Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 320 PM EST Wed Jan 03 2024 Valid 00Z Thu Jan 04 2024 - 00Z Sun Jan 07 2024 ...Great Lakes... Day 1... Cold front moving through the Great Lakes tonight will continue eastward into NYS early Thursday. 850mb temperatures will fall to around -10 to -15C (about -0.5 sigma) but water temperatures are above freezing and ice coverage is basically zero. Multi-band streamers will continue into the U.P. with light accumulation tonight East of Lakes Erie/Ontario, SW to WSW flow will support a single band over SW NY and near/into the Tug Hill ahead of the front tonight. By Thursday, front will help veer winds to NW/NNW with a transition to multi-bands that will eventually taper off tomorrow night as high pressure quickly moves in from the west. The highest snowfall will be over the Tug Hill, where WPC probabilities for at least 4 inches of snow above 50%. ...Great Basin/Four Corners... Day 1... An upper trough moving across California will continue to amplify, with a closed low developing over the Great Basin this evening, before dropping southeast through the Four Corners on Thursday. This will bring additional accumulations to the Nevada mountains as snow continues to spread across Utah and Arizona this evening. Deep southwesterly flow ahead of the trough and left exit region upper jet forcing will support areas of heavier precipitation, with pockets of heavy high elevation snow developing from northern and central Arizona to northern New Mexico and southwestern Colorado. Areas impacted are expected to include the Mogollon Rim and the White Mountains in Arizona, the northeastern Arizona and northwestern New Mexico mountains, and the San Juan and Sangre de Cristo mountains. The WPC PWPF shows at least 50 percent probabilities for accumulations of 4 inches or more across portions of these areas, largely for elevations above 6000 ft. While additional energy dropping into the base of the larger scale trough may produce some more light snow into early Friday, the threat for heavy snow west of the Rockies is expected to wane as the upper low/trough moves into the High Plains late Thursday. ...Southern to Central High Plains... Days 1-2... The upper low/trough moving across Four Corners states on Thursday is forecast to assume a negative-tilt as it moves across the southern Rockies and into the High Plains late in the day. Southeasterly flow will draw Gulf moisture into a region of enhanced lift centered over northeastern New Mexico and southeastern Colorado into the Oklahoma and Texas Panhandle region. The highest probabilities for accumulations of 4 inches or more center from northeastern New Mexico into the far northwestern Texas and western Oklahoma panhandles, where guidance continues to present a notable signal for banded heavy snow to move across the region Thursday afternoon and evening. ...Northwest... Days 1-3... Active period continues for the Pacific Northwest as a series of fast-moving mid-level vort maxes push through the region. Lead system will push through the region on Thursday with light/modest snow for the WA/OR Cascades into the northern ID ranges and NW Montana with snow levels around 3000ft rising ahead of the cold front to about 3500-5000ft from north to south. A weaker warm front on its heels will lift into the region early Friday with some additional snow to the WA Cascades into Idaho as snow levels fluctuate between systems. By Friday afternoon, a more substantial mid-level trough on the LFQ of an incoming 130kt jet will usher in a stronger cold front overnight into early Saturday. Just ahead of the front, moisture pooling will support an increase in PW values but with a source region in the north Pacific, values will only be around normal for early January (0.50" or so into the foothills of the Cascades). 700mb cold pool will be directed into NorCal where anomalies may be around -2 sigma, but the entirety of the Northwest is forecast to have below normal temperatures by the end of this forecast period 00Z Sun (probability of 700mb temperatures less than -1 sigma is at least 60% per the 12Z GEFS). This will help drive snow levels down to about 2000ft by late Saturday as precipitation slowly starts to wind down (heaviest likely overnight Friday into Saturday). Upslope enhancement will help increase snowfall over the Cascades well over a foot, perhaps down to some passes as well. WPC probabilities in the Cascades of at least 6 inches of snow on just day 3 (00Z Sat - 00Z Sun) are moderate (>40%) above about 2500ft or so and for at least 12 inches are moderate (>40%) above about 4000ft. Significant impacts to many passes are likely in the WA and OR Cascades. Farther south, moderate snow is likely into the NorCal ranges including the northern Sierra as the cold front moves through the region, eventually dropping snow levels from around 4000ft to near 3000ft. WPC probabilities for at least 6 inches of snow are moderate (>40%) over the Klamath mountains, Shasta-Siskiyous, and northern Sierra. ...Southern and central Appalachians/Mid-Atlantic... Day 3... As the upper low originating over the southern High Plains late Thursday weakens and lifts to the northeast into the mid and upper Mississippi Valley, mid-level energy will move through the base of a broadening trough that will swing across south-central U.S., into the Ohio Valley and Mid Atlantic on Saturday. At the surface, low pressure is forecast to develop over the Southeast early Saturday and track northeastward toward the Delmarva by the evening. As warm moist air moves over the top of a cold air wedge, a mixture of snow and freezing rain is expected to spread north along the southern to the central Appalachians and Piedmont early Saturday, before models suggest more of a rain-snow transition zone further to the north across northern Virginia, Maryland, and Pennsylvania later in the day. In typical fashion, where these transition zones establish themselves will depend greatly on the track of the low. Unfortunately, there remains a large degree of uncertainty regarding the track of the low and the low level cold air. Therefore, forecast confidence is limited at best. Through late Saturday, the WPC PWPF shows probabilities greater than 30 percent for snow accumulations of 4 inches or more centered from eastern West Virginia to south-central Pennsylvania. Greater than 30 percent probabilities for ice accumulations of 0.10 inch or more extend from western North Carolina through southwestern Virginia, with probabilities reaching 70 percent or more along this axis. ...Northern MN... Day 3... Mid-level vort max and a surface front will move through norther MN into the western Great Lakes on Saturday, as a weak area of low pressure develops into the MN Arrowhead. Temperatures will be relatively mild for the region and time of year (upper 20s), allowing above normal moisture to reside in the area ahead of the front (PW values +1 to +2 sigma). Periods of generally light snow are expected on Saturday which may accumulate to a few inches over the eastern Arrowhead/North Shore. There, WPC probabilities for at least 4 inches of snow are moderate (40-70%), with a broader area of low (10-40%) probabilities across the rest of the Northland. Pereira/Fracasso ***Key Messages for Jan 6-7 East Coast Winter Storm*** --An area of low pressure is forecast to develop over the Southeast early Saturday and move northeastward to the DelMarVa coast. This will bring a mix of snow and freezing rain to the interior Mid-Atlantic with rain closer to the coast. --On Sunday, the storm will track just off the New England coast with an area of snow and some coastal rain for the Northeast. --There remains a large degree of uncertainty in the track of the system and amount of cold air at the surface. A transition zone from rain to snow is likely from Virginia through New Jersey and along I-95 in Southern New England. --Breezy coastal winds are also possible as the coastal storm deepens off the NJ coast.