Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 414 AM EST Sat Mar 09 2019 Valid 12Z Sat Mar 09 2019 - 12Z Tue Mar 12 2019 ...Southwest to Southern Rockies... Three shortwaves merge and shift south off the CA coast through Sunday and create a cold core low off the Baja coast Monday. Day 1... Low pressure shifts south down the northern CA coast through tonight with energy from a remnant low over interior WA/OR being pulled in along with a reinforcing shortwave that effectively makes the collection of disturbances a positively-tilted longwave trough off the CA coast by tonight. Light to moderate precip across northern CA focuses snow above 2000ft on the Yolla Bolly/southern Klamath mountains and the CA Cascades where Day 1 probabilities for 8 inches are moderate. Day 2... The strengthening trough off the CA coast provides a stronger inland push of Pacific moisture with a swath crossing the Great Basin to UT with snow above about 4000ft. Moderate to high Day 2 probabilities for 6 inches extend east from the northern and central Sierra Nevada across the many ranges of NV to the southern Wasatch. Day 3... The newly closed low continues to shift south from southern CA Monday with Pacific moisture directed across southern CA the Desert SW to the southern Rockies with snow elevation around 5000ft, around 7000ft over the southern Rockies where the tropically sourced jet stream is focused. Day 3 probabilities are moderate for six inches over the San Bernadinos, the highest part of the Mogollon Rim with moderate probabilities for a foot over the San Juans where deeper moisture exists. ...Northern Plains to the Upper Midwest... Consistent track and QPF forecasts continue with the 00Z package for the CO low crossing the Central Plains today and turning northeast to cross Lake Michigan tonight due to influences over a northern stream trough over the Canadian Prairies and lift across Ontario Sunday as the two troughs phase. Day 1... A strong LLJ ahead of the low lifts Gulf moisture into the northern Plains today with rain east of the 850mb low up to the IA/MN border and a warm nose and marginal surface temps making for a sleet/freezing rain mix well into MN with all snow over northern MN. The extent of the super-freezing warm nose remains uncertain, but the dynamics (height falls/PVA/frontogenesis) should limit the progression of sleet/freezing rain to south-central MN, not as far north as the NAM and not as far south as the ECMWF. The 00Z GFS which is in between seems to be a good approximation for thermal profiles/ptypes for now. A stripe of moderate Day 1 probabilities for a tenth inch of freezing rain are along/left of the 700mb low track from the MN/IA border to northern MI. The timing of the trough axis pushing east along the IA/MN border coincides with nightfall making most of the snow along that border from wrap around. These wrap around bands look to extend pretty far south over eastern NE this evening and shift northeast across southern WI with the surface low track overnight. High probabilities for 4 inches are north of IA from eastern SD to northwestern WI. Farther north an inverted trough, drawn by the northern stream trough over the Canadian Prairies allows for moderate snow to spread east over central and northern MN with a pivot over northern MN as phasing begins late tonight. The highest snow totals in this event should be over central/northern MN where the ptype remains all snow and the pivot occurs making for 18+ hours of snow. Moderate probabilities for a foot are focused over west-central MN. MSP is on the fringe of the warm nose and heaviest TROWAL banding with a gradient of higher totals NW and lower SE likely. Day 2... The system lifts into eastern Ontario as it phases with the northern stream trough Sunday. Northwest flow under this expanded trough will allow continued wrap around snow over MN/WI and then MI Sunday. Lake Superior is almost completely frozen over, so little lake enhancement is expected with moderate probabilities for two inches on Day 2 limited mainly to the UP and tip of the Mitt. ...Central Appalachians...Mid-Atlantic and New England... Days 1-3 A warm front lifting north ahead of the previously noted low in the Midwest will be the focus for light to moderate precipitation spreading over the central Appalachians, northern Mid-Atlantic and New England tonight through Sunday night. This southerly flow over retreating cold air is expected to support a period of mixed precipitation, changing to rain from the central Appalachians into the Northeast, with enough cold air remaining in place to support mostly snow across northern Upstate New York into northern New England. Moderate Day 1 probabilities for two inches are over the Catskills and Adirondacks, increasing to four inches from the White Mountains to northern Maine on Day 2. The wintry mix beginning over the remnant dammed cold air east of the central Appalachian divide (VA/WV/MD/PA) should mainly be freezing rain with the Frostburg MD area and north into PA having moderate Day 1 (tonight) probabilities for a tenth inch of ice. The wintry mix following the snow farther north has moderate probabilities for a tenth inch over the the interior northern Mid-Atlantic and interior southern New England for Day 1.5 and just south of the heaviest snow swath from MA across NH into ME on Day 2. Snows in the following northwest flow allow moderate Day 3 probabilities for two inches over the Adirondacks and Green Mountains. ...Pacific Northwest... Day 3... A northern stream trough pushing southeast from the Gulf of Alaska Monday, reaching the WA/OR coast Tuesday with good timing and magnitude agreement from the 00Z ECMWF and GFS. An atmospheric river ahead of the trough reaches the WA/OR coast Monday night with much above normal moisture including one inch PW. Heavy, but progressive precip pushes into the coastal ranges and Cascades with snow elevations 2000 to 3000ft. Day 3 probabilities are moderate for 8 inches in the Olympics and northern WA Cascades with a note that this is in 12 hrs. Jackson