Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 350 PM EDT Wed Oct 19 2022 Valid 00Z Thu Oct 20 2022 - 00Z Sun Oct 23 2022 Days 1-2... The probability of snow accumulations 4 inches or greater is less than 10 percent. The probability of significant icing is less than 10 percent. Day 3... ...Pacific Northwest & Northern Rockies... There is growing confidence in a significant winter storm developing in the mountainous terrain in the Pacific Northwest and Intermountain West this weekend. An amplifying upper trough is set to dig south and east through the Northwest resulting in falling heights region-wide. Simultaneously, a cold front tracking south will usher in sufficiently colder temperatures from the top-down across the higher terrain. Warm air advection and a steady stream of positive vorticity advection ahead of the upper trough will support large scale vertical ascent within the atmospheric column. A robust 180 knot 250mb jet streak in the northeast Pacific will have its diffluent left-exit region positioned overhead Friday night into Saturday as well, further aiding in supporting rising motion aloft. By Saturday afternoon, a secondary jet streak over the northern Rockies will allow for additional diffluence beneath its right entrance region over western Wyoming and northern Utah. The atmospheric setup contains a number of anomalous and influential ingredients, most notably moisture. 00Z 10/19 NAEFS climatological percentiles for 500mb mean specific humidity are in the 97.5-99% range Friday night into Saturday, implying an abundance of atmospheric moisture at mid-levels that are close to the DGZ. 00Z NAEFS also identified IVT values anywhere between the 90-97.5% climatological percentile for much of Day 3. This steady moisture transport coincides with enhanced vertical ascent not just in the synoptic scale, but along orographically favored mountain ranges that are positioned orthogonal to the westerly mean 850-300mb flow. WPC PWFP shows the highest >4" snowfall probabilities along the Cascade Range on east to the Sawtooth, Boise, Absaroka, and Teton Ranges which are >40% for all of these regions. The heaviest totals will be primarily confined to elevations >5,000' but at lower elevations, boundary layer temperatures will be more marginal or, in some cases, simply too mild to support snow. The latest experimental pWSSI features a >30% chance for "Moderate" snowfall impacts in the tallest peaks of the ranges mentioned above. These probabilities are currently weighted toward snow rate and snow load, but given this even bleeds into Day 4, eventually snow amount will be weighted more as the winter storm unfolds deeper into Day 4 (Sunday). Overall, this winter storm will mark the first of the season for much of the region and will likely lead to deteriorating travel conditions throughout the hardest hit areas this weekend. The probability of significant icing is less than 10 percent. Mullinax