Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 443 AM EDT Thu Mar 16 2023 Valid 12Z Thu Mar 16 2023 - 12Z Sun Mar 19 2023 ...California through the Four Corners... Days 1-3... A shortwave moving slowly out of the Desert Southwest will lift over the Four Corners region today and interact with a surface front banked into the terrain and a zonally oriented modest upper level jet streak to drive weak surface low development over New Mexico. The interaction of this forcing with plentiful available moisture noted by PW anomalies nearing +2 standard deviations from the climo mean on D1, will support an area of heavy snow, primarily in the higher terrain of the Sangre de Cristos, Raton Mesa, and San Juans where upslope flow and better fgen will enhance local ascent, and total snowfall in the higher terrain is likely to be impressive. The primary shortwave driving this event will shift into the Southern Plains during D2, but broad troughing extending back into the Four Corners will allow for continued light snow through D2 although with minimal additional accumulations. WPC probabilities on D1 are high for 6 inches along the Sangre de Cristos and into the San Juans, with locally 1-2 ft likely in the highest terrain. On D3 /Saturday night and Sunday/ a complex evolution of the synoptic pattern will develop as a modest shortwave embedded within increasingly confluent mid-level flow approaches the CA coast while a surface high pressure sinks southward across the Southern Plains. The result of this setup will be an increase in available column moisture across much of the West/Southwest in response to Pacific moisture flooding onshore while Gulf of Mexico moisture advects northward on return flow around the high. This will result in two areas of higher PW anomalies, upon which modest synoptic ascent will drive periods of moderate to heavy snowfall. For D3, the WPC probabilities for heavy snow are modest, generally 5-10%, for more than 4 inches in the Sierra with more snow likely into D4, and broadly 5-10% from the Stockton Plateau and Davis Mountains northward into the higher terrain of Arizona. ...Upper Midwest & Northern Great Lakes... Days 1-3... Southern and northern stream shortwaves will phase over the Upper Midwest tonight and become embedded into a larger scale gyre as a closed low deepens across Saskatchewan and sinks into the Great Lakes Friday. This evolution will drive robust large scale synoptic ascent through height falls and PVA, which will work in tandem with increasingly coupled jet streaks to intensify a surface low pressure emerging from the CO Rockies today. This low is progged to remain progressive as it shifts northeast into the Great Lakes by Friday, but deepen considerably beneath the impressive deep layer ascent. This low will track along a surface baroclinic gradient associated with a cold front moving eastward. Ahead of this front and the associated low, moisture transport will increase in response to the amplifying mid-level flow, and moisture will advect into the Upper Midwest and Great Lakes from both the Pacific and Gulf of Mexico, surging PW anomalies to +3 standard deviations above the climo mean. This will provide plentiful moisture for an expanding precipitation shield, and the guidance is in good agreement that there will be a broad swath of heavy snow from northern Nebraska through the Great Lakes, as a frontogenetically forced band of snow develops and translates northeast on the north side of the low track. In general, this will be a progressive feature so amounts will be somewhat limited through the Central Plains and Upper Midwest noted by WPC probabilities that are around 20-40% for 4+ inches in a stripe from northeast NE through central MN. However, across the western Great Lakes including northern WI, the MN Arrowhead, and U.P of MI, there will likely be a region of heavier and longer-duration snowfall both due to more pronounced TROWAL development late D1 into D2, followed by what could be an impressive LES event for much of the Great Lakes. This appears to be most impressive south of Lake Superior D2 as a surface trough swings southward to enhance CAA and drive deepening inversion heights and more intense sfc-850 delta-Ts. As this trough swings southeast and then east into D3, LES is likely to develop in the favored W/NW snow belts downstream of all the Great Lakes. The heaviest snow across the Great Lakes is likely surrounding Lake Superior where the main synoptic snowfall will be added upon by increased LES, and 2-day WPC probabilities for more than 8 inches are above 80% along the Arrowhead, the Bayfields Peninsula, and much of the northern portion of the U.P. Locally in excess of 18 inches is possible in some areas. As LES spreads farther downstream behind the surface trough D2-D3, WPC probabilities for more than 4 inches reach 30-50% east of most of the Lakes, with the highest probabilities occurring in the Tug Hill Plateau. Weiss ...Key Messages for March 16-18 Winter Storm... --Bands of locally heavy snow will track northeast across portions of the Upper Midwest and into the western Great Lakes through tonight. --Snowfall rates within this band could exceed 1"/hr at times, which when combined with strong and gusty winds will produce difficult travel due to low visibility and snow covered roads. Whiteout conditions are possible in the heaviest snowfall. --This system will pivot east across the Great Lakes early Friday, causing a transition to more widespread lake effect snow through Saturday. --Heavy snow rates are likely within any lake effect bands, producing hazardous travel and rapidly changing road conditions. --Heavy snow accumulations are possible in any lake effect bands. The greatest accumulations are expected around Lake Superior where the combination of snow today and lake effect snow Friday may result in 1-2 feet of snow.