Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 355 PM EST Sat Feb 23 2019 Valid 00Z Sun Feb 24 2019 - 00Z Wed Feb 27 2019 ...Central Plains to the Great Lakes... Days 1-2... Powerful low pressure will continue to intensify in response to robust synoptic ascent within a coupled jet structure aloft and rapid height falls as the parent upper low deepens quickly into the Great Lakes. As this low deepens, significant moist advection noted by high PWAT anomalies advecting northward into the Great Lakes will produce a very moist column supportive of heavy precipitation. NW of the surface low, a stripe of heavy snowfall is expected, and although guidance differs slightly in the exact placement of the heaviest snow, a multi-model signal exists for more than 12 inches in a narrow SW to NE oriented band. Guidance has shown an increase in both QPF and snowfall today, with increasing frontogenesis correlating with a sharp 700mb deformation axis in conjunction with theta-e advection through the TROWAL as the WCB rotates cyclonically around the low. The overlap of robust synoptic ascent and intense mesoscale forcing will produce very heavy snow rates, with most high-res guidance showing 6-12" of snow in just a 6 hr period. This seems reasonable as theta-e lapse rates become sharply negative within the saturated DGZ, and thunder snow is likely supporting convective snow rates this intense. Have increased snowfall totals despite the relatively short duration event (6-8 hours), and a long but narrow swath of moderate probabilities for 12 inches exists from north central IA through WI and into the U.P. of Michigan. The western U.P. may be the jackpot where isolated amounts of 15 inches or more is possible as a slightly longer duration of TROWAL effects and mid-level deformation. WPC probabilities trend down quickly both east and west of this band, but any shift in the track will affect these snow amounts. East of the low-mid level circulation, strong warm air advection rides up and over retreating low level cold air from central to northeast Iowa, northern Illinois, southern to northeast WI, and finally into northern lower MI and the eastern UP of MI. In the strong mid-level warming, light snow transitions to freezing rain. There was a signal for measurable to 0.10+ inch ice amounts in the aforementioned locations, with low probabilities for 0.25+ from central WI the eastern UP/far northern lower MI. After the low passes northeast from northern Lake Michigan into Ontario, the synoptic snow decreases and northwest flow across western Lake Superior results in strong lee shore convergence underneath a band of mid level deformation. With high relative humidity and lift, several inches of snow should occur until starting to taper Sunday nigh once drier air aloft moves in from the northwest and the lee shore convergence weakens. A secondary max is expected over northwest lower MI in the lee of Lake MI. ...Pacific Northwest into the Northern Rockies... Days 1-3... Upper trough elongating off the Pacific coast will drop slowly southward while becoming repeatedly reinforced by shortwave and vorticity impulses. Beneath these features, surface low pressure will move onshore as well, one on Sunday morning and another Monday afternoon. Beneath the upper trough and driving these lows eastward, a powerful Pacific Jet will stream west to east into the Pacific Coast, producing robust synoptic ascent while concurrently helping to drive moisture spillover well into the northern Rockies. The slow southern drift of the upper trough will allow the Pacific Jet and the strong moist advection to focus into nearly the same area throughout the 3 day forecast period. The nearly zonal flow across the West will keep snow levels high, above 4000 ft south of 40 N, but generally falling snow levels north of this latitude will drop to sea level across portions of Oregon through Wyoming and points north. The prolonged and abundant moisture combined with enhanced forcing due to the surface lows and upslope enhancement on W/SW 700mb flow creates the potential for tremendous snow amounts of 3-5 feet or more from the Oregon Cascades and California Siskiyous, eastward across the Sawtooth Range and portions of NW Wyoming. The heaviest snow will be above 4000 feet. Elsewhere, WPC probabilities are high for 8 inches above 4000 feet across Oregon, Idaho, and eastern Montana, with heavy snow eventually sinking southward into the northern Sierra as well. With the snow levels falling to sea level, light to moderate snow accumulations are possible in the Portland metro area as well and WPC probabilities are moderate for 4 inches. However, a southward trend has been noted in recent guidance with the surface low track into Monday, and if this continues amounts may be lighter than current forecasts suggest. ...Mid-Atlantic into New England... Days 1-2... Warm moist advection on increasing southerly winds will spread precipitation across the Mid-Atlantic and Northeast ahead of a deep low pressure moving through the Great Lakes. During this time, the atmospheric column will be cold enough for snow only across northern New England, with the warming aloft atop a cold air wedge of high pressure producing mixed precip from the Mid-Atlantic into southern New England. The heaviest precipitation is likely during day 1, but a spoke of energy rotating the closed low driving the surface high will cause renewed precipitation on day 2 along/behind the cold front, with more snow possible across Northern New England Monday. Two-day snowfall across northern New England, especially in Maine, may be significant, and WPC probabilities are high for 8 inches, with lower probabilities for 8 inches into the terrain of NH. Elsewhere, snow amounts will be light. South of the snow/mix line, freezing rain accretion is likely in the Catskills, Adirondacks, Berkshires, and Greens, and WPC probabilities are over 50% for 0.1 inches, with low probabilities also existing for 0.25 inches. Weiss