Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 227 PM EST Fri Jan 3 2025 Valid 00Z Sat Jan 04 2025 - 00Z Tue Jan 07 2025 ...Western U.S.... Days 1-3... The busiest stretch of winter weather in the Pacific Northwest and Northern Rockies occurs over the next 24-48 hours. The region is already witnessing periods of heavy mountain snow along the spine of the Cascades and as far east as the Absaroka and Lewis Range. Some very high elevations snow is occurring as far south as the Trinity /Shasta and the northern Sierra Nevada. As a robust 500mb trough moves over the Pacific Northwest this evening, the strongest synoptic-scale ascent will be over the Northern Rockies as excellent250-500mb diffluence aloft with >90th percentile 500-700mb mean specific humidity values support a swath of heavy snow as far east as the Little Belt and Big Snowy Mountains of Montana, and as far south as the Bear River Range bordering Idaho and Utah. The Lewis Range and Bitterroots in particular also have the added help from a large 1040mb+ high over southern Canada that is prompting anomalous southeasterly flow (<1st percentile at 850mb on NAEFS tonight) to cause upslope enhancement on the eastern slopes of these ranges. By Saturday morning, the storm continues to head east with snow ending in parts of the Sawtooth and Bitterroots but peaking over the Tetons and Bear River Range. Some measurable snowfall is expected in the Wind River and Big Bear Ranges, including ranges as far south as the Colorado Rockies. At the same time, the next Pacific storm system arrives Saturday afternoon with most snowfall in the Olympics and Cascades peaking Saturday night and accumulations primarily see above 4,000ft. This plume of moisture will reach the northern Rockies Sunday morning, but mainly lead to lighter snowfall accumulations at the more remote elevations of the Blue, Bitterroots, and Lewis Range. There is another weak system that moves into Oregon late Sunday night into Monday, but snow accumulations appear minor (<4") in from the Oregon Cascades to eastern Oregon. In total, many of the higher peaks of the Pacific Northwest and Northern Rockies will see heavy snowfall through Sunday with most of the heavier amounts confined to the more remote areas of these aforementioned mountains ranges. Portions of the Cascades, Blue Mountains, and Lewis Range (primarily above 5,000ft) have high chances (>70%) for snowfall totals >12". For elevations >8,000ft in the Tetons, Bear River Range, and Absarokas, there are high chances (>70%) for snowfall >8" according to WPC probabilities. ...Great Lakes and Northeast... Days 1-2... The strengthening -NAO block over Greenland and the North Atlantic will keep a large gyre-like 500mb upper low parked over southeast Canada and the northwest Atlantic through Sunday morning. On its western flank, it will work in tandem with strong 1040mb+ high pressure over central Canada to keep a tight pressure gradient over the Great Lakes, resulting in cold NWrly flow over the Lakes. This will prompt favorable LES on northwesterly flow along with a few upper level disturbances traversing the Great Lakes through Sunday morning. The Great Lakes will gradually cool as a result, but the lapse rates will remain steep enough over and downwind of the lakes to where lake-induced instability will exceed 500 J/kg on occasion. The snow belts of the Michigan U.P., northern portion of Michigan's Mitten, from northeast Ohio on north east along the Chautauqua Ridge, and from the shores of eastern Lake Ontario to the Tug Hill will be most favored for heavy snow through Sunday AM. A prolonged WNW-NWrly fetch connecting a single band from lakes Superior, Huron, and Ontario will help produce the more significant amounts (1-2ft+) in Upstate NY between Syracuse and the Tug Hill. Latest WPC probabilities also show the Chautauqua Ridge with moderate-to-high chances (50-70%) for additional snowfall totals >8" through Sunday AM, while chances for >8" in the far eastern Michigan U.P. are lower chances (10-30%). Snowfall rates in these areas of 1-2"/hr are expected and could come in waves through Sunday. The WSSI shows Major Impacts for these areas through Sunday, indicating there are likely to be dangerous to impossible driving conditions for those impacted by these heavy LES bands. ...Northern High Plains... Days 1-2... The impending winter storm in the Northern Plains is a great example of mesoscale banding via strong frontogenetical forcing. Snow gets started this afternoon in parts of central Montana and really increases in intensity by 12Z Saturday. At this time, a 700mb low over southern Alberta and an elongated trough axis will prompt healthy mid-level divergence aloft from central Montana to western South Dakota. Closer to the surface, lower pressure over the western U.S. and strong 1040mb+ high pressure over Canada is leading to winds near the surface to be unusually strong out of the SE that is directing moisture, with origins out of the Gulf of Mexico, into the Northern High Plains. This is leading to exceptional 850mb WAA emanating out of the Southern High Plains at low level. In addition, this mean SErly flow at low-levels favors upslope enhancement through Saturday as the region becomes favorably placed on the northern flank of the 700mb low by 18Z Saturday. Lastly, SWrly 700mb winds out of the southwestern U.S. are causing their own WAA and FGEN forcing as warm 700mb temps collide with much colder air in the Northern Plains. The end result is a setup for bands of heavy snowfall setting up from the heart of "Big Sky country" on south and east into western South Dakota. The WPC Snowband Probability Tracker shows HREF guidance sporting 1-2"/hr rates beginning as early as this evening and continuing through Saturday morning. Areas as far south as northeast Wyoming also could see these heavy snow rates, primarily east of the Big Horns where upslope flow can enhance snowfall rates. As the best upper level dynamics shift southeast Saturday evening, some lingering