Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 343 AM EST Mon Nov 12 2018 Valid 12Z Mon Nov 12 2018 - 12Z Thu Nov 15 2018 ...Great Lakes... Days 1-3... Continued periods of lake effect snow are likely, especially on d2/d3 as W/NW winds develop behind a strong low pressure system moving up the Atlantic Coast. As the surface low pulls away Tuesday, a shortwave will move eastward across the Great Lakes in its wake. This will cause column winds to become W/NW through a deep portion of the column, producing favorable unidirectional shear and cold advection across the lakes. Periods of heavy lake effect snow are likely, especially southeast of Lake Superior, Erie, and Ontario. Downwind of Lake Ontario may experience the heaviest snow as instability becomes maximized and a connected fetch occurs from Lake Huron. WPC probabilities are high for more than 4 inches of snow east of Lake Ontario, with lighter amounts likely near the other lakes. ...Southern Plains into the Ohio Valley... Day 1... A swath of snow is likely to extend from near the Red River Valley of TX/OK, northeast towards the southeast Great Lakes. This area of snow is associated with lift beneath the diffluent right entrance region of a polar jet streak, as well as mid-level frontogenesis. Snow accumulations will generally be light, but WPC probabilities do exceed 30 percent for 4 inches from far northeast OK into southern MO. Locations northeast from here should see lesser snow amounts as the subtropical jet phases with the polar jet deflecting the best synoptic ascent to the southeast. Available moisture is also less into the Ohio Valley, so total accumulations there should only be 1-2 inches. ...Interior Mid-Atlantic and Northeast... Days 1-2... A positively tilted longwave trough will gradually shift eastward into the Ohio Valley Tuesday. However, a reinforcing trough over the Canadian Prairies cuts off the mid-level closed low over the southern Plains while a Nor'Easter develops with the newly focused northern stream trough. The associated surface low closes near the Mid-Atlantic coast early Tuesday morning and is north of Maine by Wednesday morning as a deep low. Significant warm and moist advection will occur ahead of this trough, with a strengthening baroclinic gradient producing the potential for snow well north of the surface low track beginning Monday night in the Ohio Valley. Rapid cyclogenesis northeast from the Mid-Atlantic Tuesday would broaden the precipitation shield to the eastern Great Lakes with areas roughly along the northern Appalachians and west under threat for snow. Significant QPF should fall as snow on the west side of the Appalachians where thermal profiles are cold enough for snow. A sharp rain/snow line is expected with mainly southerly flow east of the Appalachians through much of the QPF ahead of the low. There has been a subtle shift eastward in the model consensus this morning prompting a slight shift eastward of the heaviest snow amounts. However, probabilities for 4 inches of snow are still only high for the terrain of the Adirondacks, as well as the mountains of Vermont, New Hampshire, and especially Maine where a slight risk for 8 inches of snow exists. ...Pacific Northwest... Day 3... A shortwave trough swings off a Gulf of Alaska low and pushes across British Columbia Wednesday before dropping towards Montana late on day 3. The near zonal flow over northern WA will bring Pacific moisture and a pronounced baroclinic zone over the northern Cascades and Rockies of northern ID/MT by Wednesday afternoon. Snow levels around 7500ft can be expected on the north side of this zone with heavy snow for the north Cascades, pushing into the northern Rockies near Glacier National Park Wednesday night. ...Southern Appalachians... Day 3... Anomalously deep cutoff upper low will drift across the Mississippi Valley on Wednesday before ejecting slowly to the northeast while filling. A surface wave will develop along the Southeast coast with deep moist advection lifting into the Southern Appalachians Wednesday night. Cold high pressure to the north will only slowly retreat, and as isentropic lift intensifies, precipitation will overspread the region. Despite a warm nose rapidly lifting to the north, surface temperatures within the wedge of high pressure will remain below freezing, at least during precipitation onset, and a period of freezing rain is becoming more likely Wednesday night. There is good model agreement in freezing rain, but placement and intensity vary widely. There is enough consensus that WPC probabilities have been raised, and low probabilities now exist for 0.25 inches of accretion across the terrain of NC, VA, and WV. Weiss