Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 418 AM EST Thu Mar 09 2023 Valid 12Z Thu Mar 09 2023 - 12Z Sun Mar 12 2023 ...West Coast to the Northern and Central Rockies... Days 1-3... A deep, compact upper low centered off of the Pacific Northwest coast is forecast to settle south today and help channel a long fetch of Pacific Moisture across California into the Great Basin beginning later today. Model consensus continues to show impressive integrated vapor transport values centered across the region beginning late today and continuing into Friday, while both the NAM and GFS show PW standardized anomalies exceeding 3 standard deviations across much of central to southern California and central Nevada by this evening, with additional increases forecast overnight. This moisture will be accompanied by much warmer air, with snow levels forecast to rise rapidly to around 8000-9000 feet across the central to southern Sierra. Deep moist, onshore flow with embedded energy aloft will continue to support periods of heavy precipitation, including high elevation snow through Friday before the moisture advection begins to wane by early Saturday. The Winter Storm Severity Index (WSSI) continues to show extreme impacts in the central to southern Sierra above 8000 ft, where several feet of very heavy wet snow is expected. Heavy snow is also expected for the Shasta Cascade region and the northwestern California mountains, where snow levels are forecast to climb to around 4000 feet before dropping again as the upper low and its associated cold front move inland. The WSSI indicates extreme impacts for the higher elevations of this region as well. Key Messages for this impending winter storm can be found below. Areas of heavy snow are also expected farther north and east, from the higher elevations of the Pacific Northwest coastal ranges to the northern Rockies as subtropical moisture from the south is drawn north out ahead of the upper low. Strong upper forcing and low-to-mid level frontogenesis will support the threat for heavy snow moving east across the region today and Friday, with widespread accumulations of 1-2 feet expected, and heavier amounts likely across the higher peaks. Areas impacted through Friday will likely include the Washington and Oregon Cascades, the Blue Mountains, and the Rockies from northern Idaho and northwestern Montana to northern Utah. By late Friday and continuing into Saturday as the front continues to push southeast, areas of heavy are likely to shift farther south and east, impacting portions of the Colorado Rockies also. ...Northern Plains, Upper Midwest, Great Lakes, & Northern Mid-Atlantic... Days 1-2... A shortwave trough is forecast to move east of the central Rockies into the Plains this morning before lifting further to the east-northeast and amplifying as it moves across the lower Missouri and mid Mississippi Valleys toward the Great Lakes later today. Favorable upper jet forcing is expected to promote the development of heavier snow, with rates up to 1 in/hr, falling north of an organizing low level center as it moves from eastern Nebraska into Iowa later today and then slowly into northern Illinois during the overnight. Model consensus shows the wave continuing amplify, with a closed low developing and moving east across the Great Lakes tonight. Strong upper forcing along with low-to-mid level frontogenesis is expected to carry the threat for heavier snow into portions of the eastern Great Lakes and northern Mid Atlantic on Friday. WPC guidance shows high probabilities for snow accumulations of 4 inches or more extending from the upper Mississippi Valley to western New York and central Pennsylvania. A secondary area of higher probabilities associated with an inverted surface trough is also shown from southwestern Minnesota into eastern South Dakota. WPC probabilities indicate locally heavier amounts (8 inches or more) are more likely here as well. Another area where locally heavier amounts are more likely is along the southeastern Wisconsin shores of Lake Michigan, where some lake enhanced totals are expected. See below for Key Messages regarding this storm as well. ...Northern Plains to Upper Midwest... Days 2-3... Heavy snow is expected to develop and move east across the northern High Plains as the upper low moving into the Pacific Northwest today begins to re-amplify and move east of the northern Rockies Saturday morning. Southerly winds will draw increasing moisture into a region of strong ascent, supporting a swath of heavy snow moving east from northeastern Montana Friday night and then across North Dakota and over the Red River into northwestern and central Minnesota on Saturday. In addition to heavy snow, a tight pressure gradient is likely to support strong gusty winds and blowing snow across the region. WPC probabilities indicate that snowfall totals of 6 inches or more are likely from far northeastern Montana to the Red River Valley. For days 1-3, the probability of icing greater than 0.1" is less than 10 percent. Pereira ...Key Messages for the March 9-10 (Western U.S.)Winter Storm... --Strong Winter Storm Arrives Later Thursday: A winter storm will reach the West Coast later in the day on Thursday and provide a burst of heavy precipitation into Friday. --Heavy Rain and High Snow Levels Likely: Several inches of rain will fall in the upslope areas of the Coastal Ranges and Sierra Nevada in California. This will be a warm storm system with rain falling on existing snowpack up to 8500 feet, with the highest snow levels expected in central California. --Rain and Snowmelt May Lead to Flooding: The most significant snowmelt, and overall flooding threat, is expected below 5000 foot elevation, in areas with shallow snowpack. Creeks and streams in the foothills of the Sierra Nevada will be most vulnerable to flooding from rain and snowmelt. -- Heavy, Wet Snow at Higher Elevations: Higher elevations in northern California and in the Sierra Nevada are likely to see a heavy, wet snow. This will lead to difficult travel, and combined with an already deep snowpack, may lead to increasing impacts from the depth and weight of the snow. ...Key Messages for the March 8-12 (Northern Plains to the Mid Atlantic) Winter Storm... --A winter storm is set to track through central High Plains today then across the Lower Great Lakes tonight into Friday, bringing periods of heavy snow with snow rates up to 1â€/hr at times. --The highest snowfall accumulations are expected across southern Minnesota, southern Wisconsin, northeast Iowa, and far northern Illinois. Totals of 6-8â€+ will be common. --Plan on travel impacts including snow-covered roads, reduced visibility, and difficult travel across the impacted areas. Whiteout conditions are possible within the heaviest snow bands. --As the storm moves east, several inches of snow will be possible across the northern Mid-Atlantic including northern Ohio, western New York, and northwest to central Pennsylvania which may bring travel impacts and disruptions.