Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 354 PM EST Sun Feb 25 2024 Valid 00Z Mon Feb 26 2024 - 00Z Thu Feb 29 2024 ...The West... Days 1-3... Back to back significant systems will bring widespread heavy snow and dangerous travel to much of the West from the Pacific Northwest through the Northern and Central Rockies, and across CA and the Great Basin. A potent shortwave digging out of the Gulf of Alaska will pivot southeast along the British Columbia coast and then dig onshore WA state Monday morning. This will produce increasingly confluent mid-level flow downstream, driving impressive moisture onshore D1. As this trough continues to dig southeast, it will interact with a southern stream shortwave moving east into southern CA, especially by D2, creating a full-latitude trough which will pivot east into the Intermountain West Tuesday and then continue to advect to the Central Plains Wednesday. It is unlikely that phasing of the northern and southern streams will occur as guidance continues to progress faster with the northern stream which leaves the CA shortwave behind, but either way the confluent moisture streams from each of these impulses will be acted upon by robust synoptic ascent to drive widespread precipitation across the region. At the same time, this northern stream impulse will push an impressive arctic cold front southward, which will have the two-pronged effect of driving low-level convergence for strong ascent, while also causing snow levels to crash rapidly from around 3500 ft early, to less than 500 ft in its wake. This will cause increasingly more significant impacts to travel as snow spreads into the lowlands and valleys through much of the West, with significant travel problems likely at most of the area mountain passes from the Cascades through the Central Rockies. Additionally, strong fgen, elevated SBCAPE, and gusty winds will likely produce a linear feature, or features, of snow squalls moving along or just behind it. While additional accumulations within these squalls is likely to be modest, brief intense snowfall rates of 1-2"/hr combined with strong winds could produce near zero visibility at times. More general snowfall rates of 1-2"/hr is also expected in the terrain, most likely in favored upslope regions. WPC probabilities for more than 6 inches D1-D2 are extremely impressive for a large portion of the West. Probabilities exceeding 80% extend along the length of the Cascades and the Olympics, eastward along the Northern Rockies including the Salmon River/Sawtooth Ranges and the Blue Mountains in OR, into the Absarokas and NW WY ranges, and then southward along the Wasatch, the CO Rockies, and into the San Juans and Sangre de Cristos. With long duration of heavy snow rates of 1-2"/hr in many of these areas, snowfall totals in the higher terrain reaching 2 feet across the Intermountain West ranges, and as much as 5 feet in the Cascades. As this first trough pivots into the Central Plains, brief shortwave ridging will occur in its wake bringing an end to the forcing and a brief respite to snowfall. However, by early D3, a resumption of onshore flow will occur into the Pacific Northwest as mid-level zonal flow gradually backs to the SW ahead of yet another shortwave digging along the British Columbia coast. This backing of the low to mid level flow will cause WAA and snow levels steadily rising to around 3000 ft again. This should prevent additional lowland/valley snow, but once again significant accumulations are likely as reflected by WPC probabilities for more than 6 inches once again reaching above 80% in the WA Cascades and Northern Rockies, with locally an additional 1 foot of snow likely in the higher elevations. ...Northern Plains to the northern Great Lakes... Days 2-3... A fast moving but amplifying shortwave will race eastward from the Northern Rockies on Tuesday, reaching the Northern Plains Tuesday night and then becoming more neutrally tilted as it crosses the Great Lakes during Wednesday. Sharp but transient height falls and impressive PVA will result in dual waves of low pressure moving along the arctic cold front which will also traverse southeast beneath this impulse. During this evolution, a potent subtropical jet streak will intensify south of the amplifying trough, reaching more than 150 kts over the Central Plains Wednesday, and get pulled poleward to produce enhanced LFQ diffluence atop the greatest height falls. This favorable overlap will enhance the surface low pressure falls, and it is likely that despite the fast overall motion, intense ascent will spread across the region D2-3. This deep layer lift will act upon increasing moisture as PWs surge northeastward downstream of this trough and within intensifying moist isentropic ascent in the 290-295K layer. The dual waves will somewhat offset the meridional extension of the highest moisture, but there is some potential for a better surge of theta-e air and a modest TROWAL pivoting into the northern Great Lakes D3. Although the antecedent airmass is marginally cold enough for heavy snow, the isentropic ascent atop the front combined with the rapidly cooling airmass indicates much of the precip will be anafrontal, which will overlap with the colder air, a deepening DGZ, and periodic impressive fgen to drive heavy snow rates. This suggests a fast moving swath of heavy snow will move eastward through the Northern Plains and then Great Lakes, followed by LES downstream of Lake Superior late D3. WPC probabilities D2 for more than 4 inches reach 50-70% from eastern ND into northern MN, and 70-90% across the U.P. of MI on D3. Weiss *** Key Messages for Significant Western U.S. Storm *** --Strong winter storm and cold front The strong cold front will move into the Northwest this evening, progress southeastward on Monday into the Northern Rockies, and then into the Central Rockies by Tuesday. --Heavy mountain snow Snowfall rates of 1-2"/hr are expected over the Cascades and Northern Rockies through Monday, before spreading into the Great Basin and Central Rockies Tuesday. These snow rates combined with winds gusting 50-65 mph will produce near-blizzard conditions with significantly reduced visibility and snow-covered roads leading to dangerous travel. --Significant snow accumulations More than 2 feet of snow is expected (>80% chance) in the Cascades through Tuesday, with locally as much as 4 feet possible in the highest terrain. Elsewhere across the Intermountain West, there is a high chance (>70%) of more than 1 foot of snow in the higher elevations. Lowering snow levels will also produce some accumulations onto the valley floors. --Widespread snow squalls are likely Snow squalls are expected to develop along the path of the cold front Monday and Tuesday. Where snow squalls occur, intense snow rates will produce rapid drops in visibility and a flash freeze, resulting in dangerous travel. --Much colder air behind the strong cold front Temperatures will fall into the teens and single digits by Tuesday morning along and east of the Rockies.