Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 416 PM EDT Sun Oct 07 2018 Valid 00Z Mon Oct 08 2018 - 00Z Thu Oct 11 2018 Day 1... ...Ranges of southern MT/WY/UT/CO... Deep upper trough across the Great Basin will migrate slowly eastward while spokes of vorticity rotate through the base and eject to the northeast. S/SE flow around surface trough development in the lee of the Rockies will drive warm and moist air into the mountains. 700mb temperatures across eastern CO and into eastern WY will climb above 0C, but the mountains just NW of this axis have the potential to see significant snowfall as synoptic lift is aided by forced upslope enhancement. Snow levels across CO will likely remain above 9000 feet before lowering late, confining the highest snow amounts to the San Juans. Snow levels may be considerably lower, down to 4000 feet in WY/UT so more widespread snow is likely there with WPC probabilities indicating a good chance for greater than 4 inches of snow in the Laramies of WY, as well as the Uintas in Utah, with more than 8 inches possible in WY. ...Western SD into central ND... A shortwave will eject from the Great Basin upper trough and lift across the northern Plains. This vorticity lobe will be accompanied by diffluence within the right rear quadrant of a 300mb jet max moving atop a moist airmass as high PWAT gets advected into the region ahead of a low pressure in the lee of the Rockies. A swath of snow is probable across western SD through central ND along the 700mb deformation axis which will maximize lift despite moderate QPF. Some enhancement is possible across the Black Hills where easterly upslope flow occurs, but WPC probabilities will generally remain low for more than 2 inches of snowfall outside of the Black Hills. ...Freezing Rain... A small area ice accretion is becoming more likely from western WY into the Panhandle of NE and western SD. As 700mb warm advection commences, cold air banked into the terrain will be slow to erode due to reinforcement of cooler and drier air within a wedge of high pressure funneling down from the north. Saturation on forecast soundings is shallow, but with 700mb temps to +4C and sub-freezing surface temperatures, a 6-12 hour window exists for freezing drizzle which could accrete up to a few hundredths of an inch. WPC probabilities for one-tenth of an inch of ice accretion remain very low. Day 2... ...Mountains of UT/CO/WY/NM... Upper trough across the west will migrate eastward pushing a Rockies lee low pressure into the Plains. Lift from vorticity effects as well as jet level diffluence will produce mountain snows, especially the first half of day 2 before drier air advects into the region from the west. Accumulations of greater than 4 inches are possible in the high terrain of the San Juans, Colorado Rockies, and Uintas. Lesser amounts are likely below 7000 feet. ...Northern Plains... Low pressure in the lee of the Rockies will fill and transition to the east while 700mb warm advection persists into the Dakotas. Synoptic lift beneath a departing 300mb right entrance region and the aforementioned warm advection will be aided by isentropic ascent to produce snowfall. Temperatures initially are marginal but will cool through the morning so light snow is likely across much of western SD into southwest ND, with the highest probabilities for over 2 inches of snow focused in the terrain of the Black Hills. ...Pacific Northwest mountains into western MT... Potent 300mb jet maximum will dive 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, with widespread high elevation snow likely across the high Cascades of WA as well as the northern Rockies of MT and ID. Total QPF is expected to be light, but many locations above 7000 feet have the potential to see more than 2 inches of snow. A better probability for over 4 inches exists on the eastern side of the northern Rockies in Montana near Glacier National Park, where a prolonged period of easterly upslope flow is expected to enhance snowfall accumulations. The probability of significant icing is less than 10 percent. Day 3... ...Northern and Central Rockies... Potent shortwave and associated jet max will dig southeast into the longwave trough across the west. As this feature races eastward, it will force synoptic ascent into a moist environment, aided by weak 700mb deformation across WY. Widespread mountain snows are likely from the MT Rockies southward into Utah and Colorado. The highest amounts are likely in the high terrain of northwest Wyoming, including the Tetons and Wind Rivers, where isentropic upglide is maximized. Here, WPC probabilities are high for 8 inches of snow or more. Lesser amounts are likely elsewhere above 6000 feet. ...Northern Plains... Low pressure will develop near Iowa in response to a negatively tilting upper trough rotating through the Central Plains. As this surface low deepens and lifts into the Great Lakes, mid-level frontogenesis will increase in conjunction with the deformation axis NW of the low center. Robust ascent will accompany this feature despite weak jet level dynamics, and a stripe of heavy snow within the developing comma head is possible from far eastern ND into northern MN. Uncertainty still exists into the placement and intensity of this axis, and this is reflected by only moderate WPC probabilities for more than 4 inches of snow. The probability of significant icing is less than 10 percent. Weiss