Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 322 AM EST Sat Jan 01 2022 Valid 12Z Sat Jan 01 2022 - 12Z Tue Jan 04 2022 ...Western U.S... Days 2-3... Widespread precipitation is expected to return to the Pacific Northwest Sunday into early next week as another closed low off British Columbia opens up as it sags southeast with a favorable fetch of moisture impinging on the Oregon and Washington coasts. Lower snow levels (1000-2000 ft during the heaviest precipitation) will bring widespread heavy snow to much of the Olympics and OR/WA Cascades during the day 2 period followed by increasing snowfall for northern CA and the OR Blue Mountains, and the Bitterroot Range and Sawtooth Mountains of Idaho. For Day 2, the WPC probabilities for 8 inches or more is high for the Olympics and WA Cascades then those high probabilities spread southward into the OR Cascades and across the Idaho ranges on Day 3. 2-day total snowfall in the higher terrain of the Olympics and Cascades could exceed 4 feet, and the low snow levels will also allow for significant accumulations of more than 1 foot at Snoqualmie, Stevens, and Santiam Passes. ...Central and Southern Appalachians... Day 3... An anomalously deep shortwave trough will track through the Deep South Sunday and lift northeast toward the Mid-Atlantic Monday morning. Surface cyclogenesis is forecast to occur over the southern Appalachians to central SC and deepen as it tracks toward the NC Outer Banks. Finally, an 150+ kt 250 mb jet streak over the Northeast will put the southern Appalachians and southern Mid-Atlantic in the favored right entrance region - all of this leading to favorable forcing for ascent late Sunday into early Monday and the widespread precipitation is expected with a narrow corridor of heavier precipitation on the northwest side of the low track. The GFS continues to be a deeper/stronger solution compared to the much faster/progressive/flatter NAM and hi-res guidance which shows a weaker sfc low. The WPC progs and QPF leaned toward the GFS style solution. As the shortwave reaches the southern Appalachians, plummeting temperatures and CAA will allow for a quick changeover to wet snow initially for the higher elevations of eastern TN, western NC, and southwest VA. As the cold air reaches the SC/NC Piedmont and southeast VA, a chase of the cold air and remaining QPF leads to a tricky forecast but the potential exists for a narrow corridor of wet, heavier snow on the backside of the departing precip band. The latest WPC probabilities show the best probabilities for 1 inch for southwest/western NC for the higher elevations with just a slight signal for 1 inch spreading into southern/southeast VA. This is low-confidence forecast with lots of details to continue to iron out over the next couple forecast cycles. ...Central Plains through the Great Lakes and into Northern New England... Days 1-3 ...Significant winter storm expected for portions of Central Plains to Great Lakes... Impressive synoptic forcing developing this morning across the Plains will lead to a corridor wintry precipitation from the Plains to the Great Lakes over the next 24-36 hours. Phasing shortwave energy, upper diffluence, and developing low pressure will provide the necessary forcing for ascent to expand precipitation this morning through tonight. At the same time, a robust moisture pool noted by PWs of +3 standard deviations across the Southeast will surge northward as the LLJ rises into a TROWAL and wraps cyclonically around the system. This enhanced instability will combine with increasing frontogenesis into a very deep DGZ to produce impressive snowfall from Kansas northeastward towards Chicago. The model guidance remains fairly locked on the placement of the heaviest QPF and resulting snow though the 00Z guidance did ever so slightly trend downward with QPF such that a very slight decrease in amounts is advertised, though this doesn't change too much the overall impacts and messaging at this point. The latest HREF shows high probabilities of snowfall rates up to 1"/hr this morning across northern KS/southern NE then slight to moderate probabilities reaching the Chicago metro later this evening and this corridor is where the highest total accumulations are expected. The WPC probabilities are moderate to locally high for 6 inches in this corridor with some isolated higher amounts near 8-9" most likely across southern IA, northern MO and a possible secondary maxima around Chicago due to lake enhancement. Farther northeast on D2, the ascent and available moisture both wane, but WPC probabilities indicate a moderate chance for more than 4 inches across portions of western NY, the Adirondacks, and northern New England. South of the heavy snow, a corridor of mixed precipitation of sleet and freezing rain is expected, most concentrated over eastern Kansas through central/north-central Missouri and into portions of west-central Illinois where up to 0.10" is expected. The WPC ice probabilities for 0.10" have lowered to 20-30 percent across central Missouri - possibly due to the more intense precip rates leading to less efficient accretion. Sleet may accumulate to 0.5" in some locations after the changeover from freezing rain. Taylor ~~~Key Messages for the Central Plains to Great Lakes Winter Storm~~~ The most significant snow impacts are expected from northern Kansas to the Chicago metro area. Snowfall rates in excess of 1"/hr combined with strong winds will lead to dangerous travel and near zero visibility at times. Significant mixed precipitation including freezing rain and sleet is likely to the south of the heavier snow. Ice accretions up to 0.1", and sleet accumulations near 0.5", are possible from eastern Kansas into western Illinois. Bitterly cold temperatures will follow with dangerously cold wind chills Saturday night into Sunday morning.