Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 415 AM EDT Mon Oct 08 2018 Valid 12Z Mon Oct 08 2018 - 12Z Thu Oct 11 2018 Day 1... ...Ranges of WY/UT/CO... The models show a closed 700 mb low near the CO/UT border drifting slowly northeast and weakening Monday night, possibly into an open wave. The initial precipitation with higher elevation snow starts with 700 mb convergence and 300 mb divergence in CO and northern UT that then drifts into WY along the low level frontal boundary, with net 24 hour liquid equivalent amounts a bit higher in WY with longer duration precip. WPC probabilities indicating a good chance for greater than 4 inches of snow in Laramie and Snowy ranges of WY. Snow continues to be elevation dependent with lower amounts in valleys. ...Western SD into central ND... A shortwave will eject from the Great Basin upper trough and lift across the northern Plains near the low level front. This vorticity lobe will be accompanied by difluence within the right rear quadrant of a 300mb jet max moving atop a moist airmass. A swath of snow is probable across western SD through central ND along the 700mb deformation axis which will maximize lift, with near-sfc temps borderline for snow. Some enhancement is possible across the Black Hills where easterly upslope flow occurs, with higher snow totals partly due to higher elevations increasing chances precipitation would be snow longer. The 00z NAM suggests precip in western SD becomes snow with a burst of ascent Mon night. Most guidance indicates potential for 2-4 inches. ...Light Freezing Rain/drizzle in parts of Western SD/Western NE/Southeast WY... Patches of light ice accretion are possible from the Panhandle of NE across western SD to southeast WY. 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. The probability of a quarter inch of icing is less than 10 percent. Day 2... ...Mountains of MT/ID/northern UT/CO/WY... Potent 300 mb jet maximum will induce lift within the left exit region as it drops across the northern Great Basin, with 300 mb divergence maxima crossing southern ID and into northern UT. Precipitation will develop in response to this synoptic forcing, with widespread high elevation snow likely across the ranges of southern ID, spreading into the ranges of western WY and then northern UT. The highest amounts are likely in the high terrain of southern ID, western Wyoming, and northern UT including the Tetons, and northern Wasatch in UT. Here, probabilities show potential for 8 inches of snow or more. Further north, snow is expected to develop in the front range of the MT Rockies partly due to the 700 mb front moving across the region and partly due to return flow with an easterly component casing upslope conditions on the Rocky Mountain front. A better probability for over 4 inches exists on the eastern side of the northern Rockies in Montana in the Lewis Range/Glacier National Park. ...Northern Plains... Low pressure will develop in the MS Valley in response to a negatively tilting upper trough rotating through the Central Plains. As this surface low deepens and lifts into the upper MS Valley, mid-level frontogenesis north of the low will increase in conjunction with the deformation axis NW of the low center. Robust ascent will result, and a stripe of snow develops in eastern ND into northern MN. Uncertainty still exists into the placement and intensity of this axis, plus initial temps may be too warm for snow before cooling aloft increases confidence snow will occur, and this is reflected by low WPC probabilities for more than 4 inches of snow. Further south, temperatures are expected to be too warm for snow in much of southeastern SD and southern MN. The probability of significant (quarter inch or more) icing is less than 10 percent. Day 3... ...Ranges of northern UT and CO across WY to southern MT... As the upper trough moves east out of ID and UT, the upper jet is forecast to cross northern CO, placing WY within the area favored for greatest upper divergence. Snows are expected in advance of the upper trough early on Wed in WY and wind down once the upper trough departs onto the plains Wed night. Several additional inches are possible in the ranges of WY from the Wind River across the Snowy/Laramie mountains. Light snow persist near the cyclonic shear axis with modest low level convergence in the Tetons to ranges of southeast ID/southwest MT. ...Northern MN/adjacent eastern ND... The models show low pressure moving out of the upper MS Valley to the upper Great Lakes and across the border into Canada. The ongoing snow in northeastern ND and northern MN continues early this period and winds down as the mid level level front and convergence/deformation move north out of the region and ascent weakens. There is potential for several inches of snow in northwest MN, supported by multiple models. The probability of significant icing is less than 10 percent. Petersen