Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
145 FOUS11 KWBC 010650 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 250 AM EDT Wed May 1 2024 Valid 12Z Wed May 01 2024 - 12Z Sat May 04 2024 ...Pacific Northwest and Northern Rockies... Days 1-2... Active winter weather will continue across the northern tier of the West through late week as a large trough persists over the region. The core of this trough will be in the form of an elongated closed low centered over Saskatchewan but extending towards the northern Great Basin, and several impulses and associated vorticity lobes will swing through the flow and into the Northern Plains. These produce periods of enhanced ascent, which will wring out moisture as snow, primarily in terrain features above 4000 ft. In general, the forcing is pretty transient, and moisture is modest as noted by NAEFS PW anomalies that are slightly below normal. This suggests that any heavy snowfall will be primarily driven by where upslope flow can enhance the broad synoptic lift already occurring. The exception is still likely to be during D1 from the Lewis Range in the Northern Rockies southward through the Little Belts and maybe as far south as Yellowstone NP and the Big Horns in WY. Here, an inverted trough traversing south out of Canada will produce some enhanced E/NE upslope flow into this terrain, but more importantly this will be accompanied by some stronger mesoscale ascent as a weak theta-e ridge (remnant TROWAL) pivots south in conjunction with a modest deformation axis. The overlap of additional moisture and ascent within broad synoptic lift will drive heavier and more prolonged snowfall across this region, and this is reflected by WPC probabilities D1 and D1.5 that are 70-90% for 6+ inches reaching as far south as the Little Belts, with lower probabilities into the Big Horns. Event total snowfall in the Lewis Range could exceed 12 inches. Elsewhere, WPC probabilities are highest for 6+ inches in the OR Cascades D1.5, reaching 40-50%, with lower probabilities extending across other ranges including portions of the Absarokas. ...Central Rockies... Day 1... A fast moving Pacific jet streak arcing from the Great Basin into the Northern Plains will combine with weak height falls downstream of a northern stream shortwave to drive modest cyclogenesis in the lee of the CO Rockies Wednesday aftn. This low will deepen through Wednesday night but then lift quickly into the Central Plains by Thursday morning in response to fast progression of the driving synoptic features. Moisture across the region during this time will increase due to Pacific air flooding eastward combined with at least modestly enhanced moist isentropic upglide on southerly flow out of the Gulf of Mexico then rotating back into Colorado. The overlap of ascent into this moistening column will wring out precipitation across the terrain, with moderate to heavy snow accumulating above around 9000 ft, as the guidance has warmed a bit in the past few cycles. The general modest moisture and rapid progression of the forcing will limit snowfall accumulations somewhat, but WPC probabilities are 10-30% in the higher terrain of the CO Rockies, focused across the Park Range, before colder and drier air sinks into the region by Thursday morning cutting off precipitation. The probability of significant icing is less than 10 percent. Weiss $$