Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 356 PM EST Sat Feb 25 2023 Valid 00Z Sun Feb 26 2023 - 00Z Wed Mar 01 2023 ...California into Southern Nevada & Northern Arizona... Day 1... The anomalously deep low currently impacting Southern California is forecast to turn east this evening, moving across Southern California into the southern Colorado Basin overnight. While the heavy snow threat for Southern California is expected to wane, some additional snow showers with locally heavy accumulations are expected, especially for the higher peaks of the Peninsular Ranges. The already anomalously snow levels may dip even further as the upper low comes onshore, perhaps slipping below 1000 ft in some locations. As the low moves east, the threat for heavy snow is expected to increase across the higher elevations of southern Nevada, southern Utah, and central to northern Arizona. Deep moisture transport ahead of the low along with strong forcing, including low-to-mid level frontogenesis, is expected to contribute to heavier snowfall rates (1-2 in/hr) across the region. WPC probabilities indicate that widespread totals of 4 inches, with locally heavier totals are likely especially for areas above 5000 ft. ...Northeast... Day 1... A low-amplitude shortwave embedded within the base of a broader scale trough is forecast to move quickly east from the northern Great Lakes early Sunday into northern New York and New England by late Sunday. While widespread heavy accumulations are not expected, ample moisture and sufficient instability may be sufficient for brief, but intense showers, resulting in reduced visibility and dangerous travel conditions. As the upper wave approaches, a surface low developing over the Gulf of Maine may help to focus some locally heavier accumulations along the coast of Down East Maine. ...Upper Midwest/Great Lakes to the Northeast... Days 2-3... The upper low in the Southwest is expected to transition to an open wave and lift northeast ahead of a northern stream trough moving across the Northwest. As the wave assumes a negative-tilt, a powerful surface cyclone is forecast to develop east of the central Rockies and track east-northeast through the central Plains Sunday night and into the mid Mississippi Valley by Monday morning, before tracking farther to the northeast into the Great Lakes by late in the day. Strong southerly flow and a retreating high to the north will support a surge of warm air aloft. Mixed precipitation, becoming predominately freezing rain, is expected along the leading edge of the precipitation shield as it advances north into the upper Mississippi Valley and upper Great Lakes region. WPC probabilities indicate that accumulations of 0.10 inch or more are likely across a good portion of central Wisconsin, as well as parts of central Lower Michigan. As the system begins to slide east, drawing colder air in behind it, a changeover to snow is expected for portions of the northern Great Lakes, with parts of northern Michigan likely to see accumulations of 6 inches or more late Monday into early Tuesday. Ahead of the system, warm advection precipitation is forecast to spread northeast from the upper Ohio Valley into the Northeast beginning late Monday and continuing through the overnight. Cold high pressure remaining in place over Atlantic Canada will support a wintry mix over portions of western to central Pennsylvania and New York, with mostly snow farther north and east. A low developing near the northern Mid Atlantic coast is likely to lock in the cold air and boost snow totals as it moves offshore on Tuesday. The track and timing of this feature continue to contribute to an uncertain forecast across the Northeast. While confidence in the details is limited, it still appears likely that portions of the Northeast will see significant snowfall amounts. WPC probabilities indicate accumulations of 4 inches or more are likely for much of eastern Upstate New York, as well as much of interior New England. ...Pacific Northwest to northern and central California... Days 1-3... As the low over Southern California ejects out to the northeast, a pair of shortwaves are expected to re-establish the trough in the West, carving out a broad trough that is forecast to remain in place through the early part of next week. The initial wave and its associated frontal band are forecast to sweep across the Northwest and Northern California tomorrow. This is expected to bring some additional heavy snows to the mountains from the Olympics and Cascades to the northern Sierra. With cold air still in place, accumulating snows are also likely across the lower elevations of western Washington, Oregon, and Northwest California. This leading system is forecast to sweep out across the northern and central Rockies Sunday night, but be quickly followed by a more amplified system digging southeast from the Gulf of Alaska to the Pacific Northwest coast by Monday morning. Strong dynamics and onshore flow will bring heavy precipitation back into portions of the Northwest and northern to central California beginning Monday, with precipitation continuing through Tuesday as upstream energy continues to drop southeast and move through the base of the trough. Additional heavy snow accumulations are likely for the Pacific Northwest and Northwest California coastal ranges and the Cascades, with several additional feet likely for portions of the Sierra. Relatively lighter, but impactful snows are also expected farther east through the Intermountain West and into the northern and central Rockies. Pereira