Subj : HVYSNOW: Probabilistic Heavy Snow And Icing Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon May 05 2025 06:54:04 FOUS11 KWBC 050653 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 253 AM EDT Mon May 5 2025 Valid 12Z Mon May 05 2025 - 12Z Thu May 08 2025 ....Intermountain West... Days 1-2... The interaction of an anomalous and slow moving closed mid-level low with a surface cold front wavering across the Central Rockies will result in widespread heavy precipitation through Wednesday before everything kicks out to the east during D3. The primary mechanisms for ascent will be the intense closed low (500-700mb height anomalies below the 1st percentile according to NAEFS) and a cold front that will stall generally from the Dakotas across Colorado and into the Great Basin. Intensifying and veering low to mid level flow will encourage the impressive draw northward of moisture, leading to robust isentropic ascent at 305K from the Gulf into the Rockies on the downwind side of this feature. With moisture confluence becoming impressive, ascent through height falls and convergence along the front, aided by both strong upslope flow (especially into the Sangre de Cristos and Front Range) and elevated frontogenesis will wring out this moisture as expansive and heavy precipitation from Montana through New Mexico. Additionally, impressive easterly flow leading to the pronounced upslope flow will correspond with some reduction (and isolated folding) of the theta-e surfaces leading to some elevated instability. This enhanced forcing combined with a lowering trend in snow levels (NBM 10th% down to 7500 ft) could allow for more impactful snow into lower elevations of the foothills, especially across the Palmer Divide and Raton Mesa, including the I-25 corridor in that vicinity. After coordination with WFO BOU, opted to raise SLRs a bit in this region for the potential, with some impacts becoming more probable even east of the Front Range and Sangres. WPC probabilities across this area are high (>70%) for 6+ inches both D1 and D2 in the San Juans, Sangre de Cristos, and Front Range, with 1-2 feet of snow likely up around 9,000 ft, and locally as much as 3+ feet above 10,000 ft. While accumulations in the foothills and lower elevations down to 7,000 ft will be much less, heavy snow rates with low-SLR could cause impacts even into the I-25 corridor, especially Tuesday evening/night. Farther north, snow levels will crash behind the cold front as it sags southward, so while across WY and southern MT they too will begin around 9,000 ft, by D2 snow levels are expected to fall to as low as 6,000 ft, especially in MT and in the vicinity of Yellowstone NP. Across this area, WPC probabilities are high (70-90%) for 6+ inches D1 across the Absarokas and surrounding elevations of southern MT, with probabilities reaching 50% across the Wind Rivers, Big Horns, and in the vicinity of Yellowstone NP. The probability of significant freezing rain across the CONUS is less than 10%. Weiss $$ = = = To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .