Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 441 AM EST Fri Nov 30 2018 Valid 12Z Fri Nov 30 2018 - 12Z Mon Dec 03 2018 ...Northern to central Plains into the Western Great Lakes... Days 1-3... ...Major snowstorm likely Saturday in parts of northern Nebraska and adjacent southern South Dakota... Heavy snow is expected to occur over much of northern Nebraska to adjacent southern South Dakota north of the slow moving cyclone on Saturday. Several models continue to show support for a foot of snow in the areas where mesoscale banding sets to produce higher amounts. The snow is expected north of the closed 700 mb low Sat that tracks slowly east northeast across NE to bear the IA/MO border by Sun morning. The passage of the low allows cold air to infiltrate northwest IA and change precip over to snow Sat. Snow is expected to be the heaviest along the axis of the low level jet in the frontogenesis/deformation zone in northern Nebraska, where the 00z NAM has 1 50 kt jet maxima Sat morning, which in turn favors strong moisture advection over the frontal zone and low level convergence to induce lift. As the surface low deepens, frontogenesis will increase and lift northward into SD/MN while a deformation axis hangs back through the panhandle of Nebraska. A prolonged period of moderate to heavy snow is expected over northeast Nebraska and WPC probabilities are high for 8 inches of snow, and moderate for 12 inches. Lighter amounts are expected near the low level front across southern MN into WI, but still remains potential for several inches. Initial mixed precip could result in light icing on Sat near the ND/MN border. A secondary axis of mid-level stretching and an elongated inverted surface trough will also extend northwestward from western SD/eastern WY into east central MT. This will continue ascent in a moist 1000-500mb environment, and light to moderate snow will accumulate on Saturday into the northwest High Plains, before moving slowly across the Central Plains on Sunday. Amounts are generally lighter across the central Plains to the upper Lakes Sunday as the low level jet and low level convergence weakens, resulting in lighter QPF and resultant snow. ...Western U.S.... Days 1-3... A series of upper troughs bring snow with heavier amounts in the OR cascades down into Ca Sierra Nevada range, with secondary maxima inland in the UT Wasatch/Uintas and ranges of western CO. On day 1 several inches of snow is expected across the OR Cascades, spreading into the elevated terrain of northwest CA. The ascent is driven by upper divergence in the vicinity of a 130 kt upper jet maximum, transporting significant moisture onshore and into the terrain, with lift aided by 700 mb convergence. Lighter amounts are expected in the WA Cascades as the strength of the jet and moisture transport is weaker further north. The highest probabilities for 8 and 121 inches are in the OR Cascades on Fri (Day One). The combination of significant moist advection, synoptic ascent through jet diffluence, as well as upslope enhancement supports several inches of snow across the Uintas and Wasatch of Utah, Tetons of western WY, as well as the San Juans of Colorado and southward into northern Arizona on day 1. On day 2, the Pacific wave moves onshore an inland across the CA Sierra Nevada and then NV and UT. The heaviest snowfall is likely across the CA Sierra Nevada and then the ranges of central NV, continuing into southern Wasatch of Utah. At the end of day 2 into day 3, the wave continues out of UT into into the ranges of southern Colorado and New Mexico, where the best combination of moisture and lift coincide. Further north, the upper level jet is weaker and available moisture decreases, so lower snow accumulations are expected. ...Upstate New York and Northern New England... Days 2/3... A warm front and associated warm and moisture advection occur across interior New York and New England Saturday. Precipitation develops with potential for light icing where the cold surface high leaves sub-freezing surface temperatures until warm air gets mixed down to the surface. The strong and persistent mid level warm advection changes the precip to all rain in New York and southern New England. A few hundredths to a tenth of an inch of accretion are possible. In the highest terrain of NH and Me, where cold temperatures will hang on longer, snow is expected at the onset before changing to freezing rain and then rain. Several inches of snow are possible from the White Mountains of NH and ranges of western Maine. WPC probabilities are moderate for 4 inches in northern Maine before the change over to mixed precip types. Petersen