Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 428 AM EDT Sat Oct 06 2018 Valid 12Z Sat Oct 06 2018 - 12Z Tue Oct 09 2018 Day 1... ...Ranges of ID/MT/WY/UT/CO... An upper level trough gradually strengthens as it moves across the Great Basin. The NAM shows a closed 700 mb low developing along the front in UT. Low level convergence in the vicinity of the front and possible low leads to a QPF and snow maxima in the northern Wasatch and Uintas of UT, with a secondary maxima in the ranges of southwest MT near the front. Lighter totals are expected further east due to limited duration of snow in the ranges of eastern WY, western CO, and southern ID. The probability of significant icing is less than 10 percent. Day 2... ...Mountains of UT/CO/WY/Southern MT... An upper trough will amplify and cutoff across the desert southwest while moving only slowly to the east. The closed 700 mb low drifts east across UT towards the CO border with the 700 mb front north of the low drifting east across WY. Lift provided by the front and upper jet combines with orographic influences to generate mountain snows. Heavy snow is probable as jet level dynamics and upslope enhancement combine to create strong lift across the high terrain of the San Juans, Wasatch, Unitas, and Bighorns. WPC probabilities across the highest terrain are high for over 8 inches of snow, and over 12 inches is possible. However, uncertainty still exists into how low snow levels will get east of the low/front, and this is reflected in lower probabilities below 8000 feet. ...Western SD across Western ND... The approaching low level front and moisture convergence will cause increasing potential for heavy snow across the Black Hills. Anomalously high precipitable water will be focused on E/SE low-level flow into the terrain, causing significant upslope enhancement to synoptic lift. There remains a large spread in potential snowfall amounts with the NAM and ECMWF showing lower amounts and GFS/GEFS Mean on the high end across northeast WY into the western Dakotas. WPC probabilities are highest for heavy snow at higher elevations but low for heavy accumulations at lower elevations. Lower amounts are likely northeast into ND as initial temperatures are too warm for snow with dual QPF and temperature profile uncertainties. The probability of significant icing is less than 10 percent. Day 3... ...Ranges of UT/CO/WY/adjacent southern MT... The upper trough will drift east across Colorado and NM. Mid level forcing due to spokes of vorticity rotating around the mean trough support light to moderate mountain snows. The highest snow amounts are likely in the high terrain of the Uintas, San Juans, Colorado Rockies, and Laramie Mountains of Wyoming, where WPC probabilities are greatest for 8 inches of snow. ...Mountains of northwest MT... The models show confluent flow over British Columbia and the Pacific northwest, with a 300 mb jet streak that comes south into Wa and then OR that in turn induces upper divergence in the left exit region of the jet. The lift from the upper divergence causes precipitation to develop over the Pacific Northwest and then northern Rockies, with snow likely in higher elevations of northernmost ID and northwest MT, including Glacier National Park/Rocky Mountain front. Greater weighting was given to the stronger closed low forecasts of the ECMWF, supported by UKMET, although odds are the UKMET is too deep with its 500 mb low. ...Black Hills of SD into ND...Southeast MT Lighter snows are expected on day 3 in the western Dakotas to southeast MT as the northern stream trough drifts east with tepid 700 mb vertical motions in the region and stronger ascent in areas that are too warm for snow in the central Plains. The probability of significant icing is less than 10 percent. Petersen