Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 253 PM EST Thu Dec 30 2021 Valid 00Z Fri Dec 31 2021 - 00Z Mon Jan 03 2022 ...Western U.S... Days 1-3... Longwave trough across the West will amplify D1 in response to joint shortwaves moving through the flow and interacting near the Four Corners Saturday morning. This feature will shift eastward into the Southern Plains D2, leaving a short-wavelength but high amplitude longwave trough in its wake. This will bring an end to wintry precipitation across the region, briefly, until another shortwave drops out of the Gulf of Alaska and approaches the Pacific Northwest Coast on D3. For D1, the heaviest snow is likely in the ranges surrounding the Four Corners, including the Wasatch, Uintas, San Juans, and CO Rockies. This region will be embedded within a pronounced moisture plume advected eastward on mid-level confluence and upper level jet energy. Into this moisture, ascent will become robust through height falls, PVA ahead of a shortwave ejecting from near Baja, and RRQ jet level diffluence. Snow levels are forecast to be 5000-6000 ft, and above this level, WPC probabilities are high for 8 inches, with locally more than 2 ft likely in the highest peaks. A second area of moderate to heavy snow is likely in the OR Cascades and Blue Mountains where a northern stream shortwave will dig southward to provide height falls/ascent across the region. WPC probabilities are moderate for 8 inches on D1. On D2, ridging expands across the West, but the interaction of the aforementioned shortwaves combined with modest upper diffluence will drive continued moderate to heavy snow as far south as the Mogollon Rim and White Mountains of AZ, the Sangre De Cristos, and the Sacramento Range. WPC probabilities are moderate to high for 8 inches, with the heaviest snowfall accumulating to locally more than 2 ft. On D3, the subsequent shortwave will approach the Pacific Northwest beneath a modest Pacific jet streak to provide ascent and moisture. Snow levels will remain quite low, rising only to around 1000 ft in the warm advection ahead of the trough. WPC probabilities are high for 8+ inches of snow in the Olympics and Cascades, with several inches of snowfall likely at the important mountain passes including Snoqualmie and Stevens Passes. ...Central Plains through the Great Lakes and into Northern New England... Days 2-3... ...Significant winter storm expected for portions of Central Plains to Great Lakes New Year's Day... Merging shortwave energy from the Pacific Northwest and from near Baja will interact into a longwave trough near the Four Corners Friday and then push eastward into the Central Plains Saturday. This feature is likely to remain positively tilted as it amplifies into the MS VLY D3. Aloft, a jet streak will intensify downstream of the primary trough axis and arc into southern Canada while reaching 200+ kts. This will provide intense ascent in the RRQ, which is likely to overlap some of the most robust height falls and drive surface cyclogenesis from the High Plains of CO northeastward through the Ohio Valley and then off the New England coast Sunday. Precipitation associated with this system will expand across the Plains in response to pronounced WAA and isentropic ascent at 290-295K surging northward. Northwest of the surface low, a region of heavy snow, and mixed precipitation is likely. The guidance has come into better agreement today, with the exception being the NAM which appears to be a NW/warm outlier. Using a non-NAM blend for this update suggests a swath of heavy snowfall is likely from western Kansas northeast along the MO/IA border, towards Chicago, IL and the L.P. of MI, and into far northern New England. The presence of the strong jet streak to the northeast and impressive mid-level RH near the best 700mb fgen suggests a band of heavy snowfall will translate northeast across this area. DGZ depths are considerable, in some places as deep as 300+mb, which suggests impressive SLRs. The Cobb, NBM, and ensemble methods all suggest SLRs reaching 15-20:1, which if forcing can manifest perfectly into the DGZ could exceed 20:1 at times. Forcing will be intense through moist isentropic ascent with mixing ratios of 3g/kg, and deep synoptic ascent. However, examination of cross sections shows a limited CSI threat as theta-e lapse rates struggle to approach 0C/km. Despite that, 1"/hr snowfall rates seem probable, especially from western KS into IA/MO, and WPC probabilities for 4+ inches are high D2 as far east as Davenport, IA. During D3, this speedy system will shift the highest probabilities for 4+ inches into IL and the L.P. of MI, with lower probabilities extending into northern Maine. Local enhancements are likely SW of Lake Michigan where onshore flow from the NE should provide a favorable LES environment near Chicago. Local maxima may exceed 8 inches anywhere within this region. The biggest change with this iteration is the mixed precipitation zone. There is likely to be a period of freezing rain south of the heaviest snow axis, but guidance today has trended faster with the changeover from freezing rain to sleet due to strong low-level CAA which should provide a faster re-freeze of snowflakes after exiting the warm nose. WPC probabilities for 0.1" of freezing rain remain 20-30% from eastern KS through central MO. However, this may be overdone as forcing during the freezing rain appears robust enough to drive heavier rates which do not accrete efficiently. Additionally, a more rapid cooling should lead to more sleet, and there is a higher probability for up to one-half inch of sleet accumulation across central MO. Any freezing rain and sleet should transition to snow before ending as the CAA persists. Weiss ~~~Key Messages for the Central Plains to Great Lakes Winter Storm~~~ Accumulating snow and ice this holiday weekend from the Central Plains to the Great Lakes resulting in widespread travel disruptions. Several inches of snow are likely with potentially heavier accumulations where snowfall rates reach 1" per hour. Some locations may exceed 6-8†total amounts. Significant mixed precipitation including freezing rain and sleet is possible to the south of the heavier snow, with some ice accumulations greater than 0.10â€. Bitterly cold temperatures will follow on Sunday with highs only in the 10s to low 20s and wind chills near to below zero.