Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 400 AM EST Fri Jan 3 2025 Valid 12Z Fri Jan 03 2025 - 12Z Mon Jan 06 2025 ...Western U.S.... Days 1-3... Precipitation will continue to spread inland across western Oregon and northwestern California ahead of an occluded low approaching the coast this morning. Deep, moist southwesterly flow ahead of the boundary, along with strong forcing will support an uptick in precipitation, with orographically focused moderate to heavy amounts forecast to develop along the coastal ranges into the southern Cascades and northern Sierra Nevada. However, high snow levels are expected to confine any heavy snow accumulations to the highest peaks early on. Then, as the upper trough and associated cold front begin to move inland, snow levels will begin to lower, expanding the potential for heavy snow along the southern Cascades, the northwestern California ranges, and the northern Sierra later today. Meanwhile, a well-defined warm front, lifting across the northern Cascades, Intermountain West, and Rockies, will be a focus for organized precipitation and locally heavy mountain snow as well today. Meanwhile to the east, moisture focused by low-to-mid level convergence along the western periphery of strong high pressure extending from central Canada into the Northern Plains will support a swath of light to moderate snows extending from central Nebraska northwestward through the High Plains and into the northern Rockies. Upslope flow is expected to help enhance amounts across the central to the northwestern Montana ranges. By early Saturday, the upper trough is expected to bring high elevation snow into the central Rockies as it moves across the Great Basin into the region. This will be followed by a second system, which is forecast to bring additional orographically focused heavier amounts into the Olympics and the northern Cascades Saturday into early Sunday. Conditions will remain unsettled across the Northwest into Monday, as a third shortwave impacts the region. However, a building upstream ridge will preclude any long fetch of deep moisture and the threat for widespread heavy amounts across the region. WPC probabilities indicate that three day totals exceeding 8 inches will be mostly confined to the higher elevations of the Cascades, northern Sierra Nevada, and portions of the Rockies from northwestern Montana to northern Utah and Colorado. Widespread, but lighter amounts are expected across central and southeastern Montana, with high probabilities for accumulations greater than 4 inches covering much of the region. ...Great Lakes and Northeast... Days 1-2... The strengthening -NAO block over Greenland and the North Atlantic is aiding a mature surface cyclone to stall over Quebec and help produce persistent cold northwest flow between it and a strong surface high over Saskatchewan. This will prompt favorable LES on northwesterly flow along with a few upper level disturbances traversing the Great Lakes over the next few days. 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 could top 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 early Sunday AM. Favorable fetch connecting a band from lakes Superior, Huron, and Ontario will help produce the most significant amounts (1-2ft+) in Upstate NY between Syracuse and the Tug Hill. Snow could also be measured in feet from northeast OH, northwest PA, and western NY (south of Buffalo). Snowfall rates in these areas of 1-2"/hr are expected and could come in waves over the next few days. The WSSI shows Major Impacts for these areas with even a small Extreme area (extremely dangerous to impossible travel and widespread closures) in the Tug Hill through Sunday AM. ...Central Appalachians and Mid-Atlantic... Day 1... A compact wave of low pressure moving across the Ohio Valley early this morning will move progressively eastward beneath a modest shortwave racing southeast within the broad trough amplifying across the eastern CONUS. Immediately in the wake of this shortwave, a potent jet streak reaching 150kts will dig rapidly out of the Northern Plains, producing a narrow but intense corridor of overlapping ascent through height falls, PVA, and LFQ diffluence. This will help at least subtly deepen the low and the accompanying baroclinicity/attendant fronts as it moves eastward to off the VA coast Friday night. The system is compact and progressive, so snowfall in general is expected to be low outside of the Central Appalachians. Here, despite the modest forecast depth of the DGZ, appreciable upslope flow will drive ascent favorably into the DGZ, while some moisture enhancement occurs on post-frontal NW flow from the Great Lakes. This suggests an extended period of moderate to heavy snow, especially D1, with the higher elevations receiving the most significant snow accumulations. Event total snow here could reach double- digits in the higher terrain. Finally, although total accumulations are expected to be minimal, some snow showers or convective snow showers moving along the cold front may crest the Appalachians and move across the Mid-Atlantic states Friday afternoon as the vorticity max dives southeastward. The late afternoon timing of this vort max allows for steep lapse rates from the sfc all the way up into the DGZ, which could promote brief snowfall rates above 1"/hr as shown by low probabilities in the WPC snowband tool. Low-level temperatures will be slightly above freezing from Richmond, VA to Philadelphia, PA, but wet-bulb temperatures below 0C could allow any heavier rates to accumulate on roads and elevated surfaces, causing hazardous travel despite minimal accumulations, during the Friday aftn/eve commute. Additionally, where low-level instability is the greatest, especially