Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 402 PM EST Wed Dec 29 2021 Valid 00Z Thu Dec 30 2021 - 00Z Sun Jan 02 2022 ...Western U.S... Days 1-3... Continued active pattern across the West through Saturday, with a subtle down-tick in coverage expected at least briefly to begin 2022. A broad upper trough currently across the West will sharpen as a shortwave digs down from British Columbia into the Pacific Northwest, and a closed low west of Baja opens and ejects eastward towards the Four Corners. These features will interact Saturday near the Four Corners and then shift eastward during D3, leaving pronounced ridging across the region late Saturday. This elongating trough across the West will drive height falls, while the embedded shortwaves produce PVA, with ascent aided by periodic diffluence maxima as a subtropical jet streak arcs from CA through the Plains, and a secondary polar jet digs into the Pacific Northwest. The overlap of ascent will produce widespread precipitation, aided by surface lows and fronts, to produce widespread moderate to heavy snowfall across most of the Western terrain D1 and D2, with coverage easing to just the Southern Rockies on D3. For D1, the heaviest snow is expected across two distinct regions. The first will be across the Pacific Northwest including the WA Cascades and Blue Mountains of OR where WPC probabilities for 8 inches are high. This is in response to a cold shortwave dropping into the region and pushing a surface low southeast and onshore. With snow levels crashing to sea level, even the lowlands around Seattle, WA and Portland, OR are likely to receive snowfall, and WPC probabilities for 2 inches at Seattle are 50% where some enhanced snow rates are likely. The other area on D1 likely to receive significant snowfall is across southern CA and into the Four Corners within the greatest IVT and mid-level moisture confluence. WPC probabilities are high for 12 inches in the San Bernardinos and San Gabriels, as well as the southern Sierra, and ranges of the Great Basin, Wasatch, and CO Rockies. Locally more than 2 feet of snow is possible. By D2 the Pacific Northwest will quiet except in the OR Cascades and Blue Mountains where WPC probabilities for 8 inches are again high. Further to the southeast, the interaction of the dual shortwaves will lead to impressive height falls following mid-level divergence in the presence of above normal PWs. This should drive heavy snowfall along the Mogollon Rim, the Wasatch, San Juans and CO Rockies where WPC probabilities for 12 inches are high, and locally an additional 2 feet is possible. By D3, residual heavy snow is likely in the White Mountains of AZ and the San Juans of CO, with the remaining West quieting down as ridging blossoms from the west. ...Central Plains to the Great Lakes... Day 3... ...Significant winter storm becoming more likely to begin the New Year... Southern and northern stream shortwaves will eject out of the Western CONUS Friday and interact across the Central Plains Saturday. This will drive an amplifying longwave trough across the Intermountain West which will shift eastward with time, combining with amplifying downstream coupled jet streaks to produce surface cyclogenesis in the lee of the Rockies. This low pressure will then lift northeast from the Southern Plains into the Ohio Valley D3, with heavy wintry precipitation likely along and north of the track of the low. While guidance continues to feature a large spread in both timing and placement of this low, there is increasing confidence that heavy snow and moderate to heavy freezing rain will impact the region for New Year's Day. Well NW of the surface-850mb low, heavy snow is likely. PW pooled across the Southeast near the Gulf of Mexico is progged to reach nearly +3 standard deviations above the climo mean, and then get advected northward into the system as the trough deepens to the west. This is noted by a 40kt LLJ wrapping cyclonically into the low coincident with the WCB lifting into a modest TROWAL at the end of D3. This will be accompanied by impressive WAA. and moist isentropic ascent at 290-295K which will surge into a very cold column to drive an anomalously deep DGZ with increasing ascent. SLRs in the region of heaviest snow could exceed 15:1, which is the 75th percentile according to CIPS. Additionally, while it is too early to pinpoint the placement of any banding, both fgen banding and deformation banding as the low pulls away are likely, leading to locally enhanced snowfall. WPC probabilities have increased and now feature a better than 70% chance for 4 inches along the KS/NE border northeastward into southeast IA, with lower probabilities extending from the High Plains of eastern CO through the L.P. of MI. A secondary maximum in probabilities exists near the Lake Michigan SW lake shore where LES enhancement is likely. South of the heaviest snow, a corridor of moderate freezing rain is also likely as the WAA drives a warm nose >0C northward, while low-level NE flow maintains dry wet-bulb advection /CAA/ to keep surface temps from climbing above freezing. Despite 850mb flow backing to enhance WAA, low-level CAA will offset, and precip may transition from rain to freezing rain to snow, or in some locations persist as freezing rain for an extended duration. There remains considerable spread into exactly where the axis of heaviest freezing rain will occur, but WPC probabilities for 0.1" are as high as 20%, highest across parts of northern Missouri. Weiss ~~~Key Messages for the Central Plains to Great Lakes Winter Storm~~~ Accumulating snow and ice are likely on New Year's Day (Saturday) from the Central Plains to the Great Lakes with travel impacts expected through the holiday weekend across several states. Several inches of snow are likely with potentially heavier accumulations where snowfall rates exceed 1" per hour. Significant freezing rain is possible to the south of the heavier snow. Uncertainty remains in the location and timing of mixed precipitation. Bitterly cold temperatures for Sunday with highs only in the 10s to low 20s but wind chills near and below zero.