Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 409 AM EST Sun Feb 12 2023 Valid 12Z Sun Feb 12 2023 - 12Z Wed Feb 15 2023 ...Southern & Central Appalachians... Day 1... Deep closed low over the Southeast coastal plain this morning will exit off the North Carolina coast this evening. Exiting northern stream jet coupled with the subtropical jet across the Gulf will provide a large area of divergence and broad lift over the Southern Appalachians today. Surface low along the front will deepen as mid-level height falls approach from the west, but the triple point will quickly take over along the coast this morning. Plentiful moisture will stream in from the south/southeast over the Atlantic on the WCB around the surface low and into the terrain. Temperatures have been marginal thus far, but as mid-level cooling approaches and the deformation band organizes rain will turn to snow over higher terrain where it will accumulate several inches D1. Some areas that have slipped below freezing may see some icing as the strong WAA above the deck continues ahead of the low. Precipitation winds down early Monday as the low exits into the Atlantic. WPC probabilities of at least 4 inches of snow are low-moderate (<70%) and confined to the higher terrain above about 3500-4000ft over far western NC along the TN border and into far southwestern VA. ...Four Corners region and the Upper Midwest... Days 2-3.. Closed low will move across AZ/NM D2, with modest moisture into the Four Corners region. Upslope enhancement into the Mogollon Rim and White Mountains will yield several inches of snow, especially above about 6000ft as the low moves through. Additional ascent into the higher terrain of southwestern CO and northwestern NM (San Juans, Jemez mountains, and southern Sangre de Cristos) will yield higher snowfall totals in excess of 6 inches, but again confined to higher elevations above about 7000ft. The upper low will continue to track ENE then northeastward into D3, moving into the Corn Belt Tuesday. Surface cyclogenesis in southeastern CO will move northeastward, ahead of which will be an influx of Gulf moisture northward and around the low pressure. Marginally cold air on the northwest side of the low will support a wide stripe of light snow from SD across central/northern MN as other areas to the south will be too warm. WPC probabilities of at least 4 inches of snow are low -- generally below 40%. By Day 3, an amplifying upper trough diving south through the Pacific Northwest acts to provide exceptional lift within the column over the Great Basin and into UT, western CO, and northern AZ. Timing differences remain amongst deterministic guidance, but all guidance agrees on a strong cold front traversing the Four Corners region by Tuesday afternoon, leading to plummeting snow levels and rising SLRs. The experimental WPC PWSSI is showing probabilities ranging between 30-50% for Moderate impacts Tuesday afternoon and evening from the mountain ranges of southwest UT, north-central AZ, and the San Juans of southwest CO. These areas are also sporting >60% probabilities for snowfall >6" according to the WPC PWPF. Expect hazardous travel conditions in the higher terrain of the Four Corners region Tuesday night and into Wednesday morning as the upper trough makes its way overhead. ...Pacific Northwest through the Rockies... Days 2-3... Successive upper troughs will dive southeastward out of the Gulf of Alaska starting early Monday, ushering in much colder air for the western third of the CONUS into Tuesday/Wednesday. Though PW values will be near normal for mid-February, combination of broad lift via upper divergence in the LFQ of a 150kt jet, strong height falls/PVA, and upslope enhancement will yield modest snows for the Cascades and several inches farther east into the Blue Mountains and central ID ranges as the lead front pushes to the Divide. Snow levels will start near 3000ft but fall below 1000ft and eventually to sea level (as the precipitation ends) by D3. Lead vort will spur sfc low development over southeastern MT/northern WY into D3, with additional modest snow likely for southern central MT. Secondary vort will be the driver of the much colder air as it coalesces into another closed low over the Four Corners by early Wednesday. Light lowland snow is quite likely as the precipitation ends over western WA/OR with a few more inches for the OR Cascades D3 with the trough axis, but the bulk of the snow D3 will be over the Four Corners region. Strong height falls and westerly flow into the terrain will yield modest to significant snow for some areas as snow levels will fall to all valley floors outside the deserts in AZ though amounts will be much lighter in lower elevations. Given the track of the upper low, focus will be over southern UT/northern AZ/southwestern CO D3 where WPC probabilities of at least 6 inches of snow are >50%. Fracasso ...Key Messages for Feb 14-17 Winter Storm... --Confidence is increasing that a storm developing near the Four Corners on Tuesday will spread a swath of heavy snow northeastward across the Plains and into the Upper Midwest by Thursday. --A corridor of mixed precipitation, including sleet and freezing rain, is also possible along this storm track. --The combination of heavy snow and mixed precipitation, especially where combined with strong winds, could result in hazardous travel conditions and 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 record lows, will follow in the wake of this system.