Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 313 PM EDT Wed Mar 19 2025 Valid 00Z Thu Mar 20 2025 - 00Z Sun Mar 23 2025 ...Pacific and Interior Northwest... Days 1-3... A series of shortwave troughs will continue impacting the Pacific Northwest and Interior mountains in-of ID/MT/WY over the next 72 hrs. Current WV satellite imagery shows a fairly robust upper trough located off the Pacific Northwest coast with a well-defined diffluent structure on the eastern flank of the trough axis. The ridge that has been present downstream is beginning to feel the affects of the upstream disturbance leading to some flattening of the northern ridge extension and a realignment of the ridge further east. This general synoptic progression will lead to increasing mid and upper ascent focused over a large portion of the west coast with the primary impacts focused over Northern CA up through much of OR/WA state. The beneficiary of the heaviest precip core over the course of D1-2 will be situated within the Cascades with snow levels hovering between 3000-4000' MSL prior to the stronger height falls that will occur as the shortwave progresses towards the coast with the mean trough taking on a more negative tilt as it eventually ejects inland later this evening. The cold frontal progression associated with the disturbance will move ashore around the time of the mean trough swinging inland generating a period of falling heights with snow levels dropping closer to 2000-3000' MSL leading to snow accumulations becoming more prevalent in the higher valleys around the Olympic Peninsula and Cascades. Snowfall accumulations of at least 4" remain very high probabilistically (>70%) across the higher elevations of the Cascades and Olympics in WA/OR, as well as the Salmon, Siskiyou and Mount Shasta regions of CA. Mid-level probs between 40-60% for at least 4" are located over the Northern Sierra's with the highest chance in elevations above 7500' MSL. Further inland, the prospects for heavy snow continue later this evening as the shortwave propagates through WA/ID with the greatest forcing aligned over the Cabinet/Lewis/Bitterroot Ranges in the Interior Northwest with the southern extent benefiting the Absaroka Range of northwest WY. Similar probs of >70% for at least 4" of snowfall exist in those terrain focused regions with the impacts falling within the back-half of D1 into much of D2. As this first trough shifts eastward, forcing will weaken across the Northwest, but flow across the Pacific becomes increasingly zonal due to squeezing between broad ridging well west off Baja and a modest trough across the Gulf of Alaska. This zonal flow will maintain onshore moisture advection to support rounds of precipitation into the Northwest, with periodic vorticity lobes swinging eastward (although maintaining latitude generally north of OR) to enhance ascent. Snow levels D2-3 will fluctuate generally between 2500ft and 4500ft, with rounds of precipitation leading to periods of heavy snowfall. WPC probabilities both D2 and D3 are similar in magnitude and areal extent, reaching 70% or more for 4+ inches from the Olympics, to the WA/OR Cascades, eastward through the Salmon River/Sawtooth Ranges, and into the Northern Rockies. 2-day snowfall in some of the higher elevations could reach 2-4 feet, with up to 1 foot at the higher passes of the Cascades. Weiss/Kleebauer ...Central High Plains through the Great Lakes... Day 1... ...Significant winter storm is producing blizzard across portions of the Midwest today. Key Messages are being issued for this system... This powerful winter storm has been as-advertised with wind gusts topping 50mph that are fostering blizzard conditions, numerous cases of thundersnow, and prolific snowfall rates of 1-2"/hr beneath the TROWAL over northern Kansas and eastern Nebraska. As the 700mb low traverses northern Missouri this afternoon, a ribbon of 700mb FGEN will pivot and extend over northwest Iowa, southeast Minnesota, and as far north and east as northern Wisconsin. Blizzard conditions will be most common from eastern Nebraska to the Mississippi River between MN/WI. An additional 4-8" of snowfall are forecast in these areas with localized amounts topping 10" possible. Snow accumulations of 1-2" and wind gusts of 50-60 mph are likely in southeast Nebraska and into east-central Kansas. WPC continues to issue Key Messages for the ongoing winter storm and they are available for viewing below. The storm heads for the Great Lakes tonight with heavy snow from north-central Wisconsin to the Michigan U.P.. Similar 1-2"/hr snowfall rates are likely within the heaviest snow bands, along with wind gusts topping 30 mph. Lake enhanced snowfall is likely along the eastern half of the Michigan U.P. through Wednesday night and possibly into early Thursday morning. By 12Z Thursday, the residual TROWAL on the backside of thew weakening storm system will keep some light-to-moderate snow in the forecast across western Michigan and as far south as northern Indiana, but the progressive nature of the snow bands and warmer soil temperatures should keep any snowfall totals to minor amounts (coating to 3"). Still, wind gusts surpassing 30 mph would result in reduced visibilities that make for hazardous driving conditions Thursday morning. Aside from periods of light snow traversing the Great Lakes and Ohio Valley, most of the disruptive snowfall will have concluded by Thursday morning. Mullinax ...Interior Northeast... Days 2-3... By Thursday afternoon, a lobe of 500mb vorticity tracking towards the Southeast will foster strong upper-level ascent over the Northeast coast. At the same time, a frontal boundary approaching from the west will align itself along a more baroclinically favored area along the Northeast coast that will give rise to a developing wave of low pressure Thursday night near Long Island. With the developing low positioned beneath the diffluent left-exit region of a 250mb jet streak, low pressure will develop quickly and southerly flow off the Gulf Stream will advect rich Atlantic moisture around the northern and western flanks of the low. After initially starting out as rain, a transition to snow will ensue from the Adirondacks on east through the Berkshires, Green, and White Mountains on Friday. The storm is a quick mover thanks to the next upper trough in the active jet stream pattern making its way towards the Great Lakes. It is worth noting that the EC-AIFS does show the potential for the storm's deformation axis to linger longer over central Maine and the White Mountains, which could lead to some locally heavy snow totals in elevated terrain through Friday afternoon. Any lingering snow in parts of northern Maine should come to an end Friday night. WPC probabilities shows moderate-to- high chances (40-70%) for snowfall totals >2" in the peaks of the Catskills, Adirondacks, Berkshires, Green, and White Mountains. The tallest peaks of the Whites, most notably around Mt. Washington, could see localized amounts approach 6" through Friday afternoon. Mullinax ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png