Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 331 AM EST Mon Dec 18 2023 Valid 12Z Mon Dec 18 2023 - 12Z Thu Dec 21 2023 ...Great Lakes, Ohio Valley, and the Central to Southern Appalachians... Days 1-2... The overall forecast remains much the same, with widespread snow showers and possible snow squalls expected to impact areas from the Great Lakes to the central and southern Appalachians through today into Tuesday. Locally heavy snow accumulations can be expected, especially southeast of lakes Michigan and Erie and in the upslope regions of the Allegheny Mountains. The powerful surface low that brought widespread heavy rain to the Southeast is now moving north through the Carolinas. This system is expected to track quickly north, reaching the St Lawrence Valley this evening. The heaviest rain associated with this system is forecast to remain centered east of the region. However, a northern stream shortwave is beginning to carve out a deep, negatively-tilted trough over the western Great Lakes this morning. Guidance shows this system continuing to amplify, with a closed low developing over the Great Lakes later today. Surface observations this morning show notably colder air spreading across North Dakota and northern Minnesota. This airmass will continue to spread further south and east, moving across the still relatively warm lakes today. The resulting steepening lapse rates along with lift through the DGZ will support lake effect snow showers with locally intense rates. As shown by the WPC PWPF, widespread heavy accumulations are not expected. However, some areas are likely to see locally heavy amounts. There remains a good model signal that deep southwest flow on the backside of the deepening low will support a band of heavy snow developing off of southeastern Lake Michigan, impacting portions of southwestern Michigan and northern Indiana. HREF guidance indicates rates of 1-2 in/hr are possible as the band develops later today. WPC PWPF shows a narrow stripe of accumulations of 4 inches or more is likely, with accumulations of 8 inches or more possible. As the upper low shifts farther east, areas of heavy snow are also likely to develop southeast of Lake Erie. Moisture contributions from Lake Huron are expected to support the development heavy snow bands impacting northeast Ohio and spreading east through northwestern Pennsylvania into far western New York. WPC PWPF shows a long axis of high probabilities for accumulations of 4 inches or more, and some moderate probabilities for 8 inches or more, along the upslope regions southeast of the lake. There also remains a strong signal for locally heavy snow in the upslope regions of the central Appalachians, with the heaviest amounts likely to fall along the Allegheny Mountains of West Virginia. WPC PWPF continues to show high probabilities for amounts of 4 inches or more, with moderate accumulations of 8 inches or more centered over the area. While the PWPF indicates that lighter amounts are likely elsewhere, there remains a significant signal for convective snow showers with possible snow squalls that may produce brief periods of intense snowfall and windy conditions. This includes areas where heavy snow accumulations are not suggested by the PWPF. The NAM and GFS continue to show high snow squall parameter values spreading southeast from the upper Great Lakes through the Ohio Valley and into the central Appalachians during the morning and afternoon hours. The intense snowfall and windy conditions generated by these storms are likely to create hazardous travel conditions. Even apart from where any convective storms may develop, a tightening pressure gradient forming in the wake low moving along the East Coast and ahead of high pressure over the Plains will support strong, gusty winds across the entire region today. On Tuesday, snow is expected to diminish from west to east as high pressure over the Plains shifts east and centers over the eastern U.S. through the remainder of the period. ...Sierra Nevada... Days 1-3... An upper low currently centered west of northern California is forecast to transition to an open wave, with the remnant energy spreading inland ahead of a second low dropping southeast from the Gulf of Alaska tonight. This second system is forecast to remain offshore, but drop south along the California coast beginning Tuesday night and continuing into early Thursday. While moderate to heavy precipitation is expected to develop across much of Northern California today and persist into Tuesday, snow levels beginning above 7000ft and climbing to above 8000ft in many locations will limit the potential for any widespread heavy snowfall amounts through early Tuesday. Then as the upper low drops south, snow levels dipping back below 8000ft will slightly broaden the heavy snowfall footprint on Tuesday. By early Wednesday and continuing into Thursday, the low will begin to orient the deeper moisture and the threat for heavier precipitation further south into Southern California. However, snow levels are expected to remain between 7000-8000ft. ...Pacific Northwest... Days 1-2... Expect generally light precipitation to spread north as a shortwave associated with the remnants of the previously noted upper low lift across the region. High snow levels will further preclude any widespread heavy snow impacts, however guidance continues to raise some concerns for light icing along the eastern slopes of the southern Washington and northern Oregon Cascades into the Columbia Gorge. Trapped cold air is expected to produce areas of freezing rain, with WPC PWPF showing some low end probabilities for ice accumulations of 0.10 inch or more. Pereira