Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 435 AM EDT Mon Apr 10 2023 Valid 12Z Mon Apr 10 2023 - 12Z Thu Apr 13 2023 ...Pacific Northwest... Days 1-2... A deep low over the Gulf of Alaska will shift southeast to western Washington through Tuesday and then shifts to Idaho through Wednesday night. An atmospheric river ahead of this low and related trough is streaming into the OR coast with high moisture and high snow levels which will drift inland today. Day 1 snow probabilities are moderate to high for 6"+ for the WA Cascades and Olympics. The upper trough axis south of the deep low shifts over the WA/OR coast late tonight with the upper low moving over the WA coast Tuesday evening. Snow levels down near 2000ft are under the upper trough with much more widespread precip and Day 2 snow probabilities for 6"+ over the Cascades north from central OR (which includes all major mountain snow passes) and the Olympics with moderate probabilities over the northern OR Coast Ranges and over the Clearwater Mtns of ID. The upper low passage over western WA/OR Tuesday night brings snow levels down to 500ft or so near Seattle and Portland. Steep lapse rates noted in regional forecast soundings support convective snow rates which could allow precip loading to locally drag snow levels down to sea level. Some low Day 2 snow probabilities for 0.1" and 1" reach the lowlands around Portland, OR. One note about this case is the 500mb closed contour is 536dm while the contour from 12Z April 11, 2022 when there were several inches around the Portland metro was 526dm - a notable difference in the overall thermal profile. ...Northern Rockies... Day 3... As the upper trough pushes over the Pacific Northwest Tuesday night/Wednesday, lee-side cyclogenesis occurs over WY, promoting NEly flow over the northern High Plains through the south-central MT ranges with an increasing connection to Gulf moisture streaming up the Plains. Heavy snow from convergent flow with topographical enhancement gets going Wednesday night with Day 3 snow probabilities moderate for 8"+ over south-central MT, including the Crazy Mtns. The probability for icing over 0.1" is less than 5% Days 1-3. Jackson