Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 224 PM EST Thu Jan 26 2023 Valid 00Z Fri Jan 27 2023 - 00Z Mon Jan 30 2023 ...Lower Great Lakes, Central Appalachians, and Ohio Valley/Northeast... Day 1... The large storm that brought heavy snow to the region will be well into the Canadian Maritimes this evening, but the residual longwave trough will persist across the area, reinforced by a secondary shortwave digging across the Great Lakes tonight before pushing into New England on Friday. A modest surface trough/convergence axis will advect eastward beneath this shortwave, causing a surge in CAA with 850mb temps falling to -10C to -15C on NW winds. This will yield efficient lake effect snow (LES) in the typical W/NW snow belts downstream of Lakes Erie and Ontario, as well as eastern lake Superior D1, with additional moderate snow likely through upslope flow into the Central Appalachians of WV. Although lapse rates both atop the lakes and into the upslope regions will be modest, impressive ascent near the surface will drive strong UVVs into the deepening DGZ to support efficient snow growth and snowfall rates which may exceed 1"/hr at times. WPC probabilities for more than 4 inches of snow reach 20-40% in the LES bands, highest along the Chautauqua Ridge. Lower but still notable probabilities of 10-30% for 4+ inches exist in the WV terrain. Additionally, along this surface trough and beneath the secondary shortwave energy embedded within the amplifying longwave trough across the east, scattered snow showers and snow squalls will likely persist tonight from Lower Michigan and northeast Ohio through western PA/NY. This is due in part to an overlap of 0-2km theta-e lapse rates falling to slightly below 0, with at least pockets of SBCAPE reaching 100-200 J/kg, where low-level streamlines indicate additional moisture transport from the Great Lakes. CAMs continue to depict scattered to widespread snow showers with potential for 1"/hr snow probabilities across these areas. While total snowfall within any of these snow showers or squalls will be minimal, the combination of heavy snow rates with gusty winds of 20 kts or more could produce brief periods of severely reduced visibility resulting in hazardous travel. ...Intermountain West... Days 1-3... Expansive mid-level trough will engulf nearly the entire CONUS through the weekend, with repeated shortwave energy diving out of Canada into the West eventually resulting in ridging downstream across the east by Sunday. A lead shortwave embedded within this trough moving across the Great Lakes early D1 will stretch a cold front southwestward into the Rockies and Pacific Northwest, and this front will become banked into the eastern side of the terrain from the Northern Rockies through the Central High Plains Saturday morning. During this period, a second shortwave will dive out of British Columbia and into the Pacific Northwest Friday night into Saturday, before racing eastward along the front into the Central Plains Saturday aftn, with a more pronounced shortwave following quickly on its heels into the Pacific Northwest again by Sunday morning. This third shortwave will be the most impressive, driving an arctic airmass southward causing the front so surge into the Great Basin on D3. As the front drops south D1 and banks into the terrain into D2, the enhanced fgen associated with this evolution will help to enhance ascent which will already be impressive through the height falls and at least periods of upper diffluence. This fgen will combine with strengthening upslope flow in the wake of the front on increasing NE winds to result in waves of heavy snowfall from the Northern Idaho through Colorado and Utah. Regional soundings indicate that the best ascent will occur within or just below the deepening DGZ as the column cools, suggesting impressive snow rates of 1+"/hr at times, which will accumulate rapid as SLRs climb to as high as 15-18:1, especially on D2 thanks to the cold column. Snow levels initially around 1500-2000 ft will fall to ground level, so while the heaviest accumulations are likely to be in the terrain, lower elevation and valley snowfall is also expected. For D1 and D2, WPC probabilities for more than 6 inches of snow are high from the Northern Rockies and Salmon River range southeast through the Absarokas, NW WY ranges, Wind Rivers, and as far south as the Park Range of Colorado, extending into the Northern Wasatch by D2. Snowfall will likely be quite impressive in the higher terrain, with storm total snowfall exceeding 3 feet likely. While snow in the valleys and at the passes will likely be much less, still impactful accumulations are expected. During the latter half of D2 and D3 as the tertiary shortwave drops southward and drives the front into the Great Basin with associated height falls, heavy snow will likely spread along the Cascades and into the Sierra. There remains uncertainty into how quickly the cold air will ooze southward which has considerable implications into the SLR, but it is likely that fluffy SLRs near the 75th Baxter climatological percentile will develop across the area, leading to efficient snowfall accumulations that may exceed 6 inches in the terrain. Also on D3, this colder air could allow some light snow to spread into the lowlands around Portland, OR, but cold air following the moisture should limit accumulations, if any, in these areas. ...North-Central Plains and Midwest... Days 2-3... A shortwave diving across the Pacific northwest Friday morning will race southeastward along a cold front which will be slowly sinking southward across the Intermountain West. As this shortwave moves into the High Plains, the interaction of these height falls/PVA with the low-level baroclinic gradient and modest LFQ diffluence ahead of an upper jet streak to drive surface cyclogenesis in eastern WY Friday evening. This feature will remain progressive as it moves eastward, but at the same time will deepen across SD/NE thanks to the strengthening upper jet streak, before shearing out into more confluent mid-level flow across the Midwest Saturday night. Despite this fast evolution, downstream WAA and the ageostrophic response to the strengthening upper jet should result in a dynamic stripe of 850-600mb fgen to drive intense omega into the column. Regional soundings suggest that this will result in a cross-hair signature with the most intense UVVs intersecting the deepening DGZ as the column cools along/behind the surface front, while the low/mid level WAA will help deepen an isothermal layer beneath the DGZ to support aggregate maintenance. Guidance indicates at least a modest reduction in mid-level theta-e lapse rates driven by warm/moist advection along the 285K-290K surfaces, which could result in snowfall rates around 1"/hr, but CSI/convective snowfall potential at this time looks low. Still, the impressive fgen should produce a period of heavy snow along the SD/NE border late D1 into D2, where WPC probabilities indicate a high risk for at least 4 inches of snow, and despite the progressive nature of the system there will likely be some maxima of 6-8" where the best banding develops. Late D2 into D3, the overall forcing begins to wane as the shortwave shears into more confluent flow to the east in response to height rises across the Southeast, but a stripe of moderate snow is likely from northern Iowa through southern Michigan, including Chicago