overlapping the higher moisture content focused along the MD/PA line, isolated snow squalls are also possible. ...Northern High Plains... Days 1-2... The forecast remains on track and very similar to the previous issuance as an amplified upper level trough moving into the Pacific Northwest this evening will spread downstream ascent through increasing divergence into the Northern High Plains by Saturday morning. This feature is progged to split into dual vorticity lobes, with the primary one swinging into the Central Rockies, while a secondary impulse lifts into Alberta/Saskatchewan. Downstream of this trough, weak shortwave ridging will initially be placed over the region, but this will gradually become displaced by the approaching trough, leading to increasingly strong deep- layer ascent. This forcing will act upon a saturating column as 290K isentropic ascent maximizes in a narrow corridor from NW to SE from the foothills of central MT east into the High Plains of SD, which will effectively overlap with a stripe of robust 700-600mb fgen. The overall setup is not ideal for extremely heavy snow, but an expanding footprint of snowfall is likely, and rates will be enhanced by a cold column with very fluffy SLRs expected. Additionally, the guidance has become more excited about snowfall potential as reflected by an EFI of 0.8 to 0.9, primarily due to an extended period of moderate to at times heavy snow. WPC probabilities have increased as well, and now feature a 70-90% chance for 4+ inches across central MT, with locally up to 10 inches possible in some of the higher terrain, and a 30-50 chance for 4+ inches across far NW SD. ...Central Plains through the Mid-Atlantic... Days 2-3... Guidance remains aligned on a significant winter storm to impact a large area spanning from the Central Plains across the Ohio Valley and to the Mid-Atlantic Saturday night through Monday. While there continues to be considerable temporal and some latitudinal spread in the models, the overall evolution and footprint is quite similar, with impactful snow, sleet, and freezing rain expected. Starting Saturday afternoon, a deepening shortwave will eject from the Central Rockies and move into the Southern Plains by Sunday morning while taking on a negative tilt. At this point there is already considerable differences in the timing and intensity of the upper low, with the EC/UKMET and accompanying ensemble the fastest, the GFS/GEFS making up the slow/strong end of the envelope, and the CMC/CMCE in the middle. That being said, the answer likely lies somewhere in between but could lean towards either camp. The ensemble sensitivity analysis from 12z 1/2 would suggest the differences begin this afternoon as the trough moves ashore the West Coast, so these differences should be brought closer together in a few more forecast cycles. The WAA regime in place ahead of the storm without a strong surface high to the north- northeast would support some of the more northern solutions, but the upper- level configuration in the western Atlantic limits this potential. Regardless, significant spread in timing and intensity results in lower than typical confidence for D3. Despite uncertainty in timing and placement, confidence remains in an impactful winter storm. Regardless of the exact placement, the setup is favorable for a widespread wintry precipitation from the Central Plains/Mid-Mississippi Valley by the end of D2 through the Ohio Valley D3 and eventually Mid-Atlantic D3-D4. As the upper low tilts negatively over the central U.S. and then closes off, a subtropical jet streak will sharpen and rotate around the base of this trough leading to coupled downstream ascent. At the same time, increasing downstream moist advection on low-level flow from the Gulf of Mexico will rapidly saturate the column as isentropic ascent along the 295K surface lifts northward and then pivots cyclonically into a TROWAL around the developing cyclone. As this low intensifies, in conjunction with the strengthening WAA, the setup seems to conceptually support a laterally translating band of heavy precipitation from west to east. With DGZ depth probabilities rising to above 30% for 100mb of depth, and a an overlap of folding theta-e with -EPV in cross sections supporting CSI/CI, an impressive band with snowfall rates at least 1-2"/hr appears likely. Additionally, a secondary axis of deformation could lengthen the heavy snow in some areas with more impressive snowfall rates. There is still uncertainty as to where the transition zone between heavy freezing rain, sleet, and snow will occur but the EFI is highlighting values exceeding 0.9 with an SoT of 2 from eastern KS through southern IN, and the WSSI-P is suggesting a greater than 70% chance for moderate impacts in this same geographic window. Given the strong WAA and deep low-level cold airmass in place, sleet could become a dominant ptype from parts of eastern KS through southern IN/northern KY, with over an inch of sleet accumulation possible for some areas. In addition to impacts, although confidence in exact placement is uncertain, WPC probabilities current reflect a high risk (>70%) for 8+ inches near the NE/KS border through northern MO and into central IL, with a stripe of high probabilities for 6+ inches extending east along I-70 through IN and west-central OH. On the south side of the system, WPC probabilities indicate high chances (70-90%) for at least 0.1" ice accretion in a stripe from eastern KS through southern MO and into southern IL, southern IN and much of KY. 0.25" ice probs of 40-60% are found from southern MO to KY. This system has prompted the issuance of key messages which are linked below. Pereira/Snell ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png