Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 321 PM EST Tue Jan 29 2019 Valid 00Z Wed Jan 30 2019 - 00Z Sat Feb 02 2019 ...Great Lakes Days 1-3... A surface low will continue to track north into Ontario bringing an anonymously cold arctic airmass south into the upper mid-west, Ohio valley into the Northeast. The implications being strong winds allowing sufficient fetch over the Great Lakes that are conducive for vigorous lake effect snows. These conditions could lead to white-out/blizzard conditions in some locations. Those most impacted from lake effect will be regions aligned with the best fetch trajectory downstream of the Lakes. For lake Superior and Lake Michigan, we anticipate lake effect banding adjacent to the northern and western shore, respectively for Day 1. As the winds veer southwesterly with the low lifting northeast, expect locations adjacent to the northeast portions of Lake Erie and Ontario to receive the best lake effect snows through Day 3. It appears the DGZ will be fairly shallow for locations adjacent to Lake Superior and Lake Michigan which will result in smaller dendrites and thus lesser accumulations. Highest probabilities will be east off of Lake Ontario due to the sustained fetch along the main axis of the lake with seemingly better DGZ saturation. This combined with upslope enhancement in the Tug Hill will lead to heavy snow amounts. ...Central Appalachians to the Mid Atlantic... A deep trough with an associated surface cold front will continue to cross the Mid-Atlantic this afternoon/evening. Enhanced upslope should slowly taper off across the windward side of the mountains into the overnight given the progressive nature of the surface front/mid-level impulse and saturation within the DGZ diminishing. Therefore expect 1-3 inch snow accumulations from this system across the central Appalachians with locally higher amounts with better upslope. As the front moves east, liquid precipitation along the front will continue to transition to snow this afternoon/evening as the arctic airmass arrives. It appears a surface wave developing along the front will have little impact across the southern portion of the Mid-Atlantic. However, it may help enhance forcing and cold air advection across portions of PA/DE/NJ where higher snowfall amounts may occur. The surface wave/developing low starts to deepen offshore as it moves north and east with better upper level support. Given this, we expect lighter accumulations across the Delmarva and higher snowfall accumulations farther north along the I-95 corridor through this evening into the overnight. ...Northeast Day 1... The aforementioned surface wave that will develop into a low along the cold front which is expected to enhance forcing and CAA resulting in higher snowfall amounts as the low shifts north and east into Maine by Wednesday afternoon. Portions of the Catskills, Adirondacks, and Berkshires, southern Green and White Mountains may see significant snow accumulations by Tuesday evening, with WPC probabilities indicating a High Risk for amounts of 4 inches or more on Day 1 and moderate or higher risks for 8 inches in most ranges. The models continue to lower snow accumulations further north towards the NY/Canadian border, where downslope flow off the Adirondacks should lower amounts. As the surface low continues to lift north through northern New England Wednesday morning, the upper level trough will become negatively tilted with mid-level vorticity supporting significant snow amounts across interior Maine, where several additional inches are possible. Given the inland track of the low, this might aid in a transition of snow to rain across coastal Maine reducing overall snowfall amounts. The exact track of the low will have large implications on snowfall amounts not only along closer to the coast, but farther inland. Therefore confidence dwindles across this region but did trim back amounts a bit. The wintry precipitation should diminish as low pressure departs north out of the state Wed evening. ...California and the Southwest Days 2-3... Models are in fairly good agreement with a compact upper low dropping southeast from the eastern Pacific into Southern California is expected to produce some locally heavy, high elevation snows along the Sierra on Thursday.The slow arrival leads to primarily precip in the coastal ranges Wed night where it is too warm for snow. On Thu as the 700 mb wave and low progress towards the southern CA coast, so the enhanced moisture and lift is focused on the higher terrain of the ranges of southern CA and southern Sierra Nevada mountains, where several inches of snow are possible. The threat ends with the 700 mb wave moving out of CA into the southwest, with light snow possible in the higher elevations of southern NV, UT, and AZ as the wave approaches Fri. ...Mid MS Valley to the Ohio Valley Day 3... Several models show a mid level impulse rounding the upper level trough moving from the mid-MS Valley across the Ohio Valley then off the Mid-Atlantic coast for Day 3. This impulse will produce confluence flow carrying ample 700mb relative humility combined with sufficient vertical velocity from modest frontogenesis. These ingredients will likely support light to moderate snow accumulations across much of the region given the progressive nature of the impulse. Models have increased precipitation amounts, most notably across upslope regions of WV. Therefore, have increased amounts with a larger swath of 1-3 inches of snow possible along the low track and locally higher amounts associated with enhanced upslope in the central Appalachians. The probability for significant icing is less than 10 percent nationwide days 1-3. Pagano