Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 417 PM EST Sat Jan 06 2024 Valid 00Z Sun Jan 07 2024 - 00Z Wed Jan 10 2024 ...Northeast... Days 1-2... Major winter storm brings heavy snow to the Northeast tonight, lingering along the New England coast Sunday. A strong, positively-tilted, shortwave trough will lift from Texas to the central Appalachians tonight which will slow the surface low moving up the northern Mid-Atlantic coast tonight, causing it to linger east of New England Sunday. The interaction of this second shortwave with the strengthening SWly jet to the south (left exit region over New England) and a strong baroclinic gradient offshore will result in a rapid deepening of this low tonight/Sunday, which will enhance snowfall across the interior northern Mid-Atlantic and New England. An axis of moderate to heavy snow driven by WAA leading to waves of robust 850-600mb fgen will continue to push north over PA this afternoon before pivoting east this evening over northeast PA/central/eastern NY - then south-central New England overnight. Snowfall rates of 1.5-2"/hr will continue to be common through this interior swath. The most significant snowfall and related impacts from this event are expected from the Poconos and northwest NJ through the Catskills and into much of southern and central New England. The rain/snow line will be along the south coast of New England with areas just inland receiving multiple hours of heavy/wet snow. This will be aided by the developing TROWAL which will pivot back into New England, and the setup should support an intensifying deformation band which will lift through eastern PA/eastern Upstate NY and then may transition to a pivoting band across southern New England, and the presence of likely CSI in cross-sections suggests snowfall rates could top 2"/hr despite the snow character being on the wetter side. Uncertainty remains with snowfall in southeast Mass, but there are indications of enough cold air dropping down there late tonight, although depth still should be modest there (though very wet). The deformation band then lingers along the eastern New England coast Sunday with decent banding an potential for 1"/hr rates along coastal Maine and through Boston. Day 1 (after 00Z) WPC probabilities for >8" are over 80% in terrain including the eastern Catskills/southern Greens/Berkshires/Litchfield Hills and the Worcester Hills of Mass and also across southeast NH/far southern Maine and northeast Mass. Locally, 12-15" of snow is still likely under the heaviest bands and north from Boston with the extended snowfall Sunday. The low ejects east from the Gulf of Maine Sunday night, but some lingering bands are expected in Down East Maine Sunday evening and possibly into eastern Mass/Cape Cod. ..Pacific Coast, Great Basin, Northern Rockies... Days 1-3... A very active pattern continues through at least midweek across most of The West as a longwave trough digs down the CA Coast tonight to encompass The West Sunday, followed by a potent trough with a leading atmospheric river (AR) into the Pacific Northwest Monday/Tuesday. The longwave trough that develops through Sunday will steepen lapse rates and combine with pockets of upper diffluence and an eastward advancing cold front to drive ascent supportive of mountain and lower elevation snows from the Cascades through the Northern Rockies. Moisture will be generally normal to slightly below normal per the NAEFS, but the widespread and robust synoptic ascent will result in light to moderate snow across much of the West with snow levels falling from around 3000 ft to as low as 500 ft beneath the core of the upper trough and behind the surface cold front. Heavy snow with Day 1 probabilities for >6" are 50-80% over the central/southern Sierra Nevada and the OR Cascades as well as the higher Great Basin ranges in NV into ID. A initial wave with the AR arrives into the Pac NW Monday with snow levels starting around 1000ft that raise to 5000ft Monday evening. Day 2 snow probabilities are 40 to 70% for >6" along the WA Cascades into central OR. The potent shortwave trough/low forcing the AR arrives at the WA coast late Monday night with a powerful cold front sweeping across WA/OR and into northern CA to the northern Rockies through the day Tuesday. The best moisture will be acted upon by downstream mid-level divergence to drive ascent along with upslope flow into the Cascades and more direct diffluence within the left exit of the digging NWly jet coming in from the Pacific. Heavy snow with snow levels crashing to 2000ft Tuesday is expected across the Pacific Northwest. Day 3 WPC probabilities for >12" are 50-90% along the WA Cascades with more like 40-80% for the Olympics, OR Cascades, and much of the Northern Rockies along the MT/ID border into central ID ranges. Snow levels will begin above the passes, but will drop well below during the event, raising associated impacts. ...Four Corners States across the Plains and through the Midwest to the Appalachians... Days 1-3... An intensifying wave digging down the CA coast tonight will close off over the Four Corners states Sunday morning. This feature will continue to dig and deepen as it progresses crosses the southern Rockies late Sunday night, producing height anomalies at 500mb of near -4 sigma according to NAEFS. This extremely anomalous trough will drive impressive downstream ascent through height falls and divergence to the northeast, with intensifying upper jet streaks coupling efficiently overhead the greatest height falls to result in rapid surface