Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 401 AM EST Tue Jan 24 2023 Valid 12Z Tue Jan 24 2023 - 12Z Fri Jan 27 2023 ...Southern Plains, Ozarks, Middle Mississippi Valley, & Lower Great Lakes... Day 1-2... A deep upper low tracking into the Southern Plains this morning will be the catalyst for a blossoming area of heavy snow over the TX Panhandle and across central OK. It is possible for some heavy snow to creep its way south of the Red River into North TX, but the expectation is for the heaviest snowbands to setup from areas just south of Amarillo and north of Lubbock, then crossing into central OK. The emerging 850mb low into central TX will contain an impressive low level jet (LLJ) >60 kts. This will act as an excellent supplier of low-level moisture around the northern and western flanks of the 850mb low, resulting in a dynamic warm conveyor belt beneath the diffluent right-entrance region of a 250mb jet streak that will be intensifying throughout the day. The heaviest snowfall rates are likely to occur beneath the deformation axis, which could contain minor elevated CAPE values emerging from North TX amid the strong, southerly WAA Tuesday. The 00Z HREF depicts a cluster of CAM guidance that indicated hourly snowfall rates of 2"/hr possible from the TX Panhandle to far north-central TX and south-central OK through 18-20Z. While road and soil temperatures remain mild, should these rates come to fruition, snowfall will be able to quickly accumulate on all surfaces and lead to quickly deteriorating travel conditions. Latest WPC PWPF showed a high chance (70-80%) for snowfall >4" and moderate chances (40-60%)for snowfall >6" in central OK on Tuesday. As the 850mb low continues to consolidate over northeast TX Tuesday evening, the nose of the 850mb jet will be aimed directly at eastern OK and the Ozarks of northern AR and southern MO. Intense 850-700mb frontogenesis and robust omega within the DGZ will result in 1-2"/hr snowfall rates there Tuesday evening and into the overnight hours. WPC's Snowband Probability Tracker shows two potential windows for such heavy snowfall rates: one from the initial WAA at 850-700mb as the storm tracks to the south Tuesday evening, then again during the overnight hours as the TROWAL moves in overhead. Northern AR and southern MO is highlighted by the WSSI to contend with Major impacts from this event, suggesting the potential for considerable disruptions to daily life and dangerous, to if not impossible, travel in these areas. Latest WPC PWPF showed 60-80% chances for >8" of snowfall in northwest AR. By Tuesday night, a pair of jet streaks (one over South TX, another over the Great Lakes) will place their respective diffluent jet streak regions over the Middle MS Valley and OH Valley. The same source of moisture transported via the 60-70 knot 850mb jet will be directed to the northwest flank of the 850mb low with the pivoting axis of heavy snow setting up over central IL, central IN, northwest OH, and southeast MI. WPC PWPF shows 50-70% probabilities for >6" of snowfall in these areas late Tuesday night into Wednesday. The WSSI does show a swath of Moderate impacts for these same impacted areas, highlighting the potential for treacherous travel conditions through Wednesday. Farther east, the initial WAA thump of snowfall over eastern IN and central OH will produce heavy snowfall rates Wednesday night, but the surge of warm air via the LLJ and the imposing dry slot at upper levels will help to change snow over to rain Wednesday morning throughout the upper Ohio Valley by midday Wednesday. ...Central Appalachians, Northern Mid-Atlantic, & Northeast... Days 2-3... As the low in the Ohio Valley continues to intensify, a deep moisture plume emanating out of the Gulf of Mexico will stream up the Eastern Seaboard late Tuesday night and into Wednesday morning. Farther north, high pressure over Quebec will help to sustain sub-freezing temperatures from the central Appalachians to New England, but the high will be exiting east throughout the day. Periods of snow will ensue over the central Mid-Atlantic over the northern Mid-Atlantic due to a robust 850-700mb front tracking north through the region. As vertical velocities increase and reach the DGZ, snow will fall heavily at times across central PA, the Poconos, and northward to the southern tier of NY and the Catskills. During the heaviest periods of snowfall, snow is forecast to come down at a 1-2"/hr clip Wednesday afternoon. The uncertainty lies with how long this can this front end "thump" of snow last, as there will be an inevitable changeover to a wintry mix due to the intense WAA aloft forcing a burgeoning warm nose aloft, and the 300-700mb layer dry slot arriving from the Ohio Valley Wednesday evening. Currently, WPC PWPF shows 50-70% probabilities for snowfall totals >4" in central PA, the Poconos, and the Catskills through Wednesday evening, but there also remains 10-30% probabilities for >6". This implies there are scenarios still in play where the atmospheric column remains dynamically cooled and saturated long enough that these impressive 1-2"/hr rates could pile up quick and cause travel headaches during the Wednesday afternoon rush hour. The latest experimental PWSSI does show 40-50% probabilities for Moderate impacts in central PA around the I-80 corridor, largely driven by Snow Rate in the PWSSI algorithm. Travel is likely to be treacherous in these areas midday Wednesday to Wednesday evening, if nothing else from near-zero visibilities due to heavy snowfall rates. It is worth noting that the wedge of subfreezing air will likely stick around even as the warm nose aloft leads to a growing area of >0C temps aloft. This will likely lead to an icy wintry mix in the central Appalachians and northern Mid-Atlantic Wednesday evening and could linger into the Wednesday night hours. As the intense 850-700mb frontogenetical forcing makes it way north, the intense swatch of 1-2"/hr snowfall rates will advance north into the Interior Northeast. Areas from the Tug Hill and Adirondacks on east to the Berkshires, the Green and White Mountains, and much of Maine can expect to stay snow longer. This allows for these areas to contend with heavier snowfall totals. All these regions mentioned feature 60-80% probabilities for >6" of snowfall, but it is in southeast mainland ME and the White Mountains of NH that have 20-40% odds to see >12" of snow. Much of the interior Northeast in elevated areas feature Moderate impacts according to the WSSI, with the valleys still seeing Minor impacts, but lesser totals as a result of the lower elevation. As the dry sot arrives Wednesday night, precipitation will likely change over to a mix of sleet and freezing rain. Should the cold wedge in the surface-925mb layer stay sufficiently cold, some areas could pick up over an inch of sleet, adding to the weight of snow in some locations. The freezing rain potential will be closely monitored as well, as the WPC PWPF does show 20-40% probabilities for >0.1" of freezing rain accumulation from the Adirondacks and Berkshires on northeastward to central Maine. Should totals approach 1/4", there would be a growing concern for possible tree damage and power outages. Mullinax ...Key Messages for Jan 24-26 Winter Storm... --An expansive winter storm will move into the Southern Plains today producing areas of heavy snow from eastern New Mexico and the Texas Panhandle to Oklahoma. --The storm is forecast to strengthen and track northeastward from the Lower Mississippi Valley to the Great Lakes Tuesday night and Wednesday, producing a stripe of moderate to heavy snow from the Ozarks to the Great Lakes. --An icy wintry mix is expected to develop across portions of the Central Appalachians, with areas of heavy snow across portions of the interior Mid-Atlantic on Wednesday. --Heavy snow is forecast to spread across the interior Northeast as a second area of low pressure develops along the coast Wednesday night. --Snow-covered roads and reduced visibility will make travel difficult across impacted areas. Gusty winds and heavy snow may result in downed trees and power outages.