Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 356 AM EST Sat Dec 01 2018 Valid 12Z Sat Dec 01 2018 - 12Z Tue Dec 04 2018 ...Northern to Central Plains into the Western Great Lakes... Days 1-2... ...Major snowstorm expected Saturday across north central Nebraska and adjacent southern South Dakota... On day 1 (Sat), Heavy snow is expected to occur over much of north central to northeast Nebraska and adjacent southern South Dakota north of the slow moving cyclone on Saturday. The latest forecast guidance still supports snow totals over a foot across portions of north central to northeast Nebraska and adjacent south central SD, generally north of the 700 mb closed low that will track slowly east/northeast across Nebraska toward the Iowa/Missouri border by Sunday morning. As the system deepens and slowly moves east, strong dynamic cooling will change rain over to moderate to heavy snow across northern Nebraska and adjacent southeastern South Dakota, with potential for 1-2"/hr rates. WPC probabilities are high for 8 inches of snow and moderate for 12 inches. Frontogenesis will increase and lift northward into SD/MN where several inches of snow is also expected across portions of southern Minnesota/adjacent northwest IA. At the onset, soundings indicate that precipitation will likely begin as a wintry mix. There is potential for light icing across portions of southwest Minnesota, extreme northwest Iowa, and far southeastern South Dakota. Across Montana and Wyoming, a secondary axis of mid-level stretching in combination with a lingering inverted surface trough will continue forcing for ascent in a moist environment. Lift within the dendritic growth zone will lead to light to moderate snow on Saturday before moving into the Central Plains on Sunday. WPC probabilities are moderate for 8 inches of snow across portions of northeast Montana. On Day 2 (Sun), light to moderate snow is expected as the low level wave tracks across MI, with several inches of snow possible in the eastern to central upper peninsula of MI, with lake enhancement aiding lift from the wave. More weighting was given to the higher resolution guidance here. ...Western U.S... On day 1 (Sat), A mid-upper level trough moving onshore across the Pacific northwest will bring snow with heavier amounts in the Oregon cascades down into California Sierra Nevada range, with secondary, lighter maxima inland in the NV ranges, ,southern ID, southern UT Wasatch and northern AZ/adjacent northwest NM. Several inches of snow are expected across the Oregon Cascades, spreading across the mountainous region of northern and central California. Moisture impinging on the terrain combined with 700 mb lift will result in heavy snow in the CA Sierra Nevada range, aided by 700 mb convergence. The combination of significant moist advection, synoptic ascent through jet diffluence, as well as upslope enhancement supports several inches of snow across the southern Wasatch of Utah, and ranges of northern AZ. The highest probabilities for 8 and 12 inches are in the CA Sierra Nevada. On day 2 (Sun), the mid-upper Pacific wave moves across UT and AZ to NM, with the ECMWF even briefly forming a 700 mb low in southwest CO. The heaviest snowfall is likely across portions of the southern Wasatch of Utah and into the ranges of southern Colorado and northern New Mexico, where the best combination of moisture and lift coincide Sunday during the day into the evening. Ascent subsides with the wave passage Sunday night to Mon morning, so snow coverage/intensity should wane then. On day 3 (Mon), the models show a closed low over the eastern Pacific slowly drifting towards the coast. As the system approaches, low level onshore flow advects moisture into the ranges of northwest CA and adjacent southwest OR. After a dry start, moistening of the column and lift leads to snow developing Monday night, with light accumulations possible by Tue morning. ...Upstate New York and Northern New England... Days 1 and 2... Broad isentropic lift and warm advection associated with a warm front lifting through will lead to precipitation breaking out across interior New York and New England. With a retreating surface high pressure, shallow cold air in place will lead to the potential for light icing until warmer air arrives and precipitation changes over to all rain. Probabilities of significant icing (> 0.25") are greatest (25-30 percent) across terrain of interior New York into portions of VT, northwest MA, NH and western Maine. In the highest terrain of western Maine, where cold temperatures will remain in place the longest, several inches of snow are likely. Both the 00z NAM and 21z SREF Mean indicate 4-6 inches of snow before either a change over or end of the event as a dry slot crosses the region. WPC probabilities are moderate to high for 4 inches in northern Maine. Day 3... The approach of the low-mid level trough and arrival of colder air provides a window of opportunity of light snow in western New York, with enhancements near the low level boundary, in addition to enhancements from Lakes Erie and Ontario. The NAM and SREF Mean show a few inches are likely, a little above the global models. The probability of a quarter inch of icing is less than 10 percent day 3 (Mon). Petersen