move eastward along the front with northeasterly flow over southeastern CO, where significant snow is possible. By late Wed and into early Thu, the whole system will lift northeastward with a stripe of modest snow on the northwest side of the low across KS and into southeastern NE and into IA. Trends in the guidance have favored a bit weaker/sheared system moving through the West with a bit flatter ejection through the Plains, and had adjusted the snow axis southward as well. Though some models were much drier than the favored consensus, combination of colder air (higher SLR) and potential meso banding resulted in still modest amounts despite a bit lower probabilities overall. Nevertheless, as the hi-res guidance comes into focus there will be time to modify amounts/location. WPC probabilities for at least 4 inches of snow are high (>70%) over southeastern CO and moderate (>40%) northeastward across central KS. Fracasso ...Key Messages for Feb 14-17 Winter Storm... --A winter storm will develop near the Four Corners on Tuesday and spread a swath of heavy snow northeastward across the Plains on Wednesday. This system may continue into the Midwest and Great Lakes Thursday. --The combination of heavy snow and strong winds will produce blowing and drifting of snow, resulting in hazardous travel conditions and possible impacts to infrastructure. --A corridor of mixed precipitation, including sleet and freezing rain, is also possible along the storm track over the Plains. --Uncertainty remains with the timing and location of the storm track, which determines where the axis of heaviest precipitation and most significant impacts will occur. --Much colder temperatures, falling to well below normal values and perhaps near record lows over the Southwest, will follow in the wake of this system.