Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 414 AM EDT Mon Mar 22 2021 Valid 12Z Mon Mar 22 2021 - 12Z Thu Mar 25 2021 Days 1-3... ...Central and Southern Rockies/Southwest... A 700 mb front and developing circulation therein crosses northeast NM early today and moves northeast across the central Plains. Low level convergence and ascent continues along the track of the front and low, so the primary focus will for heavy snow is over the Sangre De Cristo Mountains and higher mesas of northeast NM and adjacent southeast CO. High available moisture and intense deep layer ascent will produce heavy snowfall today until the system moves off to the northeast. WPC probabilities are high on D1 for 6 inches in the Front Range, Palmer Divide, and northern Sangre De Cristos with locally up to 12 inches possible. While the circulation brings widespread precipitation to the central Plains, initial conditions are too warm for snow. Cooling as the systems arrives may lead to a change over to snow in portions of western KS and central NE, but limited duration caps snowfall potential. On D2/Tue, a 700 mb shortwave moves southeast from the southern Great basin across the southwest. A broad 700 mb low and associated front form, with pooling of moisture and ascent near the circulation as it cross es from southern UT and across AZ and western NM. downstream from the circulation, the surface cold front moves south from CO across NM. Boundary layer moisture convergence occurs near the front and also in windward terrain in the post-frontal environment in the Sangre DeCristo mountains. An upper divergence maxima pivots overhead in response to a strengthening jet streak. The strong ascent aided by upslope flow will produce heavy snow across the Sangre De Cristo and San Juan mountains. WPC probabilities are moderate for 12 inches in these areas D2. On Day 3/Wed., the 700 mb circulation moves northeast across northeast NM and the TX panhandle, continuing beyond into the Plains. Heavy snow continues early in the higher elevations of northeast NM/adjacent CO, but tapers as the circulation departs. The probability of 98 inches drops to low across the CO/NM San Juans and Sangre DeCristo Mountains of NM. ...Pacific Northwest through the Northern Rockies... A couple of low amplitude 700 mb shortwaves move across the Pacific Northwest and Northern Rockies This will be accompanied by abundant moisture into the region. Plentiful flow aloft in advance of the upper level trough will provide some enhanced low-level convergence, along with some upslope flow on the windward side of the terrain. Moderate precipitation is expected with several inches of snow forecast in the WA/OR Cascades, Northern Rockies, Blue Mountains of OR, and Tetons of northwest WY. As the upper trough moves slowly east across WY and UT on Tuesday/D2, moderate snow is likely in the Wind River/Big Horns/Casper Arch of WY, Wasatch/Uintas of UT. WPC probabilities are moderate for 6 inches. On Wed/D3, another mid-upper level shortwave and associated jet energy should move onshore and across the Pacific Northwest, continuing into the northern Rockies Wed night. Diffluent flow aloft near the upper trough supports upper divergence maxima. This supports snow across the Cascades with clusters of 6 to 10" of snow likely. Several inches of snow are likely across the Blue Mountains of northeast OR to the mountains of central and northern ID and northwest MT. This is echoes in the forecasts from the ECMWF, Canadian Regional, Canadian Global, UKMET, and NAM models. The long duration 850-700 mb ascent leads to slightly higher probabilities for heavy snow in the WA Cascades on Wed/D3 (up to moderate probability of 8 inches). For Days 1-3, the probability of significant icing (0.25 inch or more) is less than 10 percent. Petersen