Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 306 AM EST Thu Dec 12 2024 Valid 12Z Thu Dec 12 2024 - 12Z Sun Dec 15 2024 ...Great Lakes.. Days 1-2... The core of an anomalous upper low (-2 sigma at 500mb according to NAEFS with a 510dm center) will begin to fill and drift eastward across southern Canada on D1, with the primary mid-level trough axis pivoting into New England by 00Z Saturday. This will be replaced by brief shortwave ridging downstream of a potent trough digging across the Central Plains. Until that happens, however, the environment will remain favorable for another round of widespread and heavy lake-effect snow (LES) in the W/NW snow belts. At the start of the forecast period, 850mb temps will be generally -15C to -20C, and continue to plunge to -18C to -23C by 00Z Friday before slow warming begins. This CAA supplying the cold air will move across Great Lakes water temperatures that are still warm, generally +5 to +9C, supporting deltaT that is well above threshold for heavy LES, and will drive lake-induced equilibrium levels 10,000 - 15,000 ft, which will be well above the -10C isotherm suggesting potential lightning in the heavier bands. This will support snowfall rates of 1-2"/hr in many of the bands, with locally 3-4"/hr possible downstream of Lakes Erie and Ontario. Combined with the cold temperatures and gusty winds, significant travel impacts are expected within LES. The heaviest LES is likely D1 before a slow ease of CAA (and onset of subtle WAA) wanes LES from west to east during D2, but not before multiple feet of snow occur in the more persistent and intense banding. WPC probabilities D1 are high (>70%) for more than 8 inches in the eastern U.P., near Traverse Bay, along the Chautauqua Ridge, and into the Tug Hill Plateau, with high probabilities continuing on D2 only in the Tug Hill. ...Pacific Coast and Interior Northwest... Days 1-3... Two waves of precipitation will spread onshore the West Coast and then inland through the forecast period bringing widespread snow to the higher elevations. The first wave of moisture will be ongoing to start the period as a slowly filling closed low drops into northern CA with 500mb height anomalies falling below -1 sigma according to NAEFS. Height falls combined with the LFQ of an accompanying upper level jet streak will drive ascent onshore, leading to locally heavy snow, primarily confined to the Sierra Nevada. The strongest forcing is transient and pushes south of the area halfway through D1, but at least a short duration of 1"/hr snow rates is likely across the Sierra, generally above 5000 ft. During D1, WPC probabilities for more than 6 inches of snow are above 70% in the Sierra, but probabilities for more than 12 inches are only 10-30%. A more significant system will then approach the coast from the Pacific Friday into Saturday, leading to a more substantial precipitation event after only a brief break Thursday night into Friday. This system will again be driven by a pronounced closed low offshore, opening into a potent trough as it shifts across CA/OR late Saturday, reaching the Great Basin by the end of the forecast period. Once again, downstream divergence, warm/moist advection, and robust jet energy will combine to drive ascent. However, this second system will be more broad, reflected by 500mb height anomalies below -1 sigma across much of the Pacific Coast, combined with modestly coupled jet streaks to push a strong surface low into British Columbia and a second wave across CA. This overall more impressive system is reflected as well by GEFS and ECENS probabilities for 500 kg/m/s IVT lifting onshore exceeding 60%. Snow levels across the West with this second system will generally remain around 4000-5000 ft, with a narrow corridor exceeding 6000 ft in the strongest IVT/WAA. However strong ascent into an environment with a deep layer of lapse rates stronger than moist- adiabatic could lower snow levels beyond forecasts, and the NBM 25th percentile may be more representative, which reaches as low as 3000-4000 ft during the heaviest precipitation, and falls even lower as precip begins to shift eastward. This suggests the potential for more widespread pass-level impacts, reflected by WSSI-P above 70% for moderate impacts due primarily to snow load and snow rate in the Sierra and Shasta/Trinity region. Lesser, but still notable impacts, are possible as far north as the WA Cascades. WPC probabilities for more than 8 inches of snow on D2 are high (>70%) but confined to the Shasta/Trinity region of northern CA and the highest peak of the WA/OR Cascades. By D3, high WPC probabilities for more than 8 inches spread across the length of the Sierra and into the Sawtooth/Salmon River Ranges, while also continuing near Mt. Shasta. ...Central Plains into the Upper Midwest... Days 2-3... A closed 500mb low emerging from the Central Rockies will track eastward while deepening and pulling south in response to a potent vorticity lobe rotating around its base. The trend in the global guidance has been for this feature to be a little weaker and farther south, and while this will likely result in more interaction with a stationary front/modest low-level baroclinic zone, the resultant downstream moisture advection may be somewhat muted. Still, as this advects east into Missouri by Saturday evening, it will combine with strengthening fgen in response to surging WAA to drive moisture northward reflected by a narrow channel of +1 sigma PWs on the NAEFS tables, while also helping to create a surface low moving across the Central Plains and the Upper Midwest by the end of the forecast period. Downstream of this low, a sprawling high pressure centered over the Mid-Atlantic states will gradually retreat to the east. This will allow confluent flow to merge into the Corn Belt and then Upper Midwest, with overrunning WAA leading to a period of mixed precipitation including sleet and freezing rain, with snow farther north. Even where precipitation is snow, it may begin as a period of ZR/IP due to antecedent low RH within the DGZ (no ice growth) noted in regional soundings. There remains considerable spread in the latitudinal gain of warm air, but with the high retreating steadily and WAA increasing, solutions with a farther north depiction of accumulating snow and ice seem more reasonable, and this could result in hazardous accretions of freezing rain, and modest snowfall as well. Current WPC probabilities for more than 4 inches of snow are less than 10% with this system, but the probability for at least 0.1 inches of ice are 10-30% D2 in central IA, increasing to 50-70% D3 when locally as much as 0.2 inches is possible. Weiss