Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
656 FOUS11 KWBC 071916 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 316 PM EDT Tue May 7 2024 Valid 00Z Wed May 08 2024 - 00Z Sat May 11 2024 ...Northern Rockies... Days 1-2... The upper low responsible for periods of heavy snow in the Northern Rockies Monday night and Tuesday morning will begin to track south from southwest ND this afternoon to southwest SD by Wednesday morning. In addition, the upper low will begin weakening this afternoon and steadily weaken through Wednesday. Despite the weakening phase, there is still an anomalous NWlry IVT that is above the 90th climatological percentile (according to NAEFS) and will direct a steady diet of 850-700mb moisture into the Northern Rockies this evening and through Wednesday. Given the time of year, snow will be harder to accumulate on roads during the daytime hours, but snow will still accumulate on trees and grassy surfaces in areas where snowfall rates average close to 1-2"/hr in the mountainous terrain above 6,000ft in the Lewis Range, Big Belt, Little Belt, and Big Snowy Mountains of MT, above 7,000ft in the Absaroka of MT and Big Horns in WY, and above 8,000ft in the Absaroka, Wind River Range, and Laramie Range in WY. As the IVT diminishes Wednesday night and the 500mb low continues to weaken and tracks east, the strength of the upslope flow into these ranges will lessen, thus decreasing snowfall rates Wednesday night and into Thursday morning. Outside of the >9,000ft peaks in WY and CO, most snowfall accumulations will be over with just minor accumulations expected during the day Thursday. WPC PWPF through Thursday morning shows high chances (50-70%) for additional snowfall accumulations >18" in the Little Belt, Big Snowy, and Absaroka Ranges in MT with similar probabilities at/above 8,000ft in the Big Horns of WY. In fact, there are moderate-to-high odds (50-70%) for >24" of additional snowfall in these ranges. Similar high chance (>70%) probabilities for >8" of additional snowfall are depicted in the Wind River Range, Laramie Range, and Medicine Bow in WY above 8,000ft. The Probabilistic WSSI (WSSI-P) shows moderate-to-high chances (50-70%) for Moderate Impacts in the Big Belt, Big Snowy, Absaroka, and Big Horns through 00Z Thursday. These mountain ranges, particularly above 6,000ft in the Big Belt and Big Snowy, along with the Big Horns >8,000ft, can expect hazardous travel conditions with possible closures and disruptions to infrastructure. The Little Belt Mountains and their northern slopes are most likely to witness Major Impacts, indicated by the WSSI-P showing moderate-to-high chances (50-70%) for impacts that include considerable disruptions to daily life. This includes dangerous to impossible travel and widespread closures. It is also worth noting that blustery winds combined with periods of heavy snow will reduce visibility and also make for hazardous travel conditions. The WSSI-P Minor Impacts for Blowing Snow depict moderate chances (40-60%) in these mountain ranges. Given the heavy, wet snow and gusty winds forecast, spotty power outages and localized tree damage may occur. The probability of significant icing across the CONUS is less than 10 percent. Mullinax $$