Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 358 AM EST Tue Feb 13 2024 Valid 12Z Tue Feb 13 2024 - 12Z Fri Feb 16 2024 ...Northeast... Day 1... Over the past 24 hours, the trends in guidance showed a flatter/more progressive 500mb trough over the Ohio Valley with a stronger upper trough approaching from the Great Lakes. This squeeze play has continued to suppress the storm track and subdue the rate of intensification of the storm system. This has a dramatic effect on snowfall totals as well, considering a weaker storm system and less intense snow rates mean less dynamic cooling within the column, and with the shift south in the storm track, precipitation will fall over less elevated terrain where onset boundary layer temperatures are milder. Latest CAMs show a narrow but potent area of 850-700mb frontogenesis early this morning from the Potomac Highlands and along the Mason-Dixon line that rapidly strengthens over the Susquehanna Valley, Delaware Valley, and along the southern New England coast. The rapid cyclogenesis off the coast is still supported thanks to the "kissing jets" setup (divergent left exit region of a jet streak in the Southeast aligning with the divergent right-entrance region of a jet streak over Nova Scotia) fostering fantastic upper level divergence off the Mid-Atlantic coast this morning. So while totals have assuredly trended down, this storm can still pack a punch, particularly in coastal areas of Long Island and southern New England where heavy snowfall rates will overlap with wind gusts up to 40mph at times. Areas with elevation, specifically across south-central Pennsylvania and as far west as the Laurel Highlands, are forecast to see storm-total snowfall amounts of 3-6". As the deformation zone ramps up over eastern Pennsylvania, northern New Jersey, the greater NYC metro area, and Long Island Tuesday morning and into the midday hours, hourly snowfall rates could approach 2"/hr at their peak. Note that these rates are crucial to accumulating snowfall in these areas given the lack of antecedent sub-freezing temperatures. This will make a massive difference in determining which areas can accumulate snow quickly and which areas struggle to accumulate. The latest forecast calls for a swath of 3-6" from Lower Susquehanna Valley and over the Delaware Valley, while 6-8" are possible from northern New Jersey and the NYC metro area to the southern New England coast. Localized amounts approaching 10" are possible in parts of northern New Jersey and across southern New England. In addition, the storm will still rapidly intensify south and east of Long Island on Tuesday, making for significantly reduced visibilities from the NYC metro and Long Island to far southern New England. Isolated cases of tree damage and power outages are possible. Farther south, totals will be closer to 1-3" along the Mason Dixon line, but given the marginal boundary layer temperatures and recent warmth the past several days, most accumulations will be confined to unpaved surfaces. Areas where snow accumulates on roads will be most likely where hourly rates can approach or surpass 1"/hr. Wind gusts will remain quite strong in its wake as the storm tracks out to sea Tuesday afternoon, but all snow should be out of the picture in southern New England by 7PM or so. ...Northwest and northern California to the northern Rockies... Days 1-3... The Northern Rockies will contend with a period of moderate-to-heavy snow today thanks to a shortwave trough diving south from British Columbia and a frontal boundary being banked up along the eastern slopes. Guidance shows some modest 500-700mb moisture aloft that, with the help of upsloping westerly flow, will result in periods of snow along the Lewis Range and as far south as the Tetons. WPC PWPF sports moderate chances (40-60%) for snowfall totals >6" over the tallest peaks of the Lewis Range, while chances are higher (50-80%) for the same totals in the Tetons. The next approaching Pacific storm system, courtesy of a negatively-tilted 500 mb trough, that will direct the next surge of 850-500mb moisture into the Pacific Northwest on Wednesday, spilling into the Northern Rockies by Wednesday night. This upper trough will actually interact with the shortwave trough over British Columbia and pull it west Wednesday afternoon, culminating in the birth of a retrograding 500mb low west of Vancouver Island. This time around, this will support a deeper surge of moisture and bring about heavier snowfall into the Cascade Range, the Olympics, and as far south as the Trinity/Shasta and northern Sierra Nevada of northern California. WPC 48-hour sports high chances (>70%) of snowfall accumulations topping 12" in all these ranges, with the lone exception being the Cascades which have moderate odds (40-60%). Snow will fall heavily as far inland as the Blue Mountains of northeast Oregon, the Boise/Sawtooth of Idaho, and the Tetons of western Wyoming Wednesday night and into Thursday. In addition, a cold front advancing south through Montana and Wyoming will usher a dome of Canadian high pressure south, amplifying the easterly upslope flow into ranges such as the Absaroka and Big Horns. WPC 48-hour PWPF shows the Boise/Sawtooth with moderate chances (40-60%) and the Tetons with high chances (>70%) for snowfall totals >12" from Wednesday night and into the early morning hours on Friday. Meanwhile, WPC 24-hour