Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 312 PM EST Sun Feb 12 2023 Valid 00Z Mon Feb 13 2023 - 00Z Thu Feb 16 2023 ...Four Corners region & Upper Midwest... Days 2-3... The Four Corners region will be exceptionally snowy in the short range period starting off with an upper low moving through the Desert Southwest Sunday night and tracking across AZ and NM on Monday. 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 through Monday AM and into NM by Monday evening according to both the NAEFS and Euro situational awareness tools (SAT). While not as anomalous, there will be sufficient 850-700mb moisture flux to work with as well throughout the Four Corners region. Plenty of synoptic-scale forcing aloft with a strong 150 knot jet streak over northern Mexico and southern NM Monday morning placing 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, the San Juan, and Sangre De Cristo. The latter two ranges mentioned that WPC PWPF have with the best chances (50-70%) for snowfall totals >6". The latest WSSI shows these ranges with Minor impacts on Monday, suggesting some impacts to travel are possible. As the upper low exits the Four Corners region, the trough will begin to take on a negative tilt by Tuesday morning and the upper low will re-strengthen as it tracks northeast towards the Midwest. By 18Z Tuesday, both NAEFS and Euro SAT show 700mb heights are near the lowest observed for the climatological period of record for Feb 14 at 18Z. At the same time as this powerful 500-700mb low moves northeast, a cold front will track into the Dakotas and Upper Midwest, supplying CAA into the region. As 850-700mb moisture wraps around the strengthening cyclone over the Midwest, a comma head precipitation shield will be in the form of snow from the eastern Dakotas to the MN Arrowhead and northern WI Tuesday night into Wednesday. Latest experimental WPC PWSSI showed a swath of 60% probabilities for Minor impacts from northeast SD and southeast ND to northern MN, to go along with a 10-30% chance for snow totals >6". Given the colder temperatures once the cold front ushers in sufficiently cold temps in the boundary layer and the strong dynamics at play, it is possible to see probabilities for >6" increase in these areas in future forecast cycles. Expect hazardous travel conditions in these regions Tuesday night and into Wednesday, largely due to heavy snowfall rates and gusty winds resulting in reduced visibilities. ...Pacific Northwest, Central & Southern Rockies, & Central High Plains... Days 2-3... As one upper trough exits the West, another arrives in the Pacific Northwest tonight while a cold front associated with a northeast Pacific low pressure system tracks introduces a surge of sub-freezing temps into WA and OR. Snow levels will quickly drop to as low as 1,000-1,500 ft Monday morning across WA with similar snow levels in OR by Monday night. Levels will only continue to drop to as low 1,000 ft in these areas by Tuesday morning. This matters because there will be a steady stream of 850mb moisture funneling into the Cascades and Olympics through Tuesday morning, allowing for persistent upslope flow within an environment suitable for higher SLRs. This is a prime setup for heavy snow in these ranges with WPC PWPF showing up to 50-70% probabilities for >12" of snowfall. The Days 1-3 WSSI for Overall Impact shows a large footprint for Moderate impacts in these ranges, as well as along the Coastal Range of OR, the Siskiyou of southwest OR. Travel on roadways in these areas is likely to be treacherous throughout the first half of the week. The upper low looks to amplify Tuesday night as it tracks into the Four Corners region Wednesday morning. This upper low, according to both NAEFS and Euro SATs, will feature temperatures at the 500-700mb levels that are among the lowest observed on the CFSR period of record for Feb 15. In addition to the cold, strong vertical ascent will be directed downwind of the trough over the Southern Rockies and into the central High Plains where an influx of 700mb moisture will be present. Lastly, a cold front diving south will also introduce not only sub-freezing temperatures into the central Plains, but also provide some low level injection of 850mb moisture as well across eastern CO and western KS. This sets the stage for heavy snow from the Mogollon Rim, the San Juans, and the CO Front Range to the High Plains of southeast CO, northeast NM, and southwest KS. There have been subtle adjustments south with the QPF shield and storm track over the last 24 hours, largely due to the storm in the Midwest on Day 2 and a deepening trough in the Canadian Prairies helping to suppress the storm track and force the upper low to open up into a more progressive feature. While these details can still change, the ingredients are still present for a disruptive snow storm in portions of the Rockies and the Central Plains Wednesday and into Wednesday night. Latest WPC PWPF depicts 60-80% chances for >8" of snowfall along the Mogollon Rim of AZ and the San Juans of CO, which combined with strong wind gusts would support treacherous travel conditions. The WPC WSSI shows an expansive area of Moderate impacts in the higher terrain of southern UT, along the Mogollon Rim and in northeast AZ, and in the San Jauns of CO. Meanwhile, WPC PWPF shows 50-70% probabilities for >8" of snow in southeast CO, while the Palmer Divide has 40-50% probabilities on Wednesday. It is worth noting that currently some locations, such as the Denver metro area, reside on the northern periphery of the storm track. Any additional shift south could see snowfall totals continue to decline, but as it stands now, latest WPC PWPF does show a 70% chance for >4" of snowfall through Wednesday afternoon. For impacted areas of eastern CO, northeast NM, western KS, and potentially as far south as the OK and far northern TX Panhandles, periods of snow and gusty winds will likely result in hazardous driving conditions Wednesday afternoon, but given the lingering uncertainty in storm track, confidence in where the worst conditions transpire is low at this time. Mullinax ...Key Messages for Feb 14-17 Winter Storm... --A winter storm will likely develop near the Four Corners on Tuesday and spread a swath of heavy snow northeastward across the Plains and into the Upper Midwest by Thursday. This system may continue into the Great Lakes on Friday. --A corridor of mixed precipitation, including sleet and freezing rain, is also possible along this storm track. --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. --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.