Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 356 PM EST Tue Jan 14 2020 Valid 00Z Wed Jan 15 2020 - 00Z Sat Jan 18 2020 ...The West... Day 1... Prolonged unsettled winter pattern will persist through the week as Pacific moisture plume wavers latitudinally into the West Coast. Within this plume and associated Pacific Jet, embedded shortwaves will move into the region Wednesday and Thursday, with enhanced moisture ahead of each of these impulses. These features will spawn waves of surface low pressure as well, and combined with the associated fronts, will spread forcing and moisture into the Continental Divide. Tonight, the snowfall will be focused in Washington, where a theta-e ribbon will advect onshore associated with low-level fgen band north of an arctic front. This band will lift slowly northward, and heavy snow rates >1"/hr are likely. There remains some uncertainty into where this band will develop before lifting north, but the consensus is for it to angle across the Olympic Peninsula to just north of Seattle, lifting north into Canada Wednesday afternoon. With snow levels down into the valleys, even the lowlands should receive several inches of snow. While the heaviest amounts will be in the Olympics, where 4 feet is possible, the Cascades also have the potential for 2 feet on D1. In the lowlands, WPC probabilities for 4 inches are moderate, highest north of Seattle. By Wednesday night into Thursday, the associated mid-level energy with the D1 snow will expand southeast and onshore Oregon/California. At the same time, a warm front will lift northeast across N California and through OR/WA/ID, raising snow levels to 1000-2000 ft in WA/ID/MT, to as high as 4000-5000 ft in central CA and points east, along and south of 40N latitude. Confluent moisture flow and jet level diffluence will spread snowfall into all the terrain from the WA Olympics, east into the Northern Rockies, south to the San Juans, and back into the Sierra Nevada. WPC probabilities east of the Great Basin are high for 4 inches, while in the Cascades/Olympics/Sierra they are high for 8 inches. The upper trough driving much of the snow on D2 will shift inland into the Central Rockies D3, with mid-level ridging displacing the moist jet stream back to the north and into Canada. This will provide a modest break in heavy snow across the West, and WPC probabilities for 8 inches are moderate only in the higher terrain of the southern Sierra, as well as the Wasatch, San Juans, and Colorado Rockies into Friday. ...Upper Midwest and Great Lakes... Days 1-2... A shortwave trough will shift quickly eastward across the Northern Plains towards the Great Lakes Wednesday. Moisture confluence and merging subtropical/northern jet streams will produce strong surface low development in the upper MS Valley. This low will then race east-northeast across New England through Thursday. Deep layer ascent through jet level dynamics and height falls in conjunction with the strengthening surface low will spread increasing moisture into the region. Deep high pressure ridging north of the track over northern Ontario/Quebec will provide enhanced convergence on the north side of the low. On the south side, warm air advection ahead of the low will push a warm nose as far north as Michigan. This will cause a p-type transition from snow, to freezing rain, and eventually to rain. Due to the fast progression of this system, WPC probabilities are only moderately high on Day 2 for 4 or more inches of snow from northern MN/WI and across the U.P. MI. Further south, there is likely to be a stripe of light freezing rain accretion in IA, IL, and southern WI. However, WPC probabilities for 0.1" remain less than 10 percent. Lake effect snow is expected behind the low Wednesday night and Thursday, with the favored NW snow belts receiving the heaviest snow downwind of Lake Superior, and the effective fetch areas off Lake Michigan and Lake Erie. WPC probabilities are moderate for 4 inches of LES. ...New England... Days 2-3... A surface low moving across central New England will race into the Gulf of Maine while beginning to deepen in response to a negatively tilting mid-level trough and merging jet streaks aloft. Intense ascent in a region of PWATs reaching +1 standard deviation above the mean will produce a period of heavy precipitation before the low exits quickly by late Thursday. Much of New England will be too warm for snow, but from the Adirondacks, through northern New England, and most of Maine, a swath of heavy snow is likely. While the system will be moving quickly, there is potential as the low deepens in the Gulf of Maine that a TROWAL will wrap cyclonically around the low, aiding ascent provided by a lingering inverted trough. This could enhance snowfall across eastern Maine late D2 into D3. WPC probabilities have increased for heavy snow, and are moderate for 8 inches in the terrain of the Northern Adirondacks, Greens, Whites, and into eastern Maine. ...Southern and Central Plains... Day 3... Significant storm system will develop in the lee of the Rockies Friday in response to intensifying diffluence within a jet streak and height falls in advance of a shortwave trough. Ahead of this system, cold high pressure will dominate the central part of the CONUS, providing cold air across much of the area. As the low begins to develop, intense WAA to the east on increasing low-level southerly flow will spread precipitation northward from the Red River Valley of OK/TX into Minnesota. While the heaviest snowfall is likely just beyond this forecast period, WPC probabilities are moderate to high for 4 inches in IA/MN on Friday. S/SW of this area, mixed precipitation is likely across MO/KS/OK as the WAA produces a warm nose aloft. Eventually this regions will turn to all rain as the warm air overwhelms the low level cold air, but before that occurs a mix of sleet and freezing rain is likely. WPC probabilities for 0.1" of accretion are moderate across MO, with low end probabilities existing in KS and OK. Weiss