Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 308 PM EST Sat Dec 24 2022 Valid 00Z Sun Dec 25 2022 - 00Z Wed Dec 28 2022 ...Great Lakes through the Interior Northeast... Days 1-3... ...Intense system will maintain local blizzard conditions tonight... The strong area of low pressure reached its maximum strength this afternoon (minimum pressure) as it continues to lift northward through western Quebec. Pressure gradient remains strong over the Great Lakes due to 1040mb high pressure nosing into the northern Plains. The broad cyclonic flow will maintain lake effect snow through Sunday with additional light snow on Monday from the northern fringe of a clipper system. N-NW-WNW flow over the upper lakes will favor multi-band snows over the U.P. and western Lower Michigan while SW flow off Lakes Erie/Ontario will maintain the strong single band well north of the climo hot spots. Eastern bands will slip back south as the flow becomes more westerly with time before stalling and weakening by Monday. 1-2"/hr rates are likely tonight near Buffalo and into Watertown as that band shifts southward out of the Thousand Islands region. WPC probabilities for at least 8 additional inches over the next 48 hours are moderate (>40%) in the western U.P., but high near the Tug Hill and Buffalo. Snow will wind down by Day 3 with only 20-40% chance of at least 4 inches of snow near the Tug Hill. ...Pacific Northwest... Days 1-3... Arctic air remains in place over much of the Interior PacNW along and east of the Cascades where a wintry mix of snow, sleet, and freezing rain continues to fall. An advancing Pacific system just offshore will continue to erode some of the cold air, but even on Sunday some interior areas will see another round of ice. WPC probabilities for at least 0.10" of additional icing after 00Z tonight are generally <50%. Additional icing is likely over parts of northern Idaho and western Montana as well as the nose of warmer air aloft skirts well eastward atop the cold surface. Snow levels will rise late Sunday as another system approaches the region, transitioning the p-type to the more typical rain/snow type, only to fall again on Monday. Modest snow amounts are expected at the higher elevations through the period generally above 4000-5000ft. Farther east, moisture will wring out over the northern Idaho and northwestern Montana ranges that are well into the cold air above any warm layer. Snow levels will fall by Day 3 as the frontal system moves through the area. Focus of the moisture will also shift southward into central Idaho with modest accumulations in the mountains. Farther south. moisture plume off the Pacific will sink steadily through NorCal into central and even southern areas on Day 3 as a strong but progressive atmospheric river rushes ashore. Snow levels will be quite high above 8000-9000ft and the atmospheric river will continue beyond this forecast period. ...Northern Plains to the Mid-Mississippi Valley... Days 1-2... An Alberta Clipper will move through the Northern Great Plains Sunday and through the Mid-Mississippi Valley Monday before being sheared apart by a reinforcing trough late Monday, greatly weakening the wave before it reaches the Mid-Atlantic. A modest push of milder air atop the cold surface layer in advance of the wave will support a chance of light icing on the southern/western side of the precip shield with >50% probabilities for measurable icing stretching from eastern MT through western MO. A band of locally moderate snow is expected from North Dakota southeastward into the Midwest on the order of a couple or few inches. WPC probabilities of at least 4 inches of snow are no higher than 30% over eastern ND with higher probabilities for 2 or more inches stretching southeast from there through southern IL/IN. Fracasso Key Messages for Blizzard --Lake Effect Blizzard A powerful winter storm will continue to produce lake-effect, wind-driven snow across the Great Lakes. Enhancement from the Great Lakes will continue to produce additional areas of heavy snow. Additional heavy accumulations are especially likely in the snow belts east of lakes Erie and Ontario. Even where lesser accumulations occur, heavy snow rates combined with wind gusts over 40 MPH will result in blizzard conditions. --Dangerous Wind Chills & Ground Blizzards Frigid temperatures and strong, gusty winds will continue to impact portions of the Great Lakes and Northeast into Sunday - producing dangerous wind chills. These winds atop existing snow cover may produce ground blizzard conditions in some areas. --Extremely Dangerous Travel with Whiteout Conditions Where blizzard conditions occur, expect periodic whiteouts with near-zero visibility and considerable blowing and drifting of snow. Dangerous wind chills will create a potentially life-threatening hazard for stranded travelers. If you must travel, prepare for extreme cold. In some areas, being outdoors could lead to frostbite in minutes. Ensure outdoor animals and livestock have sufficient shelter.