cyclogenesis beginning late Sunday night over eastern NM. As this surface low deepens and tracks E/NE, it will drive increasing moisture advection downstream from the Gulf of Mexico noted by mixing ratios of 6 g/kg on the 300K isentropic surfaces surging northward into the Southern Plains Monday. This is also reflected by NAEFS PW anomalies reaching as high as +2 sigma Monday from TX into KS, with some of this theta-e ridge rotating cyclonically into a TROWAL back into the High Plains. The combination of moist mid-level ascent on confluent SW flow downstream of the primary closed low and this increasing low-level moisture, in a region of impressive synoptic ascent will likely result in an expansive area of heavy precipitation, with snow levels in the Southwest/southern Rockies of 3000ft or below, from the Four Corners all the way across the Midwest by late Tuesday. Late tonight/Sunday as the closed low digs across the Four Corners and interacts with the strengthening coupled jet structure, moisture within the mid-level SW flow will be wrung out across the terrain of the Southwest, especially from the Wasatch through the San Juans and southward along the Mogollon Rim. WAA downstream of the surface low will surge snow levels to around 4000 ft in NM/CO, but these will quickly drop back to just around 500 ft behind the accompanying cold front as the low advects eastward. Most of the precipitation should wane behind the cold front so the heaviest snow is still likely above 3000-4000 ft, which is reflected by Day 1.5 WPC probabilities of 30-80% for >8" inches across terrain in central/southern UT, along the Mogollon Rim and the White Mountains of AZ, and across the San Juans. Maxima over 12" should generally be limited to the San Juans. Due to the falling snow levels as the event pushes east, Day 1.5 probs for >2" are across terrain east from Tucson and the Sacramento Mtns. A shield of precipitation is expected to develop over the southern and central Plains early Monday as WAA overlaps the best mid-level divergence, and waves of fgen develop both downstream of the low as well as upstream as a comma-head band develops from the eastern CO/NM border up through KS. Rapidly developing bands in this comma head should quickly become heavy which will combine with strong winds from the developing low. Day 2.5 snow probs are 40-70% for >6" from the eastern third of the CO/NM border up through western KS and again over southeast Neb/northeast KS where the initial inverted trough snow pivots into the deformation bands as the low shifts east. Additional reinforcing shortwave impulses lead to further strengthening as the low shifts east across OK Monday/Monday night and then northeast through central IL Tuesday. Day 3 snow probs for >6" are 40-80% from northeast KS, across northern MO/southern IA and northwest IL. Northeast MO is the current target for a regional snow maxima due to pivoting bands as the low turns northeast which is where Day 3 snow probs for >12" are 20-30%. Warm air advection on the eastern fringe of the expanding precip shield Tuesday looks to ride over existing surface cold air left in the wake of the ongoing system over the central Appalachians. This should lead to a wintry mix from terrain in western NC WV into west-central PA. Day 3 ice probs for >0.1" are 10-30% through this stripe along the Blue Ridge in NC/VA and back over the Allegheny Highlands into the Laurels in PA. Jackson ***Key Messages for Jan 6-7 East Coast Winter Storm*** Heavy Snow in the Mid-Atlantic and New England Heavy snow will shift from the interior northern Mid-Atlantic through much of New England tonight with more moderate to locally heavy rates lingering over New England Sunday. Widespread snowfall accumulations of 6-12 inches are expected, with locally as much as 15 inches under the heaviest bands. Impacts from Heavy Snow and Wind The combination of heavy snow rates of 1-2"/hr and gusty winds will cause rapidly snow-covered roads and limited visibility, creating dangerous travel conditions. Especially in southern New England, the heavy/wet snow should cause power outages and tree damage. Minor Coastal Flooding in the Northeast Gusty onshore winds should cause minor flooding along the Mid-Atlantic and southern New England coasts during the Sunday morning high tide cycle. ***Key Messages for Jan 6-11 Major Winter Storm*** Weather Impacts From Coast-to-Coast The winter storm now tracking south through California tonight will rapidly strengthen over the Desert Southwest Sunday spilling onto the Plains Sunday night and Monday before shifting through the Midwest and portions of the Northeast on Tuesday and Wednesday. Heavy Snow Expected; Blizzard Conditions Likely Heavy snow will move across the Four Corners states Sunday before a swath of heavy snow develops over the Southern/Central Plains on Monday. Heavy snow then shifts through the Midwest Monday night and Tuesday. Given the increasing storm intensity, strong winds should create blizzard conditions over portions of the Plains into the Midwest. High Winds Ahead of the Storm Gusty winds on the warm side of the storm may exceed 50 MPH and cause damage over the Appalachians to the East Coast. Severe thunderstorms are possible along the Gulf Coast and Southeast Monday and Tuesday. Flooding and Coastal Flooding Risk in the Eastern U.S. Heavy rain Tuesday into Wednesday will likely lead to river and possibly flash flooding from the central Gulf Coast through the Northeast. Powerful onshore winds will also likely lead to coastal flooding along much of the East Coast.