Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 409 AM EDT Tue Oct 09 2018 Valid 12Z Tue Oct 09 2018 - 12Z Fri Oct 12 2018 Day 1... ...Ranges of western MT/southern ID/western WY/northern UT... Well defined upper level divergence ahead of a well-defined shortwave/possible embedded upper low moving southeast from WA into ID to near the UT/ID border is expected to support a period of ascent into the ranges of ID. A secondary area develops closer to the upper jet maxima crossing NV into UT with higher snow amounts where upslope enhancements combine with the higher magnitude upper divergence in the UT northern Wasatch range to produce a period of moderate to heavy snow. ...CO/NM Rockies... The southern stream trough presently centered over the Southwest is expected to lift out to the northeast into the southern High Plains today. The 300 mb jet maxima in Eastern CO places southeast CO and northern NM briefly in the favored right entrance region of the jet before it departs. Low level convergence and upslope flow combined with a brief period of low-mid level frontogenesis support a period of snow until the combination departs further east onto the plains later today. Consequently, forecast amounts are higher for the Sangre de Cristo mountains than other ranges further west. ...Central to Northern Plains... The models have shifted in the forecast evolution of the ejecting deep layer trough as the 700 mb wave is now forecast to become a closed low faster than yesterday's forecasts indicted. This results in both better defined ascent along a higher amplitude mid level deformation plus slightly cooler temperatures. The combination results in potential for several inches of snow in northeast CO to western and central NE. Falling heights/temps also lead to a change over to snow in western SD and into adjacent North Dakota. Uncertainties remain in QPF and also how long temperatures are cold enough for snow, with a wide range of possible snow accumulations. Consequently, the risk of four or more inch accumulations has increased to now have a moderate risk region in the central high plains. The probability of significant (quarter inch or more) icing is less than 10 percent. Day 2... ...Ranges of southern ID/WY/CO... The models indicate the upper jet crosses CO early on Wed before heading off on to the Plains with rising heights and weakening low level lift across the region later Wed. The defined upper divergence maxima crosses the ranges of northern Co to adjacent WY early Wed significant snowfall accumulations is forecast to become more widespread across the area as the aforementioned shortwave digging into the Northwest continues to amplify, carving out a new trough across the region Tuesday night into Wednesday. WPC probabilities indicate a widespread Moderate Risk for amounts of 4-inches or greater with more localized risk areas for heavier amounts across the higher terrain from central Idaho and western Montana to northern Utah and Colorado. ...Northern Plains and Upper Midwest... The southern stream shortwave will continue to lift to the north, with an associated surface low pressure moving from the Mississippi Valley across the upper Great Lakes on Wednesday. A Period of snow is still expected within the associated comma-head with a mid level deformation maxima in eastern ND to northwest MN. WPC probabilities are indicating a Moderate Risk for accumulations of 4 inches or greater and a Slight Risk for 8-inches or more extending from eastern North Dakota to northwest Minnesota. The event winds down as the low pressure departs into Canada. The uncertainty persists regarding the exact axis/orientation of snow, with the NAM/Canadian/SREF Mean a bit west of the GFS/GEFS Mean and operational ECMWF solutions. The probability of significant (quarter inch or more) icing is less than 10 percent. Day 3... ...Ranges of Co/southern WY... A longwave trough becoming positively tilted, with the northern end moving east steadily and the southern end moving more slowly east across UT towards Co allows periods of upper divergence to persist longer in Co than WY and thus higher snowfall totals are expected in most solutions in CO. Several inches of snow are possible in the ranges of western CO on Thursday, with lesser amounts in UT and WY. The probability of significant (quarter inch or more) icing is less than 10 percent. Petersen