Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 445 AM EDT Mon Mar 13 2023 Valid 12Z Mon Mar 13 2023 - 12Z Thu Mar 16 2023 ...The West... Days 1-3... The series of active, unsettled weather continues for the West Coast, particularly the state of California which is set to receive another very strong atmospheric river event Tuesday into Wednesday. A near persistent zonal flow over the West combined with repeated passing of shortwaves will keep precipitation in the forecast through the entire period however there are 2 main systems to highlight. The first being in the day 1 period as a strong cold front and mid/upper level shortwave passes through the Pacific Northwest. Modest amounts of moisture and enhanced lift from a jet streak positioned over California will provide favorable forcing for widespread precipitation. Snow levels will begin to crash as the front passes through Oregon/Washington while remaining elevated/high across CA and parts of the Intermountain West with continued warm air advection. For the day 1 period, an additional 4 to locally 12 inches of heavy, wet snow will possible from the Sierra northward through the OR/WA Cascades. Moisture spilling over into Idaho will have better support for localized amounts exceeding 12 inches. The next atmospheric river will have a greater punch with it featuring a deepening low pressure arriving late Tuesday across northern California and a plume of higher moisture. The latest CW3E IVT values look to be between 500-700 kg/m-s with this event and there is favorable overlap of moisture and lift. Snow levels generally are expected to be between 4000-5000 ft but may climb upwards to 8000 ft during the peak of the AR. This leads to high probabilities for 12 inches for the Sierra in the day 2 period, with localized max amounts liked to be measured in feet. Additional heavy snow will be found across central ID ranges as well as western Wyoming where WPC probabilities for 12 inches are moderate to high. For Day 3, the aforementioned energy will quickly pass through the central to northern Rockies. There remains some uncertainty on any potential phasing of the northern/southern energies and interactions with the frontal boundary draped across the region. For now, the best setup appears favored for the Colorado Rockies where probabilities for 6 inches are already moderate to high levels. Further north into the Northern Rockies and eventually into the Northern High Plains, there is less confidence but WPC probabilities for 4 inches are slight to near moderate. ...Mid-Atlantic and Northeast... Days 1-3... ...Major nor'easter for the interior Northeast...Continued uncertainty along the immediate coast... A deep closed upper low centered currently near Milwaukee will dive through the Mid-Atlantic into early Tuesday then remain offshore the Northeast Tuesday before pulling away. It will continue to become anomalously deep and negative tilted over the Northeast, reaching -2 to -3 standard deviations below the climatological normal by late Tuesday. To the south, low pressure organizing on a surface boundary draped across the Southeast will tap into the favorable upper diffluence and approaching upper low, turning northward as it rapidly deepens this evening and tonight of the coast of the Northeast. This deepening low will pivot closer to the MA coast later in the day Tuesday before slowing down and then gradually moving eastward into Wednesday. All of this will bring a powerful nor'easter to the region, setting up for a classic, very impactful multi-hazard system including widespread heavy snow, strong winds, and widespread minor coastal flooding potential. It is becoming increasingly likely that major to extreme impacts will occur across this area. The continued challenge to the forecast has been the thermal profiles near the surface, particularly closer to the coast including northern NYC areas and up the I-95 corridor to Boston. Temperatures will likely end up a few degrees near freezing across the area so precipitation type (rain vs. snow) will primarily influenced by the intensity of the precipitation rates with heavy, wet snow taking over with the strong dynamical cooling. Initially today through this evening, a surge of warm, more moist air will bring mostly rain to areas along the coast while higher elevation locations further inland begin mostly as snow. Once the deepening low approaches the coast of MA early Tuesday, much colder air begins to collapse down through the region and this should lead to a pretty quick changeover to wet snow closer to the coast. This will be particularly true for portions of southern New England including central to eastern MA where very intense snow bands (2" to near 3"/hr) are most likely to set up and pivot as the low approaches, slows, and even stalls for several hours Tuesday evening. The 00Z guidance has begun to cluster on the QPF amounts for the event with a large area of 1-2" liquid equivalent with some potential for near 2.5" liquid across eastern MA. While earlier model runs had trended to the south/southeast a bit, there was a slight shift back to the NW this cycle, which complicates the snow forecast for the near coastal regions including northern NYC toward Boston. Heavy snow, greater than a foot, is almost certain for the higher terrain areas further inland while heavy snow mixing with and/or changing to rain at the lower elevations and closer to the coast during less intense rates may lead to lesser snow amounts. SLRs used were generally below climo and the snow will be very wet, reflected by the snow load parameter in the pWSSI which is major to extreme levels. For this cycle, the WPC snow probabilities for at least 8 inches are above 80% for a large area from the Poconos through the much of central/upstate NY, central/western MA, a large portion of VT/NH and far southern Maine. The heaviest snow will likely occur in the terrain areas including the Adirondacks, Catskills, Greens, Berkshires, Worcester Hills, Monadnocks, and Whites where WPC probabilities for 12 inches are above 70-80%. Isolated totals greater than 24 inches are most likely for the Catskills, Berkshires, and southern Green Mountains based on the WPC probabilities. Closer to the coast, there are low probabilities for 4 inches from northern NYC to Boston and reflect the lower confidence. Guidance has been trending stronger/colder/wetter for the western Boston area where probabilities for 6 inches are now up to 40-50 percent. While confidence in major impacts and heavy snow accumulations is greatest inland across the higher terrain, the potential still exists for some significant accumulations across far northern NJ and parts of southern New England including the Boston area. Behind this system, a long duration of moderate to robust upslope flow will drive periods of heavy snow into the central Appalachians. The guidance has trended stronger with the available moisture, and is now featuring better lift reaching into the lower portions of a deep DGZ. This has led to an increase in snowfall potential with higher SLRs leading to fluffy accumulations for which WPC probabilities suggest above 60% chance of exceeding 4 inches and isolated 6"+ totals. Taylor ...Key Messages for March 10-15 (Upper Midwest to Northeast) Winter Storm... --A coastal low pressure will strengthen rapidly into a major norâ€easter that significantly impacts the Northeast this evening through Wednesday. The peak of the storm is expected tonight through Tuesday night. --Heavy snow rates (2-3â€+/hr) and strong winds will produce dangerous to impossible travel. The heavy-wet nature of the snow, combined with max wind gusts up to 50 mph, will result in scattered to widespread power outages and tree damage. Similar impacts could be felt along the I-95 corridor from southern New England to Portland. --March norâ€easters tend to favor the more elevated terrain for receiving the heaviest snowfall totals. Snowfall totals >12†are expected in the Catskills and southern Adirondacks in NY, Berkshires and Worcester Hills in MA, Monadnocks and White Mountains in NH, and southern Green Mountains in VT. Localized maximum totals of 24-30†are possible. --Widespread minor coastal flooding and beach erosion may be possible tonight 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 tonight 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 tonight at a slightly lower intensity. --Rain and snow to intensify late 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 Midweek Additional heavy rain and snowmelt to result in 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.