Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 339 AM EDT Wed Oct 31 2018 Valid 12Z Wed Oct 31 2018 - 12Z Sat Nov 3 2018 Colorado and New Mexico ----------------------- The heavy snow event across southern Colorado and northern New Mexico early Wednesday morning will be abating during the first 6 hours of the Day 1 period Wednesday. The greatest totals are likely to be realized over north-central New Mexico, in particular the Sangre de Cristo range, where the best combination of moisture and forcing for ascent will be through the midday hours. The trough axis passes through this region by the afternoon and dry air advection ensues, bringing an end to the snow. There are still some GEFS ensemble members indicating the potential for 4+ inch amounts, where the EC members showing these amounts are fewer. WPC probabilities are low-moderate for 4 inch amounts for this region. Northern Rockies and Cascades ----------------------------- The heaviest snowfall totals during this entire forecast period are expected for the mountainous terrain adjacent to Yellowstone National Park and the higher elevations of northwestern Montana. A series of shortwaves embedded within a pronounced and long duration Pacific Jet will create periods of heavy mountain snows through the end of the week. Two distinct shortwaves will move across the Pacific Northwest through Friday accompanied by favorable upper level jet dynamics. This jet stream will advect a low-mid level moisture plume across the Cascades and then the northern Rockies, and lift associated with jet level diffluence, vorticity advection, and orographic forcing will support numerous snow showers across the higher terrain, especially above 7500 feet elevation. Thermal profiles are generally warm enough to support mainly rain for the Cascades with the exception of the higher volcanic peaks, which could easily receive several inches of snow. WPC probabilities are high for greater than 4 inches of snow for the higher elevations of northwest Wyoming, especially on Day 3. Northern Plains --------------- The next short wave trough tracking across southwestern Canada on Friday will support an Alberta Clipper that is forecast to track across eastern Montana and reach the western Dakotas by Saturday morning. A swath of light to possibly moderate precipitation is expected to the north and east of the low track courtesy of warm air advection during the second half of Day 3 Friday night, and thermal profiles are currently marginal across northeast Montana and central/western North Dakota for accumulating snow. There are multiple EC and GEFS members indicating the potential for 4 inches of snow across parts of these areas, and this will be heavily dependent on near surface temperatures, which will likely be in the 31 to 35 degree range, and precipitation rates. WPC probabilities are low-moderate for 4 inches across extreme northeast Montana and northwest North Dakota. The probability for significant icing (0.25 inches or more) is less than 10 percent all 3 days. D. Hamrick