Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 259 AM EST Mon Jan 6 2025 Valid 12Z Mon Jan 06 2025 - 12Z Thu Jan 09 2025 ...Ohio Valley through the Mid-Atlantic... Day 1... ...Major winter storm continues from the Ohio Valley through the Mid-Atlantic... The major ongoing winter storm which has already dumped over a foot of snow from central KS to northern MO is forecast to rapidly slide eastward today while producing additional areas of heavy snow and freezing rain between the Ohio Valley and Mid-Atlantic. This system is being forced by a closed 500mb low which will be positioned over the IL-IN border to start the period. This closed low will weaken into a potent open wave today while pushing due east toward the Mid-Atlantic by this evening. The weakening is being aided by a separate stronger upper low diving southward towards New England and helping shear out the storm system as it exits the East Coast. At the same time, however, a potent jet streak rotating around the base of the trough will amplify towards 150 kts, helping to offset some of the loss of deep layer lift resulting from the opening of the trough. At the surface, the accompanying low pressure will skirt east across the lower OH VLY/TN VLY, with secondary development likely occurring across the Carolinas and then pressing east off the Mid- Atlantic Coast by Tuesday morning. Together, this will result in a large swath of heavy precipitation, with all p-types likely (at least at the beginning of the forecast period). Downstream of the primary surface low, strong isentropic ascent along the 290K-295K surface will draw impressive moisture northward into the system (NAEFS PW above the 90th climatological percentile) supporting the expansiveness of widespread precipitation. A stripe of very heavy snow (rates 1-2"/hr) is likely within the WAA during the first half of D1, and the conceptual model for a laterally translating band from west to east fits this system. This suggests a narrow but intense band of snow, supported by both NBM and DESI probabilities, as well as the WPC snowband tool which indicates a high potential for 1-2"/hr snow rates from northern VA and MD eastward to DE following the band weakening across southern OH right before 12z. Since this band will be translating along its long/axis (more eastward motion than north) some locally very high snow totals are possible as reflected by 40-50% chance for an additional 8+ inches of snow from northern VA eastward through southern DE, but in general WPC probabilities support a high (>70%) risk for 4+ inches along this same axis with a sharp N/S gradient on both sides. These similar probabilities are also found in western MD and WV where an upslope component following the passage of the low pressure system. Some of this snow will be additionally enhanced by the post-system comma head/deformation which will pivot across the Mid-Atlantic Monday night. The guidance continues to trend a bit stronger with this feature, and while latitudinal placement of the most significant forcing remains quite uncertain within this axis, renewed moderate to heavy snow, with much fluffier SLRs than earlier, will likely regenerate across the region adding up to a few more inches before exiting by Tuesday morning. South of the heavy snow area, this system will also pose a continuing ice threat within the warm nose/p-type transition zone. Damaging ice accretion of 0.5-0.75" has already been realized along a stretch spanning from eastern KS to KY, but thankfully precipitation will be coming to an end across this area as light snow/freezing rain pushes eastward today on the backside of the low. Intense WAA over- topping the cold surface layer will continue this morning across parts of the southern Mid-Atlantic, however. WPC probabilities for at least an additional 0.1" beginning 12z today fall between 10-40% across southern VA. This system has prompted the issuance of key messages which are linked at the bottom of the discussion. ...Great Lakes and Northeast... Days 2-3... Behind the large system moving off the Mid-Atlantic coast Tuesday morning, a push of arctic air will dig into the Great Lakes and Northeast, fueled initially by a potent shortwave dropping nearly due south from Ontario Tuesday morning. 850mb temps are progged to crash to -15C to -20C, steepening lapse rates considerably, and resulting in a favorable setup for both lake effect snow (LES) and upslope snow into the upwind terrain of the Northeast. The greatest potential for upslope snow will occur D2 /Tuesday/ as the potent shortwave dives south pulling a cold front with it. This will mark the leading edge of the coldest air, and cause impressive NW flow into a moistening column. The temperatures during this time will be very cold, so the DGZ will be quite low, resulting in effective upslope into the snow growth region. Light and fluffy snow will efficient accumulations, so despite a short duration, WPC probabilities for 4+ inches of snow are moderate (50-70%), highest in the NW Adirondacks and N Greens. Although some heavy LES may occur downstream of Lake Michigan early D2, the greater coverage and intensity of LES is likely D3 behind this front. GLERL lake-surface temperatures are still sampled to be +5C to +8C, so the very cold air moving overhead will support lake-induced instability to drive LES. At this time the LES appears less intense as the last round, but WPC probabilities D3 feature a low risk (10-30%) for 4+ inches of snow in the favored N to NW snow belts. ...Intermountain West, Rockies, and Central High Plains.... Days 1-3... Pacific shortwave diving across the Intermountain West today will surge moisture ahead of it and feature favorable divergence to promote scattered high elevation upslope snow spreading southward across the central Great Basin and Central Rockies today before reaching the Southern Rockies late D2/early D3. Several inches of snow are likely D1 over northern NV, southeastern ID, western WY/southwestern MT and then into the CO Rockies. WPC probabilities for at least 4" of snow are >50% in these mountainous regions. Into D2, height falls (and trailing height falls) will rotate through the Southwest and sharpen, eventually closing off over the lower CO River Valley late Tue. With high pressure nosing down out of the northern Plains, upslope snow will expand across the CO Front Range via an easterly low level flow along with a relatively low DGZ. The upper low will likely move into northwestern Mexico by the end of the period, helping to drag the snowfall southward as well through the Mogollon Rim, Sangre de Cristos, Sacramento Mountains, etc. For the D2-3 period, WPC probabilities for at least 4 inches of snow are 20-50% over the Front Range into the CO Rockies, San Juans, and higher elevations in AZ/NM above 8000ft or so. ...Southern Plains... Day 3... As the aforementioned upper low closes off over the Southwest late D2 it continues to dig into northwestern Mexico D3 and become anomalously deep (below the 0.5th climate percentile per the 12z GEFS). Meanwhile, a separate shortwave rounding the far eastern Pacific ridge is forecast to dive southward and interact with this feature and spawn strong southwest flow containing rich moisture and prominent upper divergence. Given the strong high situated to the north over the Central Plains and cold the low-level airmass in place, snow and mixed precipitation is expected over much of western and north-central TX. Currently, WPC probabilities for at least 2 inches are low (10-30%) through Thursday morning but these may trend up if precipitation start time trends faster. This storm will need to be monitored as 00z ensembles depict heavier snowfall impacting parts of the Southern Plains on D4. 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