Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 541 PM EDT Mon Mar 16 2020 Valid 00Z Tue Mar 17 2020 - 00Z Fri Mar 20 2020 ...California, Northern Nevada..southern Oregon....southern Idaho Days 1-2... A closed low stalled off northern CA drifts south off the central CA coast tonight before elongating and becoming two separate circulations through Tuesday. The plume of Pacific moisture streams onshore ahead of the low. Synoptic ascent through upper divergence/low level convergence, with aid from upslope flow in windward terrain will continue produce snow initially along the Sierra Nevada and refocus south to the Transverse Ranges through Tue. An additional foot of snow is expected through Tue in the highest terrain of both the norther and southern Sierra Nevada, with a break in between in the central portion of the mountains. There is a moderate risk for 6 or more inches in the highest Transverse Ranges. The models show a tripe point low develop sfc-700 mb with the low drifting north across Nevada Tue to near the CA/NV/OR border by 00z Wed. Wrap around moisture and lift over northern Nevada into far northern CA/far southeast OR should lead to moderate snow accumulations at higher elevations. Day 1 probabilities for 6 or more inches are moderate to high over terrain in far northeast CA and adjacent northwest NV. The continued slow movement of the low into southern ID leads to additional snow in far northeast CA to far northwest NV and southern OR Cascades. As the circulation elongates and weakens, amounts decreases further north into the ranges of southern ID and southwest MT. Probabilities for 8 inches are moderate near the CA/NV border just south of OR, but low in southern ID to southwest MT. ...Four Corners States to the Central/Northern Plains and Upper MS Valley... Days 2/3... As the 700 mb low moves onshore into southern CA Wed and then northwest AZ/possibly adjacent southern UT/southern NV, the falling heights and increasing south to southwest flow leads to moisture fluxes across southern CA to AZ/UT Precip breaks out Wednesday with snow expected in the higher elevations of the AZ Mogollon Rim and Kaibab Plateau (north of Grand Canyon North Rim) to the southern UT Wasatch, as well as Bryce/Zion National Parks before expanding to the San Juan/Sangre De Christos and central CO Ranges. Wed night-Thu, the system crosses the CO Rockies with lee side surface low pressure then developing in eastern CO late Wednesday night. This low will combine with sprawling high pressure centered over the northern Canadian Prairies to produce east-northeasterly upslope flow over the High Plains/eastern slopes of the WY and the Black Hills of SD where snow breaks out late Wednesday night and continues to spread across the high plains Thu. Day 3 probabilities for 8 or more inches are high for the highest terrain of the San Juans of CO to the front range of north central CO and southeast WY. Moderate risks extend east into the western NE panhandle and then drop the further east you go across the Plains due to combined QPF/precip type differences among the models. Light icing is possible Wed night-Thu in the eastern Dakotas and MN/WI/northern Lower MI. ...Northeast... Day 2... An impulse reaches the Great Lakes by late Wednesday and New England late Wednesday night. Strong warm advection with this quick moving system will limit snow fall. A low-moderate risk of 4 inches of snow remains in effect across the ranges of northwest ME. The warming aloft causes snow to change to sleet/freezing rain, and rain across upstate NY, western MA, and VT. Transitional light freezing rain is possible in the Catskills and Adirondacks of NY and Berkshires of MA. For Days 1-3, the probability of significant icing (0.25-inch or greater) is less than 10 percent. Petersen