Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 321 PM EST Mon Feb 26 2024 Valid 00Z Tue Feb 27 2024 - 00Z Fri Mar 01 2024 ...Northern and Central Rockies... Day 1... A potent shortwave will dig out of the Pacific Northwest this evening and then slide towards the Northern Plains by Wednesday morning. Although the sharpness of this feature is likely to weaken with time, it will become embedded within a larger wavelength, full latitude trough across as it sheds to the east. This will result in expansive deep layer ascent through height falls and PVA, with the resultant strengthening of the subtropical jet streak (reaching 190 kts) downstream driving additionally enhanced synoptic lift. This shortwave will drive an arctic cold front southeast through the Rockies and into the High Plains by Wednesday, which will result in locally enhanced ascent through surface convergence, while also leading to increasing upslope flow into the terrain on the post-frontal wind shift. Although this northern stream trough will outrun a southern stream impulse, this secondary feature will play an important role in the precipitation as well. Southwest flow downstream of this secondary feature will advect deeper moisture northeastward into the Desert Southwest and Central Rockies, with PW anomalies according to NAEFS exceeding +2 sigma in some areas driven by IVT of more than 250 kg/ms. This moisture will align ahead of the front to be wrung out effectively by the approaching lift, causing widespread precipitation from the Northern Rockies, through the eastern Great Basin, and most impressively into the Central Rockies. Although much drier air will follow the front, an extended duration of heavy snowfall is likely in these areas before drying occurs late D1 into D2. Snow levels ahead of the front are forecast to be 3000-5000 ft, but will fall extremely quickly behind the front to below 500 ft. While this indicates the heaviest snow will occur above 3000 ft which will experience the longest duration of snow, before precip wanes entirely even the lower valleys could see modest accumulations. This is reflected by WPC probabilities for 6+ inches reaching above 80% from the Absarokas and NW WY ranges, eastward through the Big Horns, and south into the Wasatch, CO Rockies, San Juans, and Sangre de Cristos. 1-2 feet of snow is likely in the higher terrain, and the lowering snow levels will also allow for some light accumulations down into the valleys across much of the region before snow winds down into D2. Additionally, the strong cold front moving southeast will create a favorable environment for linear snow squalls along and just behind the front. The overlap of 100-200 J/kg of SBCAPE with impressive 0-2km fgen and strong winds drives a high SnSq parameter across the Rockies/Great Basin with the frontal passes, resulting in sow squalls that will likely have snowfall rates of 1-2"/hr and strong winds to produce periods of near-zero visibility. Although additional snow accumulations within any squalls will likely be light, the rapid changes in conditions and resultant icy roads will create dangerous travel through Tuesday. ...Pacific Coast into the Northern Rockies... Days 1-3... In the immediate wake of the lead shortwave moving across the Intermountain West on D1, brief shortwave ridging will bulge across the Pacific Northwest before rapidly transitioning to zonal flow from the Pacific once again. The cause of this rapid change is yet another shortwave digging out of the Gulf of Alaska and traversing along or just offshore the British Columbia coast, resulting in downstream backing of the flow from W to SW by Thursday morning. The 500mb pattern will additionally respond by becoming increasingly pinched downstream of this shortwave and accompanying strung out vorticity lobes, driving more intense moist advection onshore, especially D2 and D3 from WA through central CA. This mid-level confluence will be directly topped by an upper level jet streak reaching 110 kts and arcing from SW to NE, enhancing onshore moisture advection while also providing additional ascent through LFQ diffluence. This moisture advection will manifest as an impressive atmospheric river (AR), with both the GEFS and ECENS ensembles indicating a greater than 70% chance of 500 kg/ms IVT shifting onshore late D2 into D3. Although the core of this IVT is expected to impact coastal OR and northern CA, the accompanying PW plume is progged to reach as far inland as the Northern Rockies and Great Basin as reflected by NAEFS IVT anomalies of +2 and PW anomalies nearing +1 well inland. The overlap of the impressive synoptic lift and this increasing moisture will result in yet another round of heavy precipitation spreading across the West, initially in the Pacific Northwest late D1/early D2 and then expanding south and east as far as the Sierra to the south and Absarokas to the east as a surface cold front sags southeast with time beneath the mid-level trough. Snow levels during precipitation onset will be much higher than with the previous event, likely starting around 4000-5000 ft, possibly as high as 6000 ft in the Sierra. The heaviest precipitation is expected during these higher snow levels due to the accompanying