Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 345 AM EST Thu Mar 04 2021 Valid 12Z Thu Mar 04 2021 - 12Z Sun Mar 07 2021 ...Colorado... Day 1... A closed mid-level low over the western AZ/UT border early this morning will track east over southern CO today before turning southeast over the southern Great Plains tonight. Enhanced upper-level divergence in the left exit region of a westerly jet over AZ/NM today, low level convergence with notable fgen on the north side of the low at 700mb and 600mb, and warm/moisture advection with a feed of tropical Pacific and some western Gulf moisture ahead of the low makes for snow, locally heavy in the higher elevations of the through this evening. Thermal profiles are rather marginal over the Denver metro area, but the consensus low track puts that metro area in the fgen region, so a band or two of accumulating snow in that region east of the Front Range. Day 1 WPC probabilities (which begin at 12Z today) are moderately high for 6 or more inches over the northern CO Rockies, particularly the Front Range from Ft. Collins to CO Spring. ...Pacific Northwest/California... Days 1-3... A closed mid-level low and reinforcing trough will slowly shift toward the OR and WA coasts today, pushing ashore Friday night. A pronounced southerly flow will advect warm air and moisture northward along the coast of northern CA and across WA/OR. As the low slowly migrates eastward, a surface cold front will linger just offshore tonight through Friday before moving onshore Friday night. 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 moderate for 8 inches in the Olympics and Day 1.5 has these probabilities expand to 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/Trinities, into the OR Cascades, and northern Sierra Nevada with initial snow levels around 5000ft decreasing to around 3500ft as the snow tapers off late Friday night. Day 2 WPC probabilities are moderate for 8 inches in the higher Klamath and Siskiyou ranges, with local maxima above 12" likely. These probabilities expand down the northern Sierra Nevada for Day 2.5. Veering flow behind the cold front cuts off Pacific moisture on Saturday with greatly diminished precip rates expected inland over the Great Basin. ...Northern New York/northern Vermont mountains... Days 1/2... Deep low pressure currently north of the Gulf of St. Lawrence retrogrades, shifting west over eastern Quebec as troughing increases over the Northeast. Areas where the overlap of wrap-around moisture and boundary layer convergence/ascent where the northwest winds turn upslope results in snow showers Thu night across the northern Adirondack and Green mountains. High mean layer relative humidity near 90 percent tonight 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 1.5 probabilities for 4 or more inches are 20 to 30 percent over northern stretches of these two ranges. For Days 1-3, the probability of significant icing (0.25 inch or greater) is less than 10 percent. Jackson