Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 338 PM EST Sat Nov 10 2018 Valid 00Z Sun Nov 11 2018 - 00Z Wed Nov 14 2018 ...Great Lakes... Days 1-3... Ridging spreads east across the Great Lakes tonight with lake effect snow from west winds tapering off for all, but Erie and Ontario which maintain west winds through Sunday. A clipper crosses the northern Lakes late tonight through Sunday with warm air advection snow over northern WI/MI and the UP persisting through Sunday. Lake effect snow becomes more patchy Sunday night and Monday as light northwest flow returns over the region. Stronger north flow begins Monday night as low pressure moves up the east coast with an upper trough axis over the western Great Lakes through Tuesday. Synoptic forced snow occurs over the eastern Great Lakes Monday night into Tuesday with the east coast low discussed below. ...Northern Rockies... Days 1-2... A positively tilted trough over the northern Rockies this afternoon pushes south to the Four Corners through Sunday. North to northwest flow and dry continental air will limit snow in the Northern Rockies to a few inches, focused on preferred upslope areas in MT and WY. ...Southern Rockies across south-central Plains... Days 1-2... An upper level trough digs into the Four Corners region early on Sunday, followed by a second impulse Monday which will reinforce the positively tilted trough across the Southern Plains. As this occurs, a low-level cold front will drop southward into NM/TX. Persistent SW 700-500mb flow within the longwave trough will begin to overrun shallow 850mb E/NE flow producing enhanced isentropic lift from NM eastward into TX/OK/KS. 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 two areas of heavy snow. The first will be in the terrain of Colorado into northern New Mexico, along the Front Range and the Sangre De Cristo Range. Here, jet level diffluence and vorticity advection will combine with upslope enhancement along the E/NE ridges to produce heavy snow. WPC probabilities are moderately high for 8 inches across the Front Range, Sangre De Cristos, with 12 inches possible. The remaining Rocky Mountain terrain, including the San Juans, have a moderate chance for 4 inches of accumulation. The other area of heavy snow will be along an axis from eastern NM through the Panhandle of TX into west OK. Here, 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. Note that the NAM/SREF continue to be displaced NW of the majority of the guidance. However, good agreement between the GFS/ECM and its ensembles support the current forecast. A band of snow is likely to extend northeast across KS and MO and possibly into the Midwest under the southwesterly jet Sunday afternoon into Sunday night. Trended this way with more QPF/snow farther northeast and will need to monitor this for generally light snow accumulations of 1-2" in this time. ...Interior Mid-Atlantic and Northeast... Day 3... Positively tilted longwave trough will gradually eject eastward into the Ohio Valley Monday into Tuesday, spawning a slow-developing surface low pressure along the Gulf Coast Monday that ejects northeast, reaching New England Tuesday. Significant moist advection will occur ahead of this trough, with strengthening baroclinic gradient producing the potential for snow well northeast of the surface low track beginning Monday night in the eastern Midwest. 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. Lower elevations like in western PA and along the OH River would not have temperatures cold enough for snow until the low passes per the 12Z GFS/ECMWF which are in good agreement. Wrap around snow and lake effect snow would take over at that time. Overall the quick moving system looks to bring most snow to higher elevations of the Northeast with WPC probabilities of 4" highest for Day 3 north from northwest PA. The probability for significant ice (0.25 inches or more) is less than 10 percent for all three days. Jackson