363 FOUS11 KWBC 131838 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 238 PM EDT Mon Apr 13 2026 Valid 00Z Tue Apr 14 2026 - 00Z Fri Apr 17 2026 ...Central Rockies... Day 1... Closed mid-level low emerging from the Great Basin will open into an amplified, but positively tilted, trough as it ejects into the Central Rockies by Tuesday evening. This will lead to a period of enhanced lift through divergence and height falls, with the LFQ of an accompanying upper jet streak helping to produce cyclogenesis in the lee of the CO Rockies Tuesday aftn. Mid-level Pacific moisture on the 700-500mb SW flow will increase column PWs to +1 to +2 sigma across CO, with some additional low-level moisture becoming available late D1 as cyclonic flow around the developing cyclone creates E/NE winds from the Central Plains to advect additional moisture westward from the Gulf moisture return. This moisture being acted upon by the increased synoptic lift (and some upslope flow on the low-level E/NE winds) will create periods of moderate precipitation Tuesday, with significant snow accumulations expected above generally 7000-8000 ft. WPC probabilities D1 are moderate to high (50-90%) for 6+ inches across the Wasatch, San Juans, and CO Rockies, with locally 12+ inches possible (30-50%) across the San Juans before precip winds down very late Tuesday night as the surface low pulls away to the east. ...Pacific Northwest through the Northern Rockies... Days 1-3... An increasingly active week of winter weather will begin today, with widespread snowfall expected across most of the terrain from the Pacific Northwest through the Northern Rockies. The period begins with an amplified/closed low over southeast Alaska dropping steadily southward to reach the British Columbia coast by 00Z/Wed, and then pivot onshore WA/OR before 00Z/Thu. This evolution will not only produce impressive height falls for ascent, but also pinch the downstream flow to help surge Pacific moisture into the region. This is reflected by IVT for which both the ECENS and GEFS indicate has a >90% chance for exceeding 250 kg/m/s, with the West-WRF indicating a narrow corridor of 500+ IVT surging onshore as well. This IVT will be driven rapidly eastward beneath a zonally oriented jet streak amplifying south of the upper low, combined with intensifying SW low-level flow driving WAA ahead of a surface cold front. Together, this will push PWs to +1-+2 sigma, although the coverage of these highest PWs will be somewhat confined to a narrow channel ahead of the cold front. As this moisture spreads east and is acted upon by the robust ascent, the result will be an expanding shield of precipitation, with locally heavier precipitation expected in the higher terrain where upslope enhancement is likely. Snow levels will vary widely through the event, but are expected to generally fall D2/D3 behind the cold front and the parent trough driving it southeast. On D1, the heaviest accumulations are likely confined to the Cascades as the event begins, which is reflected by WPC probabilities for 6+ inches of snow above 50% focused in the WA Cascades and Olympics, and generally above 4000 ft. During D2, the coverage of heavy snow expands considerably as the front and accompanying synoptic ascent shift east in tandem with the core of the IVT moving inland. This results in WPC probabilities for 6+ inches being above 50% from the Olympics, along the WA and OR Cascades, and eastward into the Blue Mountain, Sawtooth/Salmon River, and much of the Northern Rockies. Snow levels in the Rockies will fall from around 6000 ft to 4000 ft, but will crash well below pass levels, as low as 2500 ft, in the Cascades, leading to hazardous pass-travel. Finally, during D3 the snowfall will wane across the Cascades but continue to expand eastward as far as the Absarokas, Tetons, and Wind Rivers, while continuing across much of the rest of the Northern Rockies. Snow levels D3 fall to 3000-4000 ft in these areas, with WPC probabilities suggesting a 70+% chance of at least another 6 inches of snow. Storm total snowfall during this event will be considerable, forecast to reach 1-3 feet in the higher elevations. Additionally, as the cold front sweeps southeast Wednesday night and Thursday, it will be accompanied by a line of heavier precipitation as low-level RH, 0-2km fgen, and 0-2km CAPE maximize. The simulated reflectivity from the available guidance suggests there will be a line of precip with embedded heavier rates, and the snow squall parameter rises above 1. While the setup is not ideal, and we are past the climatological favored period for snow squalls, this setup does suggest the potential for convective snow showers or snow squalls across the area. Additional snowfall from any showers/squalls will be limited, but briefly heavy rates and gusty winds could cause hazardous travel. This will need to be monitored as we approach the high-res guidance windows to get a better understanding of the timing, placement, and risk, of any snow squalls. The probability of significant icing for days 1-3 across the CONUS is less than 10 percent. Weiss $$