Subj : HVYSNOW: Probabilistic Heavy Snow And Icing Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon May 05 2025 18:41:11 FOUS11 KWBC 051841 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 241 PM EDT Mon May 5 2025 Valid 00Z Tue May 06 2025 - 00Z Fri May 09 2025 ....Intermountain West... Days 1-2... A deep upper low over AZ will slowly wobble eastward over the next two days as an influx of Gulf moisture wraps in from the southeast. This will set up a major winter storm for the high elevations of the central/southern Rockies with snowfall rates of 1-3"/hr at times and total accumulations well over 1-2ft at the highest peaks. The environment will be mild overall given the late season timing and focus of colder air well south (central AZ/NM), meaning that much of the I-25 corridor will remain snow-free (except for the typical higher elevations along the corridor in the Palmer Divide and Raton Mesa). The system will become elongated and stretch eastward tomorrow which will signal the end of modest snow, but lighter snow will persist into the evening/overnight and largely end by Thursday morning. The system will come in two phases for many areas, with the lead- in WAA-driven lighter snow early D1 (tonight) and the dynamically- driven heavier snow tomorrow by the afternoon through the evening as the upper low approaches the central CO/NM border from the WSW. Models indicate a robust system with strong isentropic ascent at the 305K level from the Gulf and strong/anomalous/record easterly flow at 700mb by midday tomorrow (per the CFSR period). As can be typical with strong systems, dynamical cooling may bring down snow levels from ~8000-8500ft to 7500ft or so, reaching farther down the Front Range toward the high elevations of the I-25 corridor before then rising as the upper low pulls away. Favored areas for the heaviest snowfall are over the San Juans this evening/overnight on southerly to southeasterly flow, then transitioning to the east/upslope side of the CO Rockies and into the Sangre de Cristos. Total snow may be 2-3ft (perhaps more) at the highest elevations including Pike's Peak and along the spine of the Sangre de Cristos. Impacts will be moderate to major per the WSSI. WPC probabilities for at least 8 inches of snow over the next two days are >50% above 8500ft or so and >50% above 10,500ft or so. Depending on the time of day and amount of dynamical cooling, there are low probabilities (10-40%) of at least 2 inches of snow as low as 6500ft or so. Farther north, snow levels will continue to fall behind a cold front currently through WY with precipitation trailing to its northwest through Montana, tied to northern stream troughing/vorticity diving south-southwestward tonight/early tomorrow. This rain/snow will continue to progress southward, with snow levels around 6000-8000ft from north to south, favoring snow over portions of southern MT, the Absarokas, and into the Bighorns and Wind River Range. There, WPC probabilities of at least 8 inches of snow are at least 30% (highest in the Absarokas). The probability of at least 4 inches of snow on day 3 are less than 10%. The probability of significant freezing rain across the CONUS is less than 10%. Fracasso $$ = = = 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) .