Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 342 AM EST Tue Feb 27 2024 Valid 12Z Tue Feb 27 2024 - 12Z Fri Mar 01 2024 ...Central Rockies... Day 1... A potent shortwave will continue to dig across the Rockies this morning and then reach the Northern Plains later today. 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 continue to drive the arctic front southeast through the Rockies today and into the High Plains 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. 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% across the 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. 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 with the frontal passes, resulting in snow 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 could create dangerous travel. ...Pacific Coast into the Northern Rockies... Days 1-3... In the immediate wake of the lead shortwave moving across the Intermountain West today, 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, beginning late in the Day 1 period across the PacNW then continuing southward through central CA into Day 3 and beyond. 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. 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 D1/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 by the end of D3. 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 8 inches of snow reaching above 80% in the Olympics, WA Cascades, OR Cascades, and Northern Rockies, and then spreading rapidly southeast late in the week into the Shasta/Trinity/Siskiyou region and the Sierra, as well as into central ID and NW WY terrain. Heavy snowfall will continue across the Cascades as well through D3, and storm total snowfall above 3000 ft will likely exceed 4-5 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. For the Sierra, the latest WSSI-P is showing slight to moderate probabilities (30 to 60 percent) for Extreme impacts with possible blizzard conditions. Dangerous travel is expected. Although beyond this current forecast period, additional heavy snow is likely to continue across this area through early this weekend, finally waning later in the upcoming weekend. ...Northern Plains to the Great Lakes... Days 1-3... Several embedded shortwaves will move out into the Northern Plains today, with the initial vort max skirting up to the ND/Canadian border followed by a southern one moving eastward out of SD into the Midwest. These two features will merge, coalescing into a broad negatively titled trough over the Great Lakes that strengthens further over southeastern Canada by Thursday. Aloft, the northern stream jet will dip into the lifting southern stream jet across the Southwest today, 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 late tonight. 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) is likely to 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 6 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. HREF shows probs of >1"/hr rates up to 70% around 16Z today 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 Wednesday evening. 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 brief upslope post-FROPA. Weiss/Fracasso/Taylor Key Messages for the Intermountain West Winter Storm --Strong cold front A strong cold front will continue to progress through the region, reaching the Central Rockies today. --Near-blizzard conditions and dangerous travel Snowfall rates of 1-2â€/hr will move into the Central Rockies today. 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 Colorado Rockies southward into the San Juans, and Sangre de Cristos, there is a high chance (>70%) of more than 8 inches of snow in the higher elevations. --Snow squalls along cold front passage Snow squalls are likely along the path of the cold front from Utah to Wyoming and Colorado today. Where snow squalls occur, intense snow rates will produce rapid drops in visibility and a flash freeze, resulting in dangerous travel. Key Messages for the Major Western Winter Storm --Another powerful storm for the West A significant winter storm will move into the Pacific Northwest late Wednesday into Thursday and into the Sierra Thursday into Saturday. --Very heavy mountain snow Multiple feet of snow are likely (>60% probability) for higher elevations, especially above 5000ft, including many of the Cascade and Sierra passes. Snow rates of 1-3"/hr are possible in the mountains. --Blowing snow and whiteouts possible Strong winds will cause blowing and drifting snow with reduced visibility and difficult to impossible travel in the mountains. There is a high chance (>70%) of substantial disruptions to daily life across the Sierra crest Fri-Sat. --Lowering snow levels and much colder temperatures As the storm moves southward, snow levels will lower into some northern CA and Sierra foothill communities. Much colder air is forecast by the weekend with temperatures 10-20ÂF below normal.