Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 419 AM EST Sat Feb 09 2019 Valid 12Z Sat Feb 09 2019 - 12Z Tue Feb 12 2019 ...The West... Ridging off the west coast and up into Alaska will maintain troughing over the western US and an active and cold pattern over much of the country through at least the next week. Over the next three days, A pair of systems impacting the western U.S. are expected to produce widespread snows across the Pacific Northwest and Northern California, including lowland snows over western Washington and Oregon as well as the North Coast of California. An anomalously deep upper low will shift south along the Pac NW coast before swinging inland over the OR/CA border Sunday morning. The colder than normal temperatures aloft will continue to support significant snow accumulations at low elevations for areas east of the Cascades with heavy snow in the terrain. As the low continues to drop south, snows are expected to extend south along the coastal ranges along the Northwest Coast of California with snow levels dropping through tonight. Two-day totals have potential to produce one to two feet in the OR Cascades and the Trinity Alps of northern CA along with two to three feet in the Sierra Nevada. Farther inland, the trough north of this low progresses across ID and NV on Sunday, producing several inches of snow then, followed by the ranges of UT and CO Sunday night. The steady forward progression of the trough and front leads to a 6-10 hour window of enhanced moisture and lift, with the short duration limiting heavy snow potential. Day 2 WPC probabilities are only moderate for eight inches in the terrain of UT/WY/CO. As the low-mid level trough crosses the northern Plains Monday morning, snow breaks out in MT to ND with low probabilities for two inches as warm advection and pre-frontal convergence produces lift. The second system is a phase of northern stream energy off BC as the ridge over AK/Yukon is cut off. This potent development allows ample Pacific moisture to shift into the Pac NW while still under the upper trough making for heavy snow to lower than normal elevations on Day 3. The 00Z ECMWF is preferred with converging flow into the OR coast persisting for several hours late Monday through Monday night. Snow elevations around 1000ft over OR with make for particularly heavy snow with moderate Day 3 probabilities of 18 inches or more over the OR Cascades and Trinity Alps of northern CA. Depending on where the front sets up, the Portland metro area may also receive some heavy snow. ...Great Lakes/Central and Northern Great Plains... Day 1... Westerly flow persists across Lake Ontario with a lake effect streamer expected to persist into Sunday morning. A spot of moderate probability for four inches is just southeast of Lake Ontario for Day 1. Lake Erie is mostly frozen over, so no LES snow is expected downwind. Day 2... Mid-level vorticity from the remnant of the low crossing CA this morning rides a 130kt jet, reaching the central Plains by Sunday morning. WAA and convergence along the associated surface trough allows cyclonic flow over the middle Great Lakes for Sunday/Day 2. Banding on the north side across central WI can be expected. WPC Day 2 probabilities are moderate for central WI with enhancement off a mostly open Lake MI stretching the moderate risk area to eastern WI. Day 3... The mid/upper trough axis associated with the low currently over the Pac NW shifts east from the Rockies Monday, taking on a negative tilt. Rapid cyclogenesis over the Plains with broad cyclonic flow extending east to the Midwest makes for a winter storm on Day 3 with a stripe of freezing rain south of heavy snow expected. Moisture from the Pacific, enhanced over the Gulf of Mexico flows ahead of this low with increasing precip rates through Day 3 as the system shifts east. The 00Z GFS was much farther north than the rest of the 00Z global suite, so the 00Z ECMWF and NAM were preferred thermally. Strong surface high pressure over Ontario/Quebec provides cold air and a tight baroclinic zone/precip gradient on the north side. Day 3 WPC probabilities are moderate for over six inches along the MN/IA border. A warm nose ahead of the low approaching the Midwest allows a warm front to set up east across the lower Great Lakes where freezing rain can be expected. Moderate Day 3 probabilities for a tenth inch of ice spread east in a stripe from southern IA to northern IL and along the MI/IN/OH border. ...Northeast TX to MO and the Central Plains... Day 1... The particularly strong high over the central US this morning has driven low level subfreezing cold air across northeast TX. An upper level jet maxima from the central plains and MS Valley support ascent in the right entrance region over northeast TX into the ARKLATEX. Moist southwest return flow on the west side of the high allows precip to break out, but warm air aloft leads to a likely mix of precip types, including sleet and freezing rain to spread north from TX to MO through tonight. Pockets of freezing rain are expected today from north TX to OK/AR and becoming more widespread tonight over MO where there are low to moderate probabilities for a tenth inch of ice. Late tonight the thermal profile looks cold enough for snow to break out over NW MO and north along the NE/IA border where 1-2 inches are possible. ...Central and Southern Appalachians and Mid-Atlantic States... Canadian high pressure shifts off Cape Hatteras Sunday with return flow on the back side of the high and ahead of an approaching trough over the central Great Plains will allow for mid level warm advection-induced precip. Enough cold air is expected to be dammed that pockets of freezing rain of a few hundredths inch along with some sleet can be expected on the eastern slopes of the central Appalachians Sunday night. 1-2" snow on the front end can be expected in the higher central Appalachian crests with a potential stripe of 1" spreading east from PA to NJ. While the convergence for precip shifts north into Monday, cold air damming increases with further ice risk over the central Appalachians and east into the Piedmont. Moderate Day 3 WPC probabilities for a tenth inch of ice are northwest from the Shenandoah Valley. Light snow is expected to continue over the northern Mid-Atlantic with moderately low probabilities for 4" over central PA and low probabilities for 1" spreading east to NYC. Jackson