Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 342 PM EST Sun Nov 11 2018 Valid 00Z Mon Nov 12 2018 - 00Z Thu Nov 15 2018 ...Western Great Lakes... Days 1-3... Continued periods of lake effect snow are likely into Wednesday in persistent west winds. Winds shift more northwesterly Monday night into Tuesday with north-northwesterly over southern Lake Michigan producing a lake effect band that extends into northern IN. ...Southern Rockies across south-central Plains... Days 1-2... An upper level trough over the Four Corners will round the mean positively tilted longwave trough across the southern Rockies into the TX panhandle tonight. This will be reinforced by a second impulse on Monday that drives the long wave trough axis to the Big Bend in TX Tuesday. A cold front has pushed through the TX panhandle and persistent SW flow between 700-500mb atop the increasingly E/NE low-level flow is producing isentropic lift from NM eastward into the Southern Plains. This isentropic lift will moisten the column sufficiently for precipitation to develop across the area, and will be enhanced both synoptically and on the mesoscale to produce a swath of heavy snow in the southern high plains tonight. Ongoing heavy snow over the southern Rockies will persist through tonight until the low level wind shifts from east to north early Monday morning. Eastern NM through the Panhandle of TX/OK and into southern KS will see mid-level deformation and frontogenesis will drive ascent, coupled with the right entrance region to a departing but strengthening upper jet streak. Guidance remains in very good agreement that a stripe of accumulations of more than 4 inches will occur, with a small chance for 8 inches, focused near the Panhandle of Texas. Despite relative short duration of high 1000-500mb saturation, intense snowfall rates of 1"/hr are possible where upright convection occurs beneath the best coupled forcing in a saturated DGZ. The column is cold enough throughout for snow, and SLRs will likely exceed climatological norms. Another drift north with the main QPF axis over the TX panhandle occurred with the 12Z consensus including the preferred 12Z GFS/ECMWF. ...Southern Plains to Detroit... Days 1-2... A band of snow is likely to extend northeast from eastern KS/OK in response to the best diffluent region of the upper jet streak. A narrow band of snow is likely within this corridor from southern MO to southeastern MI/Detroit. The subtropical jet streak phases with the polar jet Monday, limiting the energy over IL/IN before phasing occurs (this happens to be around the Day 1/2 time border). Probabilities for one inches increase over NE OH and SE MI. While QPF is barely around 0.1 inches along the swath, these bands usually overachieve and two or more inches are possible in this swath. ...Interior Mid-Atlantic and Northeast... Days 2-3... 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 focus northern stream trough. The associated surface low closes near the Mid-Atlantic coast Tuesday morning and is north of Maine by Wednesday morning as a deep low. Significant 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. Wrap around snow is lake enhanced later Tuesday, but is generally limited given the surface low track along the SE New England coast. The right side of the northwesterly jet in the wake of the low lingers just north of the Great Lakes Tuesday night into Wednesday. The deep layer WSW flow will result in LES which is handled well in global guidance and features the SE shore of Lake Ontario. Low WPC probabilities for 6 inches are in this area for Day 3. ...Pacific Northwest... Day 3... A shortwave trough swings off a Gulf of Alaska low and pushes across BC Wednesday. 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. The probability for significant ice (0.25 inches or more) is less than 10 percent for all three days. Jackson