Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 414 PM EDT Tue Oct 30 2018 Valid 00Z Wed Oct 31 2018 - 00Z Sat Nov 03 2018 ...Colorado and New Mexico... Day 1... Significant snow event will persist through the first half of day 1 with heavy snow likely in the mountains of central Colorado into north central New Mexico. A potent shortwave will dig through the base of an amplifying longwave trough across the Four Corners region before shifting eastward into the western Plains late Wednesday. Mid-level low pressure focused near 700mb will sharpen and cutoff within the positively tilted trough, with E/SE flow ahead of this feature advecting anomalously high precipitable water of more than 1 standard deviation above normal into NM/CO. Increasing relative humidity within this airmass will be squeezed out by intensifying lift due to diffluence within the right rear quadrant of a departing upper jet as well as mid-level deformation, increasing 850-600mb frontogenesis, and isentropic upglide. E/NE surface winds in advance of expanding high pressure to the north will aid in lift also as upslope enhancement becomes focused on the east facing slopes. The trend in guidance has been for a general uptick, as well as a southern displacement, of the maximum axis of QPF. Heavy snowfall is likely across all mountain ranges of central and southern Colorado, as well as northern New Mexico. The southern portion of the Colorado Rockies, as well as the Sangre De Cristos and eastern San Juans all feature high WPC probabilities for 8 inches of snow, with more than 12 inches likely in the favored eastern upslope portion of the Sangre De Cristos above 9000 feet. Lower elevation snow is probable as well, as 700mb confluence pushes moisture eastward into the western high plains. Near surface temperatures are marginal, but a period of strong lift may produce snowfall rates heavy enough to overcome warm ground temperatures. Although WPC probabilities for 4 inches are 10 percent or more only above 6000 feet, some light snow accumulation is possible as low as 4000 feet in eastern Colorado and northeast New Mexico. The probability for significant icing (0.25 inches or more) is less than 10 percent. ...Northern Rockies and Cascades... A series of shortwaves embedded within a pronounced and long duration Pacific Jet will create periods of heavy mountain snows day 1 through day 3. Two distinct shortwaves will move into the Pacific Northwest through late-week accompanied by upper level jet dynamics. This jet stream will transport significant moisture into the Cascades and Northern Rockies, and lift associated with jet level diffluence, vorticity advection, and height falls will create snow across the higher terrain day 1 into day 2, and again on day 3. The first impulse will produce widespread mountain snows across the northern Rockies and the Cascades of Washington on Day 1, shifting southeast into the ranges of Wyoming and Colorado on day 2. The highest probabilities for 4 inches of snow during this period will be across the Absaroka, and Grand Teton Ranges, as well as portions of the Colorado Rockies above 8000 feet. A second impulse and pronounced jet max will lift into Washington State and British Columbia on day 3, bringing another round of snow into the northern Rockies and ranges of northwest Wyoming. WPC probabilities for 4 inches of snow are low outside of the Absaroka range on day 3. The probability for significant icing (0.25 inches or more) is less than 10 percent all 3 days. Weiss