Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 230 PM EST Tue Dec 19 2023 Valid 00Z Wed Dec 20 2023 - 00Z Sat Dec 23 2023 ...Sierra Nevada to Southern California... Days 1-3... An upper level closed low will settle southward just off the CA coast through Thursday before advecting eastward and moving onshore Baja California on Friday. A weak jet streak will rotate around the closed low and through the base of the longwave trough to push waves of modest diffluence into CA, which will accompany mid-level divergence and spokes of PVA to produce ascent across the area. Persistent S/SW flow downstream of the closed low will drive pronounced moisture onshore noted by a long duration of +2 to +3 sigma PWs on the NAEFS ensemble tables driven by high probabilities for prolonged IVT exceeding 250 kg/ms. The best forcing and moisture will gradually shift southward along the CA coast from D1 into D3, with the most intense overlap expected D2 into central CA and then D3 into southern CA, but with snow levels likely to be 7000-9000 ft, impacts should be limited to the highest terrain. WPC probabilities D1 reach as high as 40% in the higher terrain of the Sierra for more than 4 inches, but shift south and peak above 80% D2, when 24-hr snowfall will likely reach 12-18 inches in some areas around Kings Canyon and Sequoia National Parks. By D3 the highest probabilities shift even farther south, with limited potential (20-40%) for more than 4 inches in the southern Sierra and across the highest terrain of the San Gabriel and San Bernadino ranges. ...Pacific Northwest... Day 3... A shortwave trough emerging from the northern Pacific will approach the Pacific Northwest coast Friday morning and then continue progressively to the east to reach near the Northern Rockies by the end of the forecast period. During this translation, the shortwave is progged to deepen, and while it may remain positively tilted, will still produce robust ascent downstream through divergence, height falls, and PVA. Ahead of this shortwave, warm and moist advection within modestly confluent flow will direct weak IVT exceeding 250 kg/ms onshore, resulting in an expansion of precipitation into OR and WA along and just behind a surface cold front. Snow levels ahead of the front will be 5000-6000 ft, but will fall to around 3000 ft by the end of the period, but with weaker precipitation intensity by that time. This suggests that impactful snowfall accumulations should remain generally above pass levels, and while some light accumulations are possible at Washington, Stevens, and Willamette Passes, WPC probabilities of greater than 10% for more than 4 inches of snow are confined to the northern WA Cascades. Weiss