Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 306 PM EDT Fri Apr 12 2024 Valid 00Z Sat Apr 13 2024 - 00Z Tue Apr 16 2024 ...Western NY & Northwest PA... Day 1... Strong cyclonic flow on the backside of a storm system tracking north into the heart of southeast Canada will lead to a brief but potent surge of CAA across the eastern Great Lakes this evening. Accentuated beneath a TROWAL overhead, a pivoting band of heavy rain will changeover to snow as WNW winds favorably upslope into the Chautauqua Ridge in the western portion of NY's Southern Tier and strong dynamic cooling within the column ensues. Latest CAMs are showing the potential for as much as 1-2"/hr snowfall rates along the Chautauqua Ridge tonight, and snowfall rates that heavy can rapidly cool boundary layer temperatures closer to freezing to support rapid accumulations. As with most snow events this late in the season, elevation will be a massive factor in which areas see a coating to a couple inches or as much as localized amounts between 6-8" by Saturday morning. Latest WPC PWPF sports moderate- to-high chances (50-70%) for >4" of snowfall along the Chautauqua Ridge at elevations >2,000ft through Saturday morning. Areas in northwest PA, including Bradford, PA and Allegany State Park, feature low-to-moderate chances (30-50%) for >4" of snowfall. The WSSI sports Minor to even Moderate Impact potential in western NY to the south and east of Buffalo, NY. These areas in particular can expect some hazardous travel conditions tonight and into early Saturday morning. Periods of snow will gradually taper off throughout the morning, which given the mid-April sun angle will all but end the accumulating snowfall potential from late morning on. ...California... Days 1-2... A compact and potent 500mb low approaching the Golden State today will result in an unsettled weather pattern this weekend. NAEFS continues to depict the 500mb low tonight and early Saturday morning that features 500mb heights that are below the observed CFSR climatology (1979-2009) just WNW of San Francisco. This impressive upper low will accompany a strong IVT (topping out around 400 kg/m/s) that according to NAEFS will top the 99th climatological percentile over central California. The upper low will gradually weaken throughout the day, but not before it provides ample Pacific moisture flux into California that leads to heavy snow >6,000ft from the Salmon/Shasta/Siskiyou Mountains on northern California on down the spine of the Sierra Nevada Saturday and into Sunday. As the upper low weakens Saturday night and into Sunday, snow levels will gradually rise and strong synoptic-scale ascent will weaken, keeping periods of heavy snow strictly confined to elevations >6,000ft. Meanwhile, periods of moderate-to-heavy snow will still occur in parts of the northern Sierra Nevada, the Warner Mountains of northern California, and the tallest >7,000ft peaks of the Great Basin in Nevada. Snow looks to conclude in these regions by Sunday night. WPC PWPF shows high chances (>70%) for snowfall totals >8" in elevations >7,000ft in the Sierra Nevada and the aforementioned northern California ranges. The WSSI sports Moderate Impacts in the southern Sierra Nevada (elevations >7,000ft) where a combination of Snow Amount and Blowing Snow are the primary components driving the algorithm. Otherwise, most impacts as low as 5,000ft in the northern California ranges are likely be Minor Impacts, implying there could be a few inconveniences to daily life and caution is urged while driving in affected areas. For days 1-3, the probability of significant icing is less than 10 percent. Mullinax