Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 428 AM EDT Sun Mar 12 2023 Valid 12Z Sun Mar 12 2023 - 12Z Wed Mar 15 2023 ...The West... Days 1-3... While the zonal flow and moisture transport into the West continues today and tonight, the next more significant system begins to push onshore Monday into Monday night, initially over the Pacific Northwest and northern California followed by another even stronger closed shortwave hitting central California Tuesday into Tuesday night. The first of these systems will have moderate levels of IVT approaching 400-500 kg/m/s on the Oregon coast followed by even higher levels of IVT for the second system Tuesday into Tuesday night when the latest GFS/EC have upwards of 700-800 kg/m/s values across much of central/southern California. The accompanying warm air advection with these systems will likely keep snow levels elevated, generally 7000-8000 ft in the Sierra and Great Basin and as high as 4000-5000 ft farther north before the cold front trailing the shortwaves and surface low drops through the region late in the period. For D1, the WPC probabilities are high for 8 inches only across the Sierra and the very tops of the OR/WA Cascades while for D2, the probabilities are a bit lower for the Sierra and Cascades but also spread further east including the Sawtooth and NW Wyoming ranges. For D3 when the stronger AR reaches the West, the Sierra stands to pick up very heavy snowfall with intense snow rates. Accumulations on D3 alone could top several feet for the central Sierra while spillover moisture reaching the ID and western Wyoming ranges are likely to exceed 12 inches. ...Upper Midwest through the Great Lakes and into the Mid-Atlantic and Northeast... Days 1-3... Anomalously deep, closed upper low positioned over Fargo, ND early this morning will continue to slide east/southeast over the next 12-24 hours, reaching near Chicago by early Monday morning. The lower heights aloft, cooling airmass, and sufficient moisture continuing to wrap around the system will provide favorable lift within the DGZ to produce moderate to locally heavy snowfall today into tonight. The greatest accumulations will likely be across the North Shore of MN thanks to easterly lake enhancement and through northern to east-central Wisconsin where an additional 4-6" is likely with isolated additional totals of 8-10" possible based on the latest WPC probabilities. Snow rates up to 1"/hr will be possible across north-central to east-central Wisconsin. Further south, a cold wedge air mass will be banked up against the southern to central Appalachians thanks to surface high pressure positioned over Canada. Meanwhile, overrunning precipitation is expected through this morning into the afternoon. Given the cold and drier air mass in place, a wintry mix of snow, sleet, and freezing rain will be possible from western North Carolina, southwest Virginia, and eastern West Virginia where ice accumulations up to 0.1" are possible as well as a few inches of wet snow. This will all lead to a significant, major nor'easter late Monday evening as the upstream secondary shortwave trough interacts with the weaker southern stream energy. The consolidated upper trough then quickly takes on a negative tilt Monday night leading to a rapid deepening of the surface low off the Northeast coast, stalling somewhat off the coast of MA Tuesday before slowly clearing out Wednesday. This will bring a multi-hazard, complex winter storm to the region. For Day 2 (Monday-Monday night), the bulk of the accumulating snow will likely be from northeast PA including the Poconos, northern NJ, across much of central/upstate NY, and the MA Berkshires and southern Green Mtns. Boundary layer temperatures will be more marginal and the higher snow accumulations will be more elevation dependent and rate driven by dynamical cooling. The latest WPC probs show high probabilities for at least 8-12" across these areas. By Day 3, the coastal system begins to stall somewhat off the coast of MA and much colder begins to collapse across the Northeast. This will likely bring more snow toward the coast with a heavy band across southern New England and coastal Maine. The Day 3 WPC probabilities are high for at least 8" from upstate NY through coastal Maine. Altogether, the potential exists for a widespread heavy snowfall event with many areas likely to see a foot or more of snow. Isolated higher totals approaching 2 feet are possible for the Catskills, southern Adirondacks, Berkshires, and southern Greens/Whites. Taylor ...Key Messages for March 10-15 (Upper Midwest to Northeast) Winter Storm... --Moderate to heavy snow will continue across portions of the Upper Midwest through tonight, producing an additional 4-6â€+ of snow across northeast Minnesota near Lake Superior and across much of northern to east-central Wisconsin. --This system will reach the Mid-Atlantic coast Monday night leading to a rapidly strengthening coastal low, and confidence is increasing that a major norâ€easter will affect parts of the Northeast Monday Night - Wednesday. --Heavy snow rates (up to 2â€/hr possible) and strong winds from this norâ€easter will likely produce dangerous to near impossible travel. The heavy-wet nature of the snow could result in scattered to widespread power outages and tree damage. --Confidence is increasing that higher elevation areas across the interior Northeast will receive greater than 12†of snow. Localized higher amounts are possible for portions of the Catskills and southern Adirondacks in NY, Berkshires in western MA, southern Green Mountains in VT, and southern White Mountains in NH. --Widespread minor coastal flooding may be possible Monday Night through Wednesday as the low pressure stalls and deepens off the coast of New England. ...Key Messages for Mid-March (Western U.S.) Winter Storms... --Occasional precipitation this weekend and Monday Rain in low elevations and foothills, and snow at high elevations, from northern to central California, as well as northwest Nevada, and Oregon this weekend at a slightly lower intensity. --Rain and snow to intensify Monday into Wednesday Considerable flooding impacts below 5000 foot elevation along the central California coast, San Joaquin Valley, Sacramento Valley, and southern Sierra Nevada foothills into midweek. --Heavier Rain, Snow, and Snowmelt likely Mid-Week Additional heavy rain and snowmelt may lead to renewed (more widespread) flooding from Tuesday into Wednesday, particularly in low elevations and areas with shallow and warming snowpack. Creeks and streams will continue to be vulnerable to flooding from additional rain and snowmelt. --Heavy, Wet Snow at Higher Elevations Mid-Week Another round of heavy snow, measuring in feet in the higher elevations in California, particularly the Sierra Nevada, will further compound snow load impacts and issues.