Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 346 PM EDT Sat Mar 20 2021 Valid 00Z Sun Mar 21 2021 - 00Z Wed Mar 24 2021 ...Central Rockies into Central High Plains... Days 1-2... A positively tilted longwave trough will drop slowly southeast from the Pacific Northwest towards the Four Corners through Monday morning before amplifying into a negatively tilted trough over the Southern Plains Tuesday in response to sharp shortwave energy rounding the base. As this trough deepens, an upper jet streak will intensify over the Southwest leaving the diffluent LFQ atop the Central Rockies. The combination of these features will drive lee cyclogenesis Sunday night, and this low is likely to lift quickly northeast into the Central Plains by Tuesday morning. Pacific moisture being transported inland will be wrung out by the deep layer ascent, aided by WAA on SW flow and intensifying height falls. As the surface low develops over far NE NM and lifts northeast, low-level flow will turn E/NE, producing upslope enhancement into the Front Range and Palmer Divide, especially Monday morning. The heaviest snow D1-2 is likely in the Front Range and Palmer Divide, as well as the terrain west of the Front Range. WPC probabilities D1 are high for 6" in the western CO terrain, shifting eastward D2 with locally more than 12" possible in the Front Range. The thermal profiles east of the terrain are marginal for snowfall, but persistent SW flow aloft and the likelihood for a deformation axis to enhance ascent to dynamically cool the column should produce a few inches of snow along the I-25 urban corridor and into the high plains of NE CO. WPC probabilities are high for 2" across these areas, with potentially higher amounts possible. ...Pacific Northwest through the Northern Rockies... Days 1-3... An upper jet extending southwest to northeast across NV to central MT leads to a combination of a slowly moving frontal boundary plus moisture advection from the Great Basin into Wyoming on D1. Well defined and persistent 700 mb convergence maxima helps lift the available moisture, in conjunction with upper divergence maxima and orographics in windward areas to produce moderate to heavy snow. The heaviest snow is forecast in the Big Horn range of WY where WPC probabilities are high for 6", and locally more than 12" is possible. As this first system shifts southeast, a secondary jet streak will dive out of the Gulf of Alaska and onshore the Pacific Northwest coast Monday. This will spread moisture, upper diffluence within the LFQ of this jet streak, and 700mb convergence into the region. This will squeeze out heavy snow in the terrain of the Olympics, WA and OR Cascades, OR Blue Mountains, and into the Northern Rockies D2 with WPC probabilities high for 6 inches. Snow levels will be such that even at the Cascade Passes accumulations could reach 12". The jet streak and associated forcing weaken and drop southeast on D3, leading to renewed light snow accumulations in the Absarokas, Tetons, and Big Horns. For Days 1-3, the probability of significant icing (0.25 inch or more) is less than 10 percent. Weiss