Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 410 PM EST Sat Mar 11 2023 Valid 00Z Sun Mar 12 2023 - 00Z Wed Mar 15 2023 ...The West... Days 1-3... Zonal flow will continue into much of the Pacific Coast tonight and Sunday, but with weakening IVT. Still, PW anomalies within the onshore fetch will remain around +1 standard deviations according to the NAEFS ensemble tables, especially for CA and the Great Basin/Four Corners. The best moisture transport will begin to wane late D1 through D2, and while precipitation may never shut off entirely across parts of the West thanks to this onshore flow and embedded weak short waves, in general the activity will wane. However, late D2 and into D3, the next impressive AR will approach the coast south of dual shortwaves with increasing amplitude lifting eastward. The first of these will likely surge into the Pacific Northwest late Monday and Monday night, with the trailing shortwave approaching the CA coast at the end of the forecast period. An intensifying Pacific jet streak will also impinge onto the coast during this evolution late D2 into D3, with the core of the jet reaching 110-130 kts and providing additional ascent through LFQ diffluence. This will result in impressive IVT moving once again into the region, noted by CW3E GEFS and ECENS probabilities that are moderate for 500 kg/m-s. The accompanying WAA will help raise snow levels on D3 to 7000-8000 ft in the Sierra and Great Basin, and as high as 4000-5000 ft farther north before the cold front trailing the lead shortwave and surface low drops southeast during D3. For D1, WPC probabilities for more than 6 inches are high generally only in the Sierra where locally an additional 2-3 feet is possible in the higher terrain, but low probabilities for more than 6 inches do extend into the CO Rockies and San Juans as well. For D2-3, as the next AR begins to approach and come in waves beneath the dual shortwaves mentioned previously, heavy snow will gradually become more widespread but at generally high snow levels. During D2 heavy snow will impact the WA and OR Cascades, spread back into the Sierra, and into the Northern Rockies, becoming even heavier and more widespread, reaching the Sawtooth and NW Wyoming ranges. WPC probabilities across most of these ranges are high for more than 6 inches in the 2-day period, with the heaviest accumulations likely in the WA Cascades D2 where more than 2 feet is likely, and the Sierra D3 where an additional 2-4 ft or more is possible. ...Upper Midwest through the Great Lakes and into the Mid-Atlantic and Northeast... Days 1-3... A bowling ball closed low with height anomalies below -2 standard deviations from the climo mean according to the NAEFS ensemble tables will rotate southeastward from the Dakotas through the western Great Lakes on D1, and then maintain its intensity as it shifts into the interior Northeast D2. Monday night into Tuesday /D3/ the closed low will shed a potent vorticity impulse around its base, resulting in an increasingly negatively tilted trough over the Northeast with the best PVA shifting to the New England coast. Beneath this closed low moving across the Plains to the Midwest, a surface low will track southeast into the Great Lakes by Monday and then into the interior northeast Monday night. Downstream of this feature, meridional moisture transport will increase on isentropic upglide from the Gulf of Mexico, and while PW anomalies are modest, there will be sufficient moisture for heavy snow, especially on the north side of the low track where the best moist advection within the WAA will overlap with some sloped fgen to produce a swath of heavy snow and heavy snow rates reaching 1+"/hr. The most likely region for this band appears to be from eastern IA northward through WI and into the Arrowhead of MN, with additional snow enhancement occurring into the Iron Ranges due to upslope and lake moisture on D1, and WPC probabilities are 40-70% for more than 6 inches NW to SE from northeast MN through central WI where some lake enhanced moisture could also contribute. However, the heaviest snowfall during this period is likely in the Iron Ranges where locally up to 18" of snow is possible. Also during D1, a wave of low pressure moving eastward across the TN VLY will encounter better moisture advection to produce an expanding shield of precipitation from the Central Appalachians northward. This will result in an overrunning precipitation event as the surface high retreats in Canada but maintains a wedge down the coast. This will yield a mixed precipitation event in the higher terrain of the Appalachians, with precip changing from snow to mix to rain, and even a light mix possible to I-95 near Washington-Baltimore. In this corridor, there exists a 10-30% chance for 0.1+" of freezing rain in the higher terrain from western NC through eastern WV, with a similar footprint but just a bit farther north of 30-50% chances for 4 inches of snow or more. However, the more significant event will develop Monday evening as the aforementioned secondary vorticity lobe sheds from the closed low over the Great Lakes, leading to secondary low development east of NJ in a Miller-B type setup. The guidance is in very good agreement that this low will then rapidly strengthen as it pivots northeast off the New England coast, and then get 'captured' by the upper low to swing W/NW and stall near eastern MA before slowly pivoting eastward at the end of the forecast period. The guidance has trended southeast with this track, and while it still appears the heaviest snow will be NW of I-95 from NYC to Boston due to onshore flow spreading warm air along the coast (the exception probably coastal Maine), this is taking on the appearance of a major nor'easter for parts of the interior Northeast and New England, with cold air collapsing eastward late D3 possibly bringing heavy snow to the coast as well. All the major probabilistic data suites, including the WSO, pWSSI, EFI, and CIPS analogs, suggest heavy snow for much of the area away from the coast during D3, and with low SLR and strong winds likely, this could become an extremely impactful event for some areas. WPC probabilities D3 for 6+ inches are already above 50% for much of eastern Upstate NY and interior New England except northern Maine, with probabilities exceeding 70% for 1 foot or more for the Catskills, Berkshires, southern Greens, southern Adirondacks, and into the Worcester Hills and much of southern and central NH. While uncertainty continues in the track and speed of this low, WSE plumes indicate the potential for more than 20 inches of snow in the hardest hit areas. Weiss ...Key Messages for March 10-15 (Upper Midwest to Northeast) Winter Storm... --A band of heavy snow will move across the Upper Midwest tonight through Sunday with snowfall rates of 1â€/hr possible. Where this combines with strong winds, travel will become dangerous due to snow covered roads and severely reduced visibility. --Additional snowfall of more than 6†is likely from Minnesota through central Wisconsin. Localized totals of 12-18†are possible in the Minnesota Arrowhead. --This system will shift to the Mid-Atlantic coast Monday night leading to a rapidly strengthening coastal low pressure, and confidence is increasing that a major norâ€easter will affect parts of the interior Northeast Tuesday - Wednesday. --Heavy snow rates and strong winds from this norâ€easter will likely produce dangerous travel. The heavy-wet nature of the snow could result in scattered to widespread power outages and tree damage. --While the exact track of this low is uncertain, it is becoming likely that some areas inland from the major cities will receive more than 12†of snow. ...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.