Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 414 AM EST Tue Jan 16 2024 Valid 12Z Tue Jan 16 2024 - 12Z Fri Jan 19 2024 ...Northeast... Day 1... Trough axis rounding the sprawling deep low centered over northern Ontario will shift east from the Midwest to the eastern Great Lakes this morning before turning left and crossing the Northeast this evening. Rapid surface cyclogenesis occurs with the coastal low zipping north from Cape Hatteras early this morning to the Gulf of Maine this evening riding in the right entrance region of a SWly jet topping 150kt by midday. The tight baroclinic zone along the Northeastern Seaboard which the low rides up will shift inland a bit with backing low level flow making for some wintry mix in areas that have received snow over the past day. Day 1 ice probs for >0.01" are 40-60% along the I-95 corridor from northern VA through Mass with 10-20% probs for >.1" over eastern CT/RI. Deformation axis snow bands develop ahead of the mid-level trough axis this morning over the interior northern Mid-Atlantic, strengthening as they move up through New England through this afternoon as the low intensifies. 00Z HREF mean hourly snow rates increase to 1-2"/hr over VT/NH and much of Maine this afternoon into the evening. Day 1 snow probs for >4" are 40-80% over the northern three New England states with 50% probs for >8" over northern Maine. The dry slot in this developing low expands today and allows precip to end overnight in New England. ...Prolonged Great Lake Effect Snow... Days 1-3... An enormous upper low center pinwheels over northern Ontario into Wednesday with continued cyclonic flow across the Great Lakes and some renewed heavy lake effect snow (LES) today after the passage of an upper trough axis this morning in favored belts downwind of Lakes Superior, Michigan, Erie, and Ontario through mid-week. A series of mid-level disturbances cross the Lakes through Friday, which, along with continued cold air, sustained at least occasional LES and some lake enhanced snow through that time. Day 1 snow probs for >6" are 40-80% in the Keweenaw Peninsula of MI and in the single band reformation areas south from Buffalo and on the Tug Hill with synoptic snow driven by a coastal low generally stays south of Lakes Erie/Ontario this morning. Lighter flow and forcing farther north favors eastern U.P. snow belts as well as the Tug Hill where there are higher Day 2 probs for >8". Flow shifts northerly Thursday as the upper low ejects east bringing the threat for LES to northwest Indiana, but relief for most areas that have been hammered over the past few days. ...Northwest, Northern Rockies, Northern Plains... Days 2-3... The Arctic air-mass that has firmly entrenched itself over the Pacific Northwest, the northern Rockies, and northern High Plains will stick around through Thursday while a pair of Pacific storm systems direct heavy precipitation at the region today through Wednesday night. The first round arrives this afternoon as an IVT of >500 kg/m/s (above the 90th climatological percentile according to NAEFS) forces Pacific moisture and warm air aloft to overrun the stubborn sub-freezing air-mass within the boundary layer. Freezing rain will occur along the Columbia River of western WA/OR today, expanding through The Gorge and over the Columbia Basin. There is a risk for 0.25-0.75" ice near The Gorge with Day 1.5 ice probs for >0.25" 40-70% there. Further significant tree damage and power outages are possible, with this coming on the heels of the ice storm a few days ago. Snow levels surge to 9000ft over the Cascades in the moisture plume today. However, the second wave, a sharp, positively-tilted trough shifting south from BC, will lower heights and snow levels across the Northwest Wednesday with widespread heavy snow above about 3000ft in the Cascades and down into the Valleys of central/northern ID/western MT/WY/ and northern UT where Day 2 snow probs for >6" are high. Strong high pressure building in from Alberta will also support upslope flow not only in the Northern Rockies, but also the central Montana plains but the Big Snowy and both the Little and Big Belt mountains of central Montana. The Cascades will rack up the most snowfall through Thursday with WPC PWPF showing very high chances (>80%) of measuring >18" through Thursday afternoon. The Bitterroots, Lewis Range, and Tetons also sport high chances (>70%) for >18" of snow through Thursday. The Big/Little Belt and Big Snowy mountains of central Montana sport moderate-to-high chances (50-80%) for >12" of snowfall through Thursday afternoon. Travel will be treacherous within these mountains' passes. By Wednesday night, a shortwave trough tracking southeast from southern British Columbia will foster sufficient vertical ascent aloft, along with the aid of upslope NE-Erly surface winds to support periods of snow in southern Montana, eastern Wyoming, and east through much of South Dakota/Nebraska. The latter of these regions will be ideally placed beneath the left-exit region of a strengthening 250mb jet streak, resulting in potentially banded heavy snow in north central Nebraska and south-central South Dakota. The Day 2 snow probs are high for >4" over the central MT Plains with low chances (10-20%) for >4" on Day 2.5 along the central SD/Neb border. ...Mid-South... Day 3... The lower MS Valley gets another risk for a wintry mix may have to contend with another fast moving icy setup on Thursday. Day 3 ice probs for >0.1" are still negligible, but Minor ice accumulations on surfaces that will remain well below freezing through mid-week could still lead to hazardous travel impacts. Jackson ***Key Messages for Eastern Winter Storm*** to be updated shortly