Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 509 AM EDT Sat Mar 14 2020 Valid 12Z Sat Mar 14 2020 - 12Z Tue Mar 17 2020 ...Oregon and California... Days 1-3... ...Prolonged Heavy Snow with Several Feet Expected for the Northern and Central Sierra Nevada through Monday night... A cold core low drifts south off the OR coast today before stalling and slowly filling off the far northern CA coast tonight through Sunday before drifting south down the rest of the CA Coast through Wednesday. This directs Pacific moisture ahead of the low into southern OR today, and northern/central CA through Monday night. Prolonged synoptic ascent through upper divergence, jet streak diffluence, low level convergence, with aid from upslope flow in windward terrain and cold air will produce widespread heavy snow in the ranges of northern to central CA above about 2000ft Days 1 and 2 before weakening as it shifts down the rest of the Sierra Nevada to southern CA. Prolonged heavy snow is expected in this stalled pattern with nearly identical areas of high Day 1 and Day 2 probabilities for 18 or more inches over the Sierra Nevada centered on Lake Tahoe with upwards of 6 or more feet possible for the highest terrain. Day 1 and 2 probabilities for 8 or more inches are moderate to high for the Shasta Siskiyous and Klamath mountains. The filling low progresses south on Monday with Day 3 snow probabilities for 12 or more inches moderate to high for the central and southern Sierra Nevada. ...Northern Rockies...Northern Plains...to Lake Superior... Days 1-3... Ongoing heavy snow in north-central MT and the northern Rockies continues in the wake of the cold-core low shifting south off OR. Near-surface cold air sinking southward and interacting with warm advection aloft is intensifying 850-700mb frontogenesis which combined with upslope flow for the eastern slopes will maintain the heavy snow. Day 1 WPC probabilities are moderate for 8 or more additional inches over north-central MT and high for the Absaroka/Tetons/Wind River. A stripe of precip across ID/MT/WY maintains precip with 3000ft snow levels with Day 2 probabilities high for 6 or more inches for the highest Tetons and around Yellowstone NP. A shortwave impulse moves east from MT across ND to northern MN Sunday into Monday, allowing generally light snow to develop in ND on low level convergence and ascent. The decreasing amplitude of the wave and weakening 700 mb convergence leads to lighter snow amounts the further east you go across the northern Plains. Day 2 probabilities feature moderate probabilities for 2 or more inches centered over eastern ND then over the Keweenaw Peninsula on Day 3 where lake enhanced and effect snow begins. ...Central Plains across the Midwest to the Central Appalachians... Day 1... An upper trough ejecting northeast across the central Rockies early this morning will shift east across the Midwest today, reaching the central Appalachians tonight. Gulf-sourced moisture is wrapped around an east-west oriented low level trough axis from central KS across central MO to southern IL/IN under a developing 130kt jet streak. Warm sector convection should sap most of the moisture before it reaches the colder north, limiting snowfall potential. That said, 700mb frontogenesis is potent now over eastern NE/southern IA where a band of locally moderate snow has developed and will shift east today across the Midwest. Some weakening is expected and the midday sun angle here in mid-March should further limit snowfall potential. Day 1 WPC probabilities for 2 or more inches are moderate over southern IA and north-central IL, then lower across IN and OH before increasing to moderately high over the northern Allegheny Plateau centered on western MD tonight where terrain enhances snow. For Days 1-3, the probability of significant icing (0.25-inch or greater) is less than 10 percent. Jackson