Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 427 AM EDT Sun Oct 07 2018 Valid 12Z Sun Oct 07 2018 - 12Z Wed Oct 10 2018 Day 1... ...Ranges of southern MT/WY/UT/CO... An upper level trough will amplify over the Great Basin. This will initiate low-level cyclogenesis with warm advection occurring east of the low into CO and WY. Synoptic forcing due to vorticity advection and upper difluence will combine with the frontal circulation to produce mountain snows. Warm temperatures of above 0C at 700mb across much of CO and southeast WY will limit snow to the highest terrain. WPC probabilities for greater than 4 inches of snow are highest in the Uintas of UT and San Juans of CO, with lighter amounts likely across the other mountain ranges. ...Western SD into southwest ND... A sharpening 500mb trough will induce warm/ moist air advection into the High Plains. A shortwave will eject from the mean trough to lift northeast through the western Dakotas coincident with strengthening mid-level frontogenesis and 700mb deformation to provide ascent. Initial temperature are too warm for snow so the duration of snow is uncertain. Amounts are likely to be low most areas with the best chance of higher totals in the high terrain of the Black Hills. The probability of significant icing is less than 10 percent. Day 2... ...Mountains of UT/CO/WY... Anomalously strong upper trough will migrate slowly eastward across the southwest and four corners region into the CO/NM Rockies, with the 70 mb front drifting east across ND and WY, with post-frontal cooling in WY allowing a transition to snow. The potential exists for significant accumulation of greater than 6 inches in the high terrain of the San Juans, Uintas, Laramies, and Snowy Range. Lesser amounts are likely below 6000 feet. ...Western SD into Central ND... Warm moist advection ahead of a closed mid-level low will combine with upper diffluence and the low level frontal circulation to produce lift and snow into the High Plains. Light snow accumulations are possible, with near surface temperature uncertainties leading to differences in accumulations, with the NAM Conus Nest and 00z GFS a few inches higher than the parent NAM and ECMWF. In the terrain of the Black Hills, upslope enhancement will increase snowfall potential, and WPC probabilities are highest here. ...Pacific Northwest mountains into western MT... A 300mb jet maximum will push southeast from Canada, inducing lift within the left exit region as it drops into the Pacific Northwest. Precipitation will develop in response to this synoptic forcing, initially across the Cascades of Washington before shifting into the northern Rockies. Accumulating snowfall is expected across the Rockies in the vicinity of Glacier National Park, with total amount limited by the short duration of snow this period. The probability of significant icing is less than 10 percent. Day 3... ...Ranges of MT/ID/UT/CO/WY/northern NM... The initial upper trough will gradually fill and begin to eject eastward in response to a potent shortwave and jet maximum digging across the Great Basin. The new upper trough amplifying as it crosses from ID and NV across western MT, southeast ID, and northern UT results in cyclonic vorticity advection and increasing upper difluence/embedded upper divergence in the left exit region of the approaching jet to support mountain snows. The highest WPC probabilities are across the high Rockies in Colorado, and in the northern Wasatch of Ut/Uintas/ranges of western WY and western MT, including the Rocky Mountain front, where several inches of snow is possible Tue-Tue night. ...Northern Plains... The Upper trough will gradually deamplify and lift northeast. Lift is expected within the frontal circulation for snow within the deformation zone across Eastern WY across South Dakota and into North Dakota. The models still diverge considerably into timing and location of QPF and duration of how long it is cold enough for snow so WPC probabilities remain low for 4 or more inches of snow. The probability of significant icing is less than 10 percent. Petersen