Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 342 AM EDT Wed May 15 2019 Valid 12Z Wed May 15 2019 - 12Z Sat May 18 2019 ...Western U.S.... As an anomalously deep upper low/trough moves across the region, winter is forecast to return to portions of the Sierra, Great Basin and northern Rockies. Overall, models are in good agreement showing an upper low closing off over the eastern Pacific Wednesday night before moving inland near the California-Oregon border on Thursday. As it moves inland on Thursday, this anomalously deep system is expected to carve out 500mb heights more than 2 standard deviations below normal across much of northern and central California. At the onset of precipitation, snows levels will be high - initially confining the threat for heavy accumulations to areas above 8,000 ft. WPC guidance for Day 1 (ending 12Z Thursday) indicates a Slight Risk for accumulations of a foot or more for portions of the central Sierra above 8,000 ft. As the system moves inland, snow levels will drop significantly along the Sierra into the Great Basin on Thursday. WPC guidance on Day 2 (ending 12Z Friday) shows a Slight Risk for accumulations of a foot or more for areas as low 5,000 ft along the western slopes of the central and southern Sierra, with a Moderate Risk for areas above 7,000 ft. Significant snow accumulations are possible farther east across the higher elevations of the Great Basin, with WPC guidance indicating a Slight Risk or greater for accumulations of 4-inches or more across the central Nevada ranges. As the upper low/trough pivots to the northeast, snows will diminish across California and Nevada while developing farther to the north across portions of the northern Rockies Friday into early Saturday. Increasing upper divergence coupled with low level convergence and southwesterly inflow will support widespread precipitation across the region. With snow levels falling as the upper low lifts north, the potential for locally heavy accumulations will increase Friday into early Saturday across portions of the central Idaho, northwestern Wyoming and southwestern Montana ranges. The probability of significant icing is less than 10 percent. Pereira