Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 345 PM EST Sat Dec 22 2018 Valid 00Z Sun Dec 23 2018 - 00Z Wed Dec 26 2018 ...Pacific Northwest/Northern Rockies/Northern and Central Great Basin/California.... Extended periods of snow are expected to occur in the WA/OR Cascades and WA Olympics, with potential for a foot of snow over the next couple of days. On Sat night-Sun, a period of snow is expected across the WA Olympics and along the Washington and Oregon Cascades as 700 mb theta-e advection and convergence produce ascent that is aided by orographic lift and upper divergence in the left jet exit region. The upper divergence fades Sunday in the Wa Cascades as the upper trough moves further inland, which it turn brings the snow into the Blue Mountains. WPC Day 1 probabilities indicating that the greatest threat for heavy amounts will generally be for areas above 4000 ft. On Sunday night into Monday, several inches of snow are expected in the OR Cascades and into the ranges of northwest CA (like the Siskiyous) and western WA as the upper level jet maxima stream onshore from the eastern Pacific across OR, with a well defined upper divergence maxima crossing into OR Monday morning. The jet extends inland during the day Monday, so snow will develop across interior portions of OR and adjacent southwest ID. Heavy accumulations are possible across the OR Cascades during the Day 2 period, with a high risk of 8 inches noted in the probabilities. On Monday night-Tue, a vigorous shortwave amplifies as it moves onshore into CA and then into NV Tue. Low level convergence near the developing 700 mb front combines with mid level deformation and upper divergence to produce snow across the CA Sierra Nevada Mountains, with snow then developing into the mountains of NV as the mid level trough moves east from CA into NV. The heavier snows are expected with longer duration ascent and closer proximity to the Pacific moisture source in the CA Sierra Nevada Mountains, where up a to a foot of snow is possible this period. ...Upper Midwest to the Northeast... A shortwave trough crosses the upper Midwest and into the upper Great Lakes on Sunday, producing light amounts across northern Minnesota into northern Michigan along the path of 700 mb convergence in the UP of MI. The wave continues across the Great Lakes Sunday night into Monday, with light snow developing in advance of the wave as it crosses PA into western NY. A brief period of lake enhanced snow occurs in the lee of lakes Erie and Ontario, with upslope snow showers in western PA and WV. A progressive, modest wave of low pressure brings light accumulations across much of central and interior eastern NY to southern and central New England. On day 3, the models hint at a wave developing in the Mid MS Valley, with light snow breaking out in the zone of low level frontogenesis north of the pressure, with light accumulations possible in eastern NE to Iowa and southern WI/northern IL. WPC probabilities showing little threat for widespread amounts exceeding 4 inches on day 3. The probability of significant icing (0.25 inch or more) is less than 10 percent. Petersen