easterly flow may keep periods of snow in the forecast through the overnight hours, but snow should finally taper off by Sunday morning. WPC probabilities show a wide swatch of high chance probabilities (>70%) for >4" of snow from northern Montana to northeast South Dakota. There are also low-chance probabilities (10-30%) for snowfall >8" in localized portions of central Montana, northeast South Dakota, and northeast Wyoming. The WSSI shows widespread Minor Impacts through the Northern High Plains with some spotty Moderate Impact areas in north-central Montana. ...Central Plains through the Mid-Atlantic... Days 1-3... ...Widespread significant winter storm to develop tonight across the Central Plains and then move through the Mid-Atlantic early next week... A closed low moving into the Pacific Northwest will fill as it moves towards the interior, and then weaken briefly during its dive into the Four Corners region. However, as it exits the Central Rockies Saturday night it will re-amplify, becoming a closed low once again into the Central High Plains. The models diverge at this point, with still modest differences in timing and intensity, but the trends in all the 3 global ensemble systems is for a deeper and slightly slower upper low by Sunday morning, and some clustering has occurred towards a more uniform solution. If the deeper/slower solution continues, it would conceptually result in stronger downstream warm advection and a more strong north push of precipitation, but downstream confluence is also quite strong suggesting a sharp northern gradient in precipitation and some subsidence to squelch the northward extent/shift. While the exact track will be crucial to the axis of greatest impacts, the increased clustering of solutions suggests this axis is becoming more defined. As the upper low shifts almost due east through Monday with continued deepening, it will force a strengthening jet streak aloft to help enhance ascent. This will cause the accompanying surface low to deepen slowly as it moves east, leading to a large swath of heavy snow, sleet, and freezing rain from eastern Kansas through Maryland/Virginia. While confidence is high in an impactful winter storm, the details are still in question. As the low pivots east, downstream moisture advection will begin to intensify as reflected by intense 290K isentropic ascent blossoming across Kansas Saturday afternoon and then continuing eastward in advance of the low. This will spread PWs that reach as high as +2 sigma across the Central Plains/Ozarks, with subtly lower anomalies into the Mid-Atlantic. The accompanying theta-e ridge is then progged to lift cyclonically into a TROWAL, especially Sunday when it becomes quite strong from KS into IN, before weakening in response to the filling of the upper low and shunting of the moisture to the east. The overlap of this TROWAL with impressive WAA will help deepen the DGZ (SREF probabilities for 100mb of depth now reach 70% in KS/MO) to increase the potential for heavy snow rates. Additionally, cross sections indicate a threat for CSI/CI where folded theta-e surface intersect with -EPV, resulting in the potential for thundersnow (through CI) and snowfall rates of 2+"/hr. The greatest risk for banded snow appears to be within the WAA from Missouri to Indianapolis as the setup matches the conceptual model for a laterally translating band, but then behind the low an extremely impressive deformation axis is likely which would have higher SLRs and the greater potential for excessive convective snow rates. This deformation will likely weaken towards the east as the upper low begins to fill, but still strong WAA/fgen ahead of the low within the narrow translating band to the east will support impressive rates of 1-2"/hr at times. South of this band, the WAA will overrun the cold surface temps, but forecast soundings indicate an impressive E/NE dry advective flow beneath the warm nose to maintain sub-freezing wet bulb temperatures. This will support an axis of heavy sleet and freezing rain, generally from southern MO through northern KY, where sleet accumulations above 1", and freezing rain accretions of 0.25-0.5" are likely. As the system moves east and weakens a bit, there is even more spread in the track, but still a heavy corridor of snow and mixed precipitation is likely from Ohio to Delaware. The wave is opening during this time, but the downstream WAA/fgen remains impressive especially in the 850-700mb layer, and a continuation of the upstream translating band is likely right through the Mid-Atlantic states. The intensity and accompanying snowfall is likely to be reduced from upstream, but could still feature intense rates and heavy snow from west to east, reflected by the recent EFI reaching 0.9 with a co-located SoT of +5, highest in northern VA. The placement of this band remains in question and guidance has trended south today, but where it sets up, 1"/hr snows are likely at times. With the event just getting started late D1, WPC probabilities D1 are minimal for any significant ice or snow. However, by D2, the system ramps up, and WPC probabilities for 6+ inches reach above 50% from western KS through the St. Louis metro area, and locally 12+ inches of snow is likely, especially in far NE KS or near Kansas City, MO. During D3 the 6+ inch probabilities race eastward, and Sunday night/Monday features a moderate risk (50-70% chance) of 6+ inches from eastern Illinois through the Washington, D.C. metro area. South of the heavy snow, a swath of significant, even isolated damaging icing, is likely, with WPC probabilities for 0.25" reaching 50-70% from southern MO through western KY, pushing into eastern KY and the central Appalachians on D3. Some places, especially in eastern MO, southern IL, or western KY, could receive 0.5" of ice resulting in damage to trees and power lines. This system has prompted the issuance of key messages which are linked below. Mullinax/Weiss ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png