848 FOUS11 KWBC 260655 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 255 AM EDT Fri Apr 26 2024 Valid 12Z Fri Apr 26 2024 - 12Z Mon Apr 29 2024 ...Great Basin into the Rockies... Days 1-2... A mid-level shortwave will surge onto the CA coast with an elongated vorticity lobe strung out along its southern periphery. This entire feature will pivot eastward into the Great Basin by Saturday morning, with amplification into a closed low expected over the Four Corners. This will drive increasing ascent across much of the Intermountain West, with the most intense lift moving across the Central Rockies Saturday into Sunday. In this area, forcing will be provided via an overlap of impressive height falls, downstream divergence, and increasing LFQ diffluence as the subtropical jet streak pivots around the base of the trough and arcs poleward. This overlap of ascent will drive surface cyclogenesis in the lee of the Rockies as well, with E/NE flow behind the associated cold front also producing upslope flow into the terrain of CO/WY. At the same time, mesoscale ascent will maximize as a potent deformation axis and accompanying mid-level fgen drive lift into the DGZ, which will fuel the potential for CSI and even CI as reflected by cross-sections and good model agreement of an axis of theta-e lapse rates less than 0C/km collocated with -EPV. This indicates the likelihood for some areas receiving snowfall rates in excess of 1"/hr, and the WPC prototype snowband tool suggests locally 2+"/hr rates are possible. All of this lift will work across an environment with widespread PW anomalies above +1 sigma according to NAEFS as moisture lingers from a recently departed low, and in response to persistent onshore flow from the Pacific spreading moisture eastward. Although snow levels will be moderate at around 8000 ft (using the NBM 25th percentile as a guide), strong and dynamic ascent should permit at least some snow into the lower elevations. The heaviest snow is likely above 8000 ft, especially in the vicinity of the Front Range on D2, where WPC probabilities for more than 6 inches are 80-90% or more, and with sufficient upslope flow wringing out the moisture, where it remains all snow, some areas could see as much as 3 feet of accumulation when added up over D1 and D2. Other heavy snow areas in CO include the San Juans and remaining CO Rockies where WPC probabilities D2 reach 50-70% for 6+ inches. Elsewhere across the Intermountain West, WPC probabilities D1 for more than 6 inches are above 80% around Yellowstone NP and across the Wind River Range, and 50-80% in the Uintas, Wasatch, and higher terrain of eastern NV. During D2 the focus shifts into CO, but some additional moderate snowfall accumulations are possible in the Uintas and Wind Rivers. ...Pacific Northwest... Day 3... Confluent westerly flow across the Northern Pacific will gradually back to the SW ahead of an approaching shortwave D3, and this impulse is likely to pivot onshore the WA/OR coast late Sunday night while amplifying. This shortwave will be accompanied by modest upper level diffluence in the LFQ of a 90 kt jet streak, enhancing ascent into the area. This confluent flow and overlapping jet streak will also surge moisture eastward, with an arc of 150-250 kg/ms IVT supporting an expanding precipitation shield beginning late Sunday aftn and expanding into Sunday night. Snow levels across the Cascades should generally be above 4500 ft limiting total impacts, but WPC probabilities for more than 6 inches peak above 60% in the northern WA Cascades and highest terrain farther south towards OR. The probability of significant icing is less than 10 percent. Weiss $$