Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 250 PM EST Fri Mar 01 2024 Valid 00Z Sat Mar 02 2024 - 00Z Tue Mar 05 2024 ...West Coast & Northern/Central Rockies... Days 1-3... **Significant multi-day winter storm to produce extremely heavy snowfall for the Sierra Nevada with widespread snow eastward to the Rockies** Today, the highly anomalous upper trough (200-500mb heights that are outside the CFSR 1979-2009 climatology) will continue to dig south just off the West Coast while an embedded 500mb shortwave trough spawns a compact surface low just off the Oregon Coast this afternoon. This disturbance will accompany a powerful 120kt 500mb jet streak whose divergent left exit region will become ideally placed over north-central California by this evening. Combined with the intense PVA ahead of the 500mb vort max, the impressive 500mb jet streak dynamics, and the barrage of rich 850-700mb moisture, strong vertical ascent aloft and along topographically-favored terrain will generate prolific snowfall rates along the Sierra Nevada above 4,000ft late Friday afternoon and through Friday night. The 00Z HREF shows high probabilities (>70%) for 3"/hr snowfall rates after 00Z Saturday that persist into Saturday Any time between 00Z Sat - 00Z Sun, there are at least 9-12 hr windows where >3"/hr snowfall rates are occurring somewhere along the spine of the Sierra Nevada. Add in the >80 kt 500mb wind speeds over the Sierra Nevada and blizzard conditions are anticipated Friday evening into Saturday. Meanwhile, farther inland, the left-exit region of a 150kt 250mb jet streak will become placed over southern Idaho and into western Wyoming. The steady stream of Pacific moisture will continue to support heavy snow through Friday night and into Saturday. By Saturday, the axis of highest 850-700mb moisture flux will be directed from central and southern California into the heart of the Intermountain West. Strong jet streak dynamics aloft and impressive upslope flow will keep heavy snow in the forecast at elevations >2,000ft in the Cascades and Olympics, >3,000ft in the Sierra Nevada, and >6,000ft in the Sawtooth, Tetons, Wind River, and Bear River Ranges. The impressive 700-500mb wind field will also be responsible for strong wind gusts that cause both blowing/drifting snow and whiteout conditions. The cold front will become stationary from the Sierra Nevada and Wasatch to the Colorado Rockies which will remain in place through Sunday. This front will help funnel a relentless slug of Pacific moisture into these mountain ranges through the remainder of the weekend. By Sunday evening, snow will have concluded in the Cascades with light snow lingering in the Northern Rockies, but heavy snow will stick around from the central and southern Sierra Nevada to the Wasatch, the Tetons/Wind River Range, and northern Colorado Rockies. Monday is a near carbon-copy setup with westerly mean 850-300mb flow directing additional Pacific moisture, and resulting moderate-to-heavy snow across the northern Sierra Nevada, Oregon Cascades, and as far inland as the Wasatch, Sawtooth, and Tetons. In summary, this winter storm is likely to be one of the most impactful of the winter season in the Sierra Nevada with no shortage of heavy snow throughout most mountain ranges in the Northwest and both the northern and central Rockies. The WSSI shows Extreme Impacts forecast along the Sierra Nevada, in the mountains of northern California and along the Coastal Range of northern California and southwest Oregon above 3,000ft, and the Bear River Range in northern Utah. The Sierra Nevada are likely to witness the most extreme impacts with as much as 5-12ft of snow, blizzard conditions that result in significant blowing/drifting snow and whiteout conditions, and wind gusts topping 60 mph (locally as high as 100 mph in the tallest peaks) that can cause tree damage and power outages. Compounding the damaging wind threat is Snow Load, which the WSSI also depicts Extreme Impact potential at elevations >6,000ft. Elsewhere, Major Impacts (considerable disruptions to daily life) are depicted along the southern Oregon Cascades, the Olympics, the Sawtooth of southern Idaho, the higher terrain of the central Great Basin in Nevada, the Tetons, the Wasatch, and Wind River Range. Snowfall in these areas will also be measured in feet (2-4ft with localized amounts topping 5ft expected) in most of these areas. ...Northern High Plains... Days 2-3... As the 250mb jet streak and strong 500mb PVA works its way into the northern Rockies and High Plains on Saturday, a surface low will deepen over east-central Montana while easterly 850mb winds on the northern flank of the low wraps in 850mb moisture flux. This will prolong the periods of heavy snow in parts of the western Montana Rockies into the day on Saturday. Meanwhile, the deformation axis will primarily be located over the southern Canadian Prairies, but a warm front draped west to east from North Dakota to northern Minnesota will also be a focus for periods of snow and even an icy wintry mix in these areas Saturday afternoon and evening. Strong 300K isentropic glide via southwesterly flow will be the primary supplier for an icy wintry mix in these areas. Latest WPC PWPF sports low chances (10-20%) for >0.1" of ice accumulations in parts of north-central North Dakota Friday night into Saturday morning. As the warm front lift north, the bulk of the wintry precipitation should lift north into southern Canada, but there is still the possibility for this front to stick around longer near the International Border. By Sunday morning the storm will be racing towards the Red River of the North with heavy snow on the northwest flank of the low beneath the TROWAL. Most global guidance suggests the TROWAL pivots along the US/Canada border to produce >6" snowfall totals in northeast Montana and northern North Dakota. However, some CAMs suggest the bulk of the snow could remain north in southern Canada with more minor accumulations still on the table. The latest WPC PWPF sports moderate-to-high chances (50-70%) for >6" snowfall totals in far northeast Montana and northwest North Dakota with the higher end of those listed probabilities closest to the US/Canada border. Northwest North Dakota has low chances (10-30%) for snowfall totals >12". Northwest North Dakota is also where the WSSI-P highlights moderate chances (40-50%) for Moderate Impacts between Sunday morning and Monday morning. ***Key Messages for the Major Western Winter Storm*** --Heavy Mountain Snow Affecting Many Passes Multiple feet of snow are expected (90+% chance) for higher elevations, especially above 5000 ft, including the Sierra Nevada passes (such as I-80) where 5-12 ft of snow is forecast. Extremely heavy snow rates exceeding 3 inches per hour are likely. --Blizzard Conditions in the Sierra Nevada Strong winds gusting 60-80 mph, locally over 100 mph at the highest peaks, will cause significant blowing and drifting snow and whiteout conditions, making travel impossible in the Sierra Nevada. Substantial, long-lasting disruptions to daily life are expected in the higher elevations through Sunday. --Widespread Damaging Wind in the Western U.S. Wind gusts of 55+ mph are forecast across much of the West, particularly across higher elevations and the Intermountain West, where 75+ mph gusts are possible. These winds will cause difficult travel, and likely down trees and power lines, resulting in widespread power outages. --Cold Air Will Lower Snow Levels This Weekend As the storm moves south, snow levels will lower to bring accumulating snow down into many valleys. Much colder air is forecast by the weekend, with temperatures 10-20 degrees below normal. Mullinax