PWPF sports moderate-to-high chances (50-70%) for snowfall totals >6" in the Absaroka of southern Montana and low chances (10-20%) in the Big Horns of Wyoming. ...Central Rockies & central Great Plains... Day 3... A fast moving 500mb vorticity maximum will track into the Great Basin early Thursday morning and direct a conveyor belt of 700mb moisture into the heart of the Intermountain West. Mean 850-300mb winds within the column out of the west and sufficient PVA will provide ample lift (both synoptically-forced and via topographically-favored terrain) to support periods of snow in the northern Colorado Rockies Thursday afternoon and into Thursday night. Latest WPC PWPF shows moderate-to-high odds (50-70%) for >8" of snowfall along the Park Range of northern Colorado. Farther east, broad 200-500mb troughing over the Intermountain West and downsloping winds into the central Great Plains will lead to lee cyclogenesis over southeast Wyoming and eastern Colorado Thursday afternoon. This is occurring at the same time as a cold front marches south from the northern Great Plains and southerly 850mb moisture flux streams north from the southern Great Plains. What unfolds is an area of impressive 850mb frontogenesis over western Nebraska and southern South Dakota Thursday afternoon that moves south and east into the heart of Nebraska by Thursday night. Global guidance remains unsure as to whether this 850mb frontogenesis sets up farther north or south, but there is growing consensus that it will lead to a swath of moderate-to-heavy snow in parts of the central Great Plains. Latest WPC PWPF shows low-to-moderate chances (20-40%) for >4" of snowfall from the Black Hills to right along the SD/NE border just west of 100W late Thursday into early Friday morning. Meanwhile, in northeast Nebraska and northwest Iowa, PWPF show moderate odds (40-60%) for >4" Thursday night into Friday. This setup, given the impressive mesoscale signal, could lead to localized amounts surpassing 8" should the combination of vertical ascent within the column, modest DGZ saturation, and sufficiently cold boundary layer temperatures come to fruition. ...Central Great Plains, Great Lakes, & Interior Northeast... Days 2-3... A frontal wave over the central Great Plains will strengthen as a weak 500mb trough over the Central Plains and a more potent upper trough over the Northern Plains will create a strengthening 500mb jet streak oriented NW-SE from eastern Montana to central Iowa by early Thursday morning. The divergent left-exit region of the 500mb jet streak will provide plenty of upper level support to spawn an organized surface low along the frontal boundary draped over the Midwest Wednesday night. A swath of snow will develop on the northern flank of the surface low where 850mb frontogenesis will be strongest starting Wednesday afternoon in southern South Dakota and tracking into the Great Lakes by Thursday morning. Latest WPC PWPF shows moderate chances (40-60%) for snowfall totals >4" across southern South Dakota, which conveniently also aligns well with the Probabilistic WSSI (WSSI-P) showing moderate chances (40-60%) for Minor Impacts. The WPC PWPF sports lesser chances (10-30%) for >4" of snowfall across southern Minnesota and southern Wisconsin. By Thursday morning, the more potent upper trough over the Upper Midwest will further amplify the 250-500mb mean trough and strengthen the jet streak draped across the Upper Midwest. The left-exit region of the 500mb jet streak will continue to escort the surface low east and further strengthen the low as it does so. The same swath of heavy snow forecast will track through southern Wisconsin Thursday morning, then into central Michigan during the day, and find its way into the Interior Northeast Thursday night. Not only will their be snow to the north of the 850mb front, but colder air spilling in behind the storm will trigger some lake-enhanced snow bands early Friday morning. Latest WPC PWPF shows moderate chances (40-60%) for snowfall totals >4" across the northern half of the Michigan Peninsula, but with the help of lake-enhanced snows, WPC PWPF now shows moderate-to-high chances (50-70%) for snowfall totals >6" in parts of the Adirondacks and over the Tug Hill. Moderate chances (40-60%) for snowfall totals >4" are also depicted in parts of western New York. Mullinax ***Key Messages for Feb. 12-13 Winter Storm*** --Nor'easter rapidly strengthens today Low pressure strengthening off the central Mid-Atlantic this morning will track south of the southern New England coast later this afternoon. A tight gradient of heavy snow is expected northwest of this low through the densely populated Northeast urban corridor. Intense snow bands with rates as high as 2" per hour are likely within the heaviest band of snow from the central Appalachians through southern New England. --Significant Travel Impacts in the Northeast This fast-moving Norâ€easter is still forecast to produce a swath of snow exceeding 6" in the central Appalachians, eastern Pennsylvania, the New York City metro area, and southern New England. Strong winds with gusts up to 40 MPH and the high rates of heavy/wet snow should damage trees, power lines, and disrupt travel. --Moderate Coastal Flooding Moderate coastal flooding is forecast during high tide today along the northern Mid-Atlantic oceanic coasts and possibly portions of the New England coast.