Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 406 PM EST Tue Jan 08 2019 Valid 00Z Wed Jan 09 2019 - 00Z Sat Jan 12 2019 ...Great Lakes, Northern New York, through northern New England... Days 1-3... As a 700 mb wave crosses northern NY and VT tonight, low pressure develops and moves into Maine. The 12z guidance show a weak initial wave tonight with the stronger low developing in eastern NY tonight and moving across MA and up the ME coast tomorrow. Snow is forecast to develop in the mid level deformation/frontogenesis zone in central NY, crossing the the Catskills of southeastern NY and Berkshires of western MA and then northeast in tandem with the mid level circulation up into interior Maine. The heaviest snowfall is likely in Maine due to longer duration of cold air on Wednesday, combined with enhanced precipitation. In northern Maine, WPC probabilities show a moderate risk for 8 inches through Wednesday. Light icing is possible in the precip type transition zone in Maine, interior NH, and CT River Valley of VT east of the Green Mountains. Uncertainty of snow amounts in eastern NY to western MA relates to precip type starting as rain and changing over to snow as the low level circulation passes late tonight/early Wed morning. As the cyclone shifts eastward, cold air advection develops and moist west-northwest boundary layer flow supports snow developing in the lee of Lake Ontario and windward terrain of the western Adirondacks and Green Mountains. Once the great lake moistened airmass turns upslope in WV, another area with several inches of snow is depicted in the central Appalachians centered over West Virginia. The high res windows show about half an inch liquid equivalent in the favored terrain, which should translate to about 6 to 8 inches of snow in the mountains of WV. Post frontal lake effect snow is likely in a cold advection pattern with low level convergence in the lee of lakes Superior and northern Lake Michigan, followed by Lakes Erie and Ontario. The persistence of high relative humidity and leeshore convergence along with increasing instability supports several inches of snow downwind of the Lakes. The highest amounts are likely downwind of Lake Erie and Ontario both due to long duration trajectories crossing Lake Huron/Georgian Bay that moisten the airmass, as well as upslope enhancement just east of these Lakes. On Day 3, Friday, light snow develops with a frontal wave moving east from the upper MS Valley towards the upper Great Lakes. Modest available moisture and ascent leads to light amounts. ...Northern California/Pacific Northwest/Northern Rockies... Days 1-2... A slow moving deep low pressure will drift northeast across the Pacific. East of the low deep layer south to southwest flow advects moisture and warm air into California and the Pacific Northwest, with moisture continuing over into the Rockies. Heavy snow is confined to the higher elevations of the Sierra Nevada and Siskiyous/Shastas of northern CA, Oregon and Washington Cascades. There is potential for 1-2 feet of snow at the highest elevations. Lesser amounts are likely into the ranges from the Northern Rockies as the strength . The models soundings indicate a signal for snow in portions of central to northeast WA and into eastern OR for snow to transition to a period of freezing rain in areas in and east of the Cascades. Cold surface temperatures reinforced by precipitation result in isentropic lift. The southerly component of flow results in warming and a changeover. Some uncertainty still exists due to the duration of each precipitation type. As the warm front lifts north Wed night, the potential threat area shrinks with time. The building deep layer ridge downstream of the next system minimizes winter precipitation potential Friday. Day 3 Co/NM... The models forecast a 700 mb wave moving east from the Great Basin across the CO/NM Rockies and then the high Plains. Return low level flow advects moisture into the region, but model QPF amounts are generally 1 to 2 two tenths of an inch, so snow amounts should be light. Downstream over northern AR/southern MO, precip develops Fri, with in situ cold air providing a threat for light snow, sleet and freezing rain to develop. This area is forecast to expand and increase in amounts shortly after this period into the weekend. The probability of a quarter inch of icing is less than 10 percent Day 2 and Day 3. Petersen