Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 523 PM EST Thu Mar 04 2021 Valid 00Z Fri Mar 05 2021 - 00Z Mon Mar 08 2021 ...Pacific Northwest/California... Days 1-3... A closed mid-level low will remain offshore the OR and WA coasts tonight through Friday. A pronounced southerly flow will advect warm air and moisture northward along the coast of northern CA and across WA/OR. A prolonged period of low level pre-frontal convergence combines with the warm/moist advection ahead of the front to produce prolonged low elevation rain and high elevation snow across the OR/WA Cascades and WA Olympics through Friday. Day 1 WPC probabilities are high for 8 inches in the Olympics and low in the high WA Cascades. Friday afternoon through Friday night, the cold front moves onshore in California and progresses inland. The surge in moisture and low level convergence with the front drives a period of snow across the Klamath/Shasta Siskiyous/Trinity Alps, into the southern OR Cascades, and northern CA Sierra Nevada with snow levels decreasing to around 3500ft as the snow tapers off late Friday night. Day 2 WPC probabilities are moderate to high for 8 inches in the higher Klamath and Siskiyou ranges, with local maxima above 12" likely. These probabilities gradually decline the further south down the Sierra Nevada Mountains. Veering flow behind the cold front cuts off Pacific moisture on Saturday with greatly diminished precip rates expected inland over the Great Basin. On Day 3/Sat night-Sun, the next in the series of slow moving lows drifts southeast well off the WA coast. As the low drifts closer, the next round of 850-700 mb warm/moisture advection reaches the coast and spreads inland Sun morning. The next round of snow is likely to develop over the WA Olympics, with locally heavy snow possible as steep lapse rates near 7-8 deg C/km combine with forecast strong ascent in the 850-700 mb layer to cause a period of heavy snow rates. ...Colorado/adjacent northern New Mexico... Day 1... A closed mid-level low over eastern CO this evening is expected to move southeast to Oklahoma tomorrow. Enhanced upper-level divergence/lower convergence couplets wrapping around the low combine with mid level frontogenesis, and moisture advection supports snow. Several inches of snow i expected in the Sangre DeCristo Mountains of southeast CO to adjacent northeast NM, and along the Palmer Divide. Adjacent areas of the high Plains could get an inch or two before ending. Day 1 WPC probabilities are low for 6 or more inches over the Sangre DeCristo Mountains. The event ends tomorrow as the low moves away into OK and drying aloft advects across CO. ...New York/Vermont/Pennsylvania... Days 1/2... Deep low pressure drifts west over eastern Quebec, allowing moisture to wrap around the circulation across northern New York and Vermont through Thu. Areas where the overlap of wrap-around moisture and boundary layer convergence/ascent where the northwest winds turn upslope results in snow showers Thu across the northern Adirondack and Green mountains. High mean layer relative humidity near 90 percent late tonight through Thu supports an extended period of snow showers. On Friday as the low pulls away, drying aloft develops across the region along with sinking motion, supporting the snow tapering in coverage/intensity. Day 1 probabilities for 4 or more inches are 30 to 40 percent over the northern Green Mountains and 20-30 percent over the northern Adirondacks. For Days 1-3, the probability of significant icing (0.25 inch or greater) is less than 10 percent. Petersen