Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 239 PM EST Wed Dec 13 2023 Valid 00Z Thu Dec 14 2023 - 00Z Sun Dec 17 2023 ...Southern Rockies into the High Plains... Days 1-2... A strong closed mid-level low will deepen across the southern Great Basin tonight and then maintain intensity as it drifts eastward over the Four Corners states before opening into the Southern Plains on Friday, before re-amplifying into the Central Plains by Saturday. This potent system will drive impressive synoptic ascent through downstream divergence/height falls and periods of intense PVA, which will overlap with waves of upper level diffluence within the LFQ of a jet streak rotating through the base of this trough to produce widespread and significant ascent across the region. This lift will impinge upon an increasingly saturated column as robust moisture and theta-e advection occur downstream from the primary upper trough, reflected by PW anomalies from the NAEFS ensemble tables reaching +4 sigma. The intense ascent acting upon this moist southerly flow will result in waves of precipitation spreading northward from TX/NM into CO/KS/NE. The column remains thermally marginal for heavy snow outside of terrain features, but in areas where the column is cold enough for snow, or where dynamical effects (through intense snowfall rates) can cool the column sufficiently, heavy snowfall accumulations are likely. The WPC prototype snow band tool suggests a high probability for snowfall rates of 1-2"/hr in the terrain of the San Juans and Sangre de Cristos, where upslope flow will contribute to ascent, as well as into the High Plains and even into parts of the OK/TX Panhandles and western KS. These snowfall rates are likely due to the potential for CI as noted by folded theta-es surfaces within a region of -EPV evident in cross sections. Where these intense rates occur, snowfall accumulations will likely be heavy, and WPC probabilities for more than 4 inches on D1 are above 80% in the High Plains of NM, as well as across the Sangres and San Juans, where locally as much as 20 inches of snow is possible. Farther to the north and east, WPC probabilities for 2+ inches reach as high as 50%, with locally higher amounts probable as convective rates cause rapid accumulation in some areas. The guidance has also begun to show a potential secondary band of snow developing Friday morning across western KS and into the OK/TX Panhandles as the upper low re-amplifies and results in a strengthening deformation axis to its west. This axis may overlap effectively with some 850-700mb fgen to enhance ascent and cause a pivoting band of snow to shift eastward into Friday aftn. Confidence in this evolution is low, and the best ascent does not appear to intersect the favored snow growth region. However, the potential for some additional snow accumulation has increased for D2, and the WSE plumes for 6-hr snowfall show a secondary max with a lot of spread centered around 06Z/12Z Friday. Well north of the heaviest snow, an axis of light freezing rain remains likely through tonight as the WAA slopes above the slowly retreating cold high pressure to the east. The guidance has trended with some subtly stronger push of this cold air west into the High Plains, so freezing rain accretions have increased with this update. However, the deep southerly flow should still erode this high during Thursday morning, and WPC probabilities are 50-60% for 0.01", and less than 5% for 0.1" across western KS. ...Northern Plains and Upper Midwest... Days 2-3... An amplifying shortwave digging out of Saskatchewan will shift into ND/MN late D2 into D3 while deepening into a closed low. Downstream ascent through height falls and divergence will be at least modestly aided by diffluence within the RRQ of a distant jet streak. Moist advection surging out ahead of this wave will manifest as intensifying 285K isentropic ascent, lifting cyclonically within a WCB, with moisture convergence occurring within the mid-levels as southerly flow gets sheared into the westerlies across MN. At the surface, a wave of low pressure will likely develop beneath the mid-level feature, with a warm front draped to its east serving as a boundary for enhanced ascent through modest fgen. While the forecast thermal profile is marginal to support heavy snow, and precipitation itself is still progged to be modest, there is increasing potential in a laterally translating band of precipitation from west to east Friday into Saturday, which if intense enough could dynamically cool the column to produce a stripe of heavy snow. WPC probabilities above 10% for more than 2 inches of snow are currently confined to the Arrowhead of MN where moist flow off Lake Superior upsloping into the terrain may enhance snowfall. However, WSE plumes indicate the potential for some higher snowfall amounts (still less than 3") as far west as eastern ND, so this event will need to be monitored for additional strengthening/higher snowfall potential. Weiss