Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 354 AM EST Wed Nov 22 2023 Valid 12Z Wed Nov 22 2023 - 12Z Sat Nov 25 2023 ...Northeast... Day 1... Coastal low moving through southeastern New England this morning will track toward/into the Gulf of Maine before trailing energy along the front tug the main low back to the east by early Thursday. With the Miller-B transition nearly complete as of early Wed (old primary low over southeastern Ontario weakening), much of the southwesterly WAA aloft will lessen into northwestern New England, with the focus turning to cyclogenesis just off the MA coast. Some freezing rain and sleet are likely early in the period over northern NH into northwestern ME, with a general rain/snow demarcation between coastal and interior Maine, respectively. The coastal low will attempt to form a deformation band over eastern Maine this afternoon as it wraps up and deepens, but the trend has been for the trailing wave to rob the new parent low of some of its moisture flux up and around the low, which should take much of the precipitation out of the region just after 00Z. For day 1, WPC probabilities of at least 4 inches of additional snow are greater than 50% over northern Maine. ...Northern Rockies to the Central Rockies and High Plains... Days 1-3... Upper ridging will build into British Columbia by Thursday, favoring digging troughing into the Pacific Northwest and northern Rockies. By early Friday the trough is expected to close off into an upper low over northern Utah where it will slowly move eastward toward the western central High Plains by early Saturday. The models have wavered slower then quicker with the system, and also nudged southward with the upper low, so confidence decreases with time especially into D3 with the southward and eastward expanse of the snow. At the surface, a strong cold front will dive southward out of Canada today bringing in much colder air to areas along and east of the Divide, falling below freezing as much of the snow expands in coverage over Montana. WPC probabilities for at least 4 inches of snow on day 1 are moderate (around 30%) near Glacier NP. Strong high pressure in northwestern Canada will move southeastward over the next few days, helping to maintain an easterly fetch into the terrain southward into CO, setting up an impactful storm on Thursday/Thanksgiving into Friday. By day 2 (early Thanksgiving morning), upper divergence on the RRQ of the northern stream jet coupled with increasing PVA out of the Great Basin as the trough closes off will combine with increasingly easterly flow and 700mb FGEN to greatly expand the snowfall across Wyoming behind the cold front, with snow increasing to over 1"/hr in some locations. Shadowing will occur on the lee side of the bigger mountain ranges, with windward sides seeing much higher accumulations. WPC probabilities for at least 6 inches of snow on day 2 are high (>70%) over the Bighorns southward to Laramie Mountains as well as over the Absarokas and Wind River Range. Lower probabilities extend westward across the southern Idaho ranges and eastward to the Black Hills and western NE. Upper low will help drive increased snow over the northern Wasatch into the Uintas, where probabilities are moderate (>40%) for at least 6 inches of snow late Thursday into mid-day Friday. On day 3, upper jet to the south of the upper low will round out through AZ/NM and extend into the southern Plains, aiding in broader lift over the western central High Plains. Southwesterly flow over the Four Corners region will favor the San Juans as the surface front hangs up just east of the terrain, providing lower-level convergence. There, WPC probabilities for at least 6 inches of snow are high (>70%) with moderate probabilities (40-70%) of at least a foot. Areas east of the mountains will likely see lesser amounts on the order of a few inches, along and east of I-25 in CO. Over the western central High Plains, there remains a lot of uncertainty in QPF amounts (and thus snowfall) with the bias-corrected blends notably lower than many of the deterministic models. For now, WPC probabilities of at least 4 inches of snow through 12Z Sat over eastern CO and western NE/KS are low (<30% or so), but have room to trend higher. Fracasso ***Key Messages for Thanksgiving Snowstorm*** --Much colder temperatures for the Rockies A strong cold front over Canada will push southward today bringing in much colder air to the northern Rockies. Light snow will gradually expand in coverage over Montana as the cold air sink southward. --Heaviest snow over Wyoming Thursday Snow will increase on Thanksgiving over Wyoming and progress southward into Colorado overnight into Friday. There is a high (>70%) chance of at least 6 inches of snowfall through early Saturday over parts of Wyoming. Lighter amounts are forecast elsewhere in the Rockies and over the High Plains. --Impacts to travel from snow and wind Snowfall rates of 1" per hour will make for difficult travel on busy interstates such as I-25, I-80, and I-90. Blowing snow will locally reduce visibility. --Flash freezes may affect road travel The Arctic front may cause flash freezes in Utah, Southern Wyoming, Colorado, and the Nebraska Panhandle Thursday (Thanksgiving) evening as temperatures rapidly drop below freezing.