WAA which will keep these snow levels elevated while driving the more intense ascent for expanding precipitation. However, the cold front sagging southeast will result in a slow decline in snow levels, falling to around 1500 ft in the Cascades by the end of D3, but still remaining elevated above 5000 ft in the Sierra at that time. Impactful snow is still possible as these snow levels crash, but regional soundings and ensemble clusters show decaying column moisture during this time, so lower-elevation snowfall is likely to be much less than at higher elevations. This is reflected by WPC probabilities for more than 6 inches of snow reaching above 80% in the Olympics, WA Cascades, OR Cascades, and Northern Rockies D2, and then spreading rapidly southeast during D3 into the Shasta/Trinity/Siskiyou region and the Sierra, as well as into central ID and NW WY terrain. D3 snowfall will continue across the Cascades as well, and by D3, storm total snowfall above 3000 ft will likely exceed 4 feet in the higher elevations of the Cascades, and 1-2 ft elsewhere. Heavy snow rates of 1-2"/hr combined with strong winds will result in major impacts to travel across these areas as well. Although beyond this current forecast period, additional heavy snow is likely to continue across this area through D4 and D5, finally waning later in the upcoming weekend. ...Northern Plains to the Great Lakes... Days 1-3... A multi-stream shortwave will move out of the northern High Plains early Tuesday, with a northern vort max along the ND/Canadian border and a southern one moving eastward out of SD into the Corn Belt. These two will coalesce into a negatively-tilted trough over the Great Lakes that will continue to strengthen into Ontario and then Quebec as an upper low by early Thursday. Aloft, the northern stream jet will dip into the lifting southern stream jet across the Southwest early Tue, with broad upper divergence in the LFQ of the 180kt jet favoring surface cyclogenesis. This will take low pressure from SD across MN into MI by Tue evening/overnight. Despite the lack of high pressure over Canada out ahead of the system and only marginal temperatures today, increased northerly flow around the deepening cyclone will draw in colder air around the system, favoring all or nearly all snow from ND into northern MN. The best moisture flux will be into the Great Lakes with strong W to WSW flow at 850mb across the central Plains (~50kts) that curves northward, but dynamics support at least some higher theta-e air wrapping back into the low via a TROWAL (albeit, briefly) as the system maintains a progressive pace. Combination of favorable FGEN, lift into the DGZ, and timing (overnight into early morning hours) could support a narrow heavier axis of snowfall within a broader swath of light to modest snowfall across eastern ND into northern MN. In addition, due to the tightening pressure gradient and strengthening winds, near-blizzard conditions will be possible across far eastern North Dakota and northwestern Minnesota with the combination of strong winds and snowfall creating near-zero visibility at times. The latest Winter Storm Severity Index is highlighting Moderate level impacts, driven mostly by the blowing snow and ground blizzard potential. WPC probabilities for at least 4 inches of snow are moderate (40-70%) from eastern ND into northern MN, and high (>70%) over northern MN around International Falls. Within this area, WPC probabilities of much higher snowfall (>12 inches) is low (10-20%), suggesting the potential for enhanced banding within this broader zone. 12Z HREF shows probs of >1"/hr rates up to 70% around 16Z Tue over northern MN as well. As the low wraps up into Quebec by D3, the strong cold front will bring in much colder air to the Great Lakes region, favoring some lake effect snow over especially the Michigan U.P. starting Wednesday on northerly flow then into the eastern Great Lakes (western NY) as the flow backs to NW by early Thursday. Heights will be fairly quick to rise from NW to SE across the Great Lakes, so the LES may be short-lived and largely end by the end of this period (00Z Thu). WPC probabilities for at least 4 inches of snow on D3 are highest (30-50%) over central NYS around SYR on WNW flow and also into the Chautauqua ridge. In addition, with colder air helping to change rain to snow, WPC probabilities for at least 4 inches are low (10-30%) over northern New England with wrap around snow and also into the central Appalachians via upslope post-FROPA. Weiss/Fracasso Key Messages for the Intermountain West Winter Storm --Strong winter storm and cold front A strong cold front will continue to progress through the region, reaching the Northern Rockies today and the Central Rockies Tuesday. --Near-blizzard conditions and dangerous travel Snowfall rates of 1-2"/hr will move into the Great Basin and Central Rockies on 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 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 along cold front passage Snow squalls are likely along the path of the cold front from Montana to Idaho today and Wyoming to Utah and Nevada on 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 throughout the Intermountain West.