Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 409 AM EST Mon Dec 14 2020 Valid 12Z Mon Dec 14 2020 - 12Z Thu Dec 17 2020 ...Major Winter Storm to Impact the Mid-Atlantic and southern New England states Wednesday and Thursday... Day 1... ...Great Basin to Central Rockies... A digging trough over the CA/NV border will shift to southern AZ/NM tonight with mainly moderate snows in terrain. Day 1 snow probs are moderate for 4 or more inches for central UT ranges and the CO Rockies. ...Central Appalachians into the Northern Mid Atlantic... A well-defined upper trough currently over the TN valley quickly lifts northeast over the southern Mid-Atlantic today with surface low pressure deepening and tracking through the central Carolinas. Strong upper jet forcing, along with low-to-mid level frontogenesis will support widespread moderate to heavy precipitation west of the low, with the strong dynamics and low level northerly flow supporting rain changing to snow in the heaviest bands on the far northwest side of the precipitation shield across the portions of the central Appalachians, with Day 1 snow probs low to moderate for 4 or more inches across the higher terrain of eastern WV. While widespread heavy amounts are not expected elsewhere, there is good potential for light amounts even for valleys in banding with low Day 1 snow probs for 2 or more inches from the eastern WV Panhandle to south-central PA and terrain in the northern Mid-Atlantic (Poconos and Catskills) and Berkshires/Worcester Hills of MA. Day 2... ...Pacific Northwest to Northern Rockies... A potent upper low reaches the northern British Columbia coast on Tuesday with the associated cold front pushing across WA/OR. Ample Pacific moisture accompanies this front and favorable forcing aloft will support the return of organized precipitation across WA/OR and to ID/MT with heavy snow accumulations possible across the higher elevations of the Olympics, WA Cascades, and northern ID and northwest MT Rockies where there are moderate to high probabilities for 6 or more inches of snow. ...Central to Southern Plains... The trough over NM tonight shifts east across the southern Plains Tuesday/Tuesday night. Snows develop along an north-south, elongated trough/frontogenetical axis extending from western Kansas into central Nebraska Tuesday morning then by late Tuesday precipitation reorients farther south - closer to the center of a mid-to-upper level low moving along the Kansas-Oklahoma border. Overall, expect accumulations to be light to moderate with snow probs on Day 1.5 20 to 30 percents for 4 or more inches over western KS and south-central Neb and Day 2 probabilities low over northwest OK and south-central KS (which has been rather active over the past couple weeks with snow. Day 3... ...Appalachians/Mid-Atlantic/southern New England... QPF preference with this system is the 00Z ECMWF with some 00Z GFS (which is much more suppressed from the north than consensus) along with some 18Z GEFS and 12Z ECENS. The wave that crossed the southern Plains Tuesday/Mid-South Tuesday night is reinforced/takes on a negative tilt over the TN Valley and promotes Carolina coastal low development Wednesday. This quickly brings about a major winter storm Wednesday as a 1036mb surface high over Quebec provides ample low level cold air for the onset. Significant ice accumulations and heavy snow can be expected. Precipitation from the Southeast sourced disturbances will quickly expand and intensify from the southern Appalachians and Carolinas early Wednesday to across the Mid-Atlantic the rest of Wednesday. A classic cold air damming scenario from the high to the north and overrunning low/mid-level flow with a cold surface ridge extending from eastern Canada through the Mid-Atlantic east of the mountains. The developing coastal low will help to maintain the low level cold air as it tracks from the Carolinas to southeastern VA Wednesday then off the northern Mid-Atlantic coast Wednesday night. Guidance is in decent agreement on front end snow in the cold air with warm air advection ahead of/right of the track leading to a rather climatological track and thermal setup with the rain/mix/snow line near the Fall Line/I-95 corridor from VA to NYC Wednesday. There continues to be overall agreement on the potential for freezing rain particularly over the upper VA/NC Piedmont and Blue Ridge areas then northeast up the Baltimore/Washington metro area with significant ice accumulations most likely over southwest and central VA (where there are 30 to 40 percent probabilities for a quarter inch of ice or more for Day 3). Heavy snow is expected to develop Wednesday morning over the Allegheny Mountains/central Appalachians in WV/VA and the spread across the northern Mid-Atlantic and through southern New England by Wednesday night. Day 3 snow probs are moderate to high for 6 or more inches from southeast WV to southern New England with moderately high probabilities for a foot or more focused from the eastern WV Panhandle across east-central PA to northern NJ and southern NY. This threat then extends through southern New England into Thursday, beyond the current forecast period. A tight gradient in wintry accumulations is expected from central VA up the I-95 corridor in northern VA/MD/DE/NJ to NYC and details on that are still to come. ...West Coast to northern Rockies... The next feature in the long line of activity pushing into the Northwest CONUS is a low with reinforcing shortwave arriving into the WA and northern CA coasts Wednesday night. Elevated moisture content and higher snow levels come with the heavy precipitation. Day 3 snow probabilities are moderate for 8 or more inches along the Cascades and Northern Sierra Nevada as well as the Olympics and Klamath Mtns. Jackson