Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 502 AM EDT Fri Oct 27 2023 Valid 12Z Fri Oct 27 2023 - 12Z Mon Oct 30 2023 ...Central & Southern Rockies to the Central High Plains... Days 1-3... Friday night, a pair of shortwave troughs; one over the Desert Southwest and another digging south through the interior Northwest, will work in tandem with an impressive upper ridge over the Southeast to form a robust 200mb jet streak over the Midwest by Saturday (NAEFS shows 200mb wind speeds up to the 99.5 climatological percentile over the Midwest and Great Lakes). Strong and extended periods of upper level ascent will ensue over the central Rockies and High Plains thanks to the jet streak's diffluent right-entrance region overhead and persistent PVA through the weekend. To the north, a dome of high pressure centered over "Big Sky" country will anchor an expansive area of sub-freezing temperatures from the Pacific Northwest to the Upper Midwest. Over the central High Plains, Saturday morning will witness moist SW flow in the 700-300mb layer that coincides with WAA aloft. At lower levels, a steady diet of 850-700mb moisture flux that runs directly into a secondary cold front over the Central Rockies. By midday Saturday, the cold front is forecast to race south through the central High Plains and force surface temperatures to plunge below freezing while southwesterly flow at 700mb fosters a classic overrunning event in place. An enlarging DGZ aloft Saturday evening and into the overnight will also result in rising SLRs that will be supportive of heavier snowfall rates across CO, southern WY, and central NE. The front advances as far south as west-central Texas in the form of a "Blue Norther" by Sunday with the overrunning set up taking shape as far south as northwest TX, prompting an icy wintry mix to take shape there as well. WPC probabilities are highest in the CO Rockies and the southern most ranges of WY with high probabilities (70-80%) for snowfall totals >8" through Sunday morning. Meanwhile, there is a medium chance (40-60% probabilities) for snowfall >4" from Cheyenne to central NE. Central NE in particular bears watching as guidance has been keying on strong 700mb frontogenesis aloft that could foster heavy snow bands during the day on Saturday. While WPC probabilities of 10-20% suggest lesser chances for >8" of snowfall in the immediate Denver/Boulder metro area, should heavier snow bands generating 1-2"/hr rates come to fruition as some guidance members indicate, snowfall totals >8" in the Denver/Boulder metro are within the envelope of potential outcomes for this event by the time snow comes to an end Sunday. It is worth noting the Probabilistic WSSI (WSSI-P) shows a heightened chance (60-70%) for Moderate Impacts in the Denver/Boulder metro area late Saturday into Sunday morning. Impacts would primarily be to hazardous travel conditions that include snow covered roads and poor visibilities. This would also be the region's first impactful winter storm of the season thus far. The WSSI sports Moderate to Major impacts in the higher terrain of the CO Rockies while there is an expansive swath of Minor impacts from the I-25 corridor of southern WY and central CO on northeast into the Central Plains. The Southern Rockies can also expect measurable snowfall from this winter storm as WPC 24-hr snowfall probabilities depict higher chances (60-80%) for snowfall totals >6" in the Sangre De Cristo of southern CO and northern NM on Sunday. There are also some Minor impact areas in southern KS and western OK with low chances (10-20%) for >0.10" of ice accumulation in south-central KS through Sunday afternoon. Mullinax