Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 453 PM EST Fri Feb 02 2024 Valid 00Z Sat Feb 03 2024 - 00Z Tue Feb 06 2024 ...Southwest/Rockies... Days 1-2... A negatively-tilted upper trough will continue to progress across the Southwest/southern Rockies tonight, with a closed low expected to develop over the southern High Plains by early Saturday. Strong lift supported in part by upper level jet divergence and low-to-mid frontogenesis interacting with deepening moisture will support the development of heavy snow across portions of the southern and central Rockies beginning today and continuing into Saturday. Moisture associated with the Pacific AR moving through the Southwest, along with increasing moisture from the Gulf of Mexico that will be directed by the strengthening low over the High Plains, will contribute to the increase in moisture. The heaviest accumulations through late Saturday are expected to focus over the Colorado and northern New Mexico ranges, with the WPC PWPF indicating widespread accumulations of 8 inches or more, and locally heavy amounts of a foot or more over the higher terrain. With the exception of the Palmer Divide and the Raton Mesa region, the heavy snow is forecast to remain west of I-25. Meanwhile to the north, locally heavy, high elevation snow supported by deepening moisture along a low-to-mid level boundary is expected across portions of the northern Rockies. Enhanced convergence supported by a slow-moving, low-to-mid level wave is expected to help drive some locally higher totals from the central and southern Idaho into the western Montana and northwestern Wyoming ranges late today into Saturday. WPC PWPF indicates widespread totals of 4 inches or more are likely across this region, with locally heavier totals of 8 inches or more across the higher terrain. Upslope flow on the north side of a wave developing east of the Divide will begin to help focus the heavy snowfall threat more solely on the Glacier NP region late Saturday into early Sunday, when additional accumulations of 8 inches or more are likely. ...California and Nevada... Days 2-3... Following today's AR, a second, even stronger, AR will approach the CA coast Saturday night and then shift onshore Sunday. Max IVT within this second AR will likely exceed 500 kg/ms and shift onshore southern/central CA with impressive IVT exceeding 250 kg/ms persisting well east of the Sierra Crest and spilling over onto the Great Basin. This AR will be driven by impressively confluent mid-level 700-500mb flow southeast of a closed low, angling intensifying winds to drive the intense moisture advection eastward. This will be overlapped by a robust and zonally oriented 170kt subtropical jet streak. Periodic spokes of vorticity shedding inland combined with the LFQ of the upper jet will provide strong deep layer ascent, resulting in a swath of heavy precipitation beginning early Sunday and persisting into Monday downstream of a potent surface low which will only gradually weaken as it approaches the northern CA coast by Monday morning. Snow levels during precip onset will be modestly low, around 3000-4000 ft, but will rise steadily within the accompanying warm air advection, reaching above 6000 ft in the Sierra on Monday. With warming snow levels, the snow will likely become heavy and wet with low SLRs, so impacts are expected to be substantial, especially in the Sierra as reflected by high probabilities for major impacts in the WSSI-P due to snow load and snow amount. Snow begins to accumulate Saturday night in the Sierra, but will become much more widespread and heavy on Sunday. By late Sunday, WPC PWPF shows high probabilities (greater than 70 percent) for accumulations of a foot or more for areas above 5000 ft in the northern Sierra and above 6000 ft in the southern Sierra. Heavy snow is likely to continue into early Monday in the Sierra with rates expected to begin drop late in the day as the axis of deeper moisture shifts further south. Storm total amounts are likely to reach multiple feet in many Sierra locations above 5000-6000 ft. The probability of significant icing greater than 0.10" is less than 10% for days 1-3. Key Messages for the Feb 4-7 Atmospheric River into California are below: Strong Atmospheric River into California- The next Atmospheric River (AR) event is expected to impact California Saturday night into Tuesday. This AR will be of longer duration and stronger than the one that moved through California Thursday. Heavy Mountain Snow- Expect particularly heavy snow across the Sierra Nevada, with several feet above snow levels of 5000-6000 feet. Disruptions to daily life, including difficult to impossible travel conditions are expected. Heavy snow above 6000-7000 feet is expected for southern California terrain. Excessive Rainfall and Flooding- Heavy rainfall will impact lower elevations and coastal areas, with significant rainfall totals from central to southern California including the Transverse Ranges. This should result in considerable flash, urban, and small stream flooding with debris flows and mudslides. River flooding is also likely to occur. Strong Winds and High Surf- Powerful, damaging onshore winds are expected over the central California coast and Coastal Ranges, with potentially damaging high surf. Pereira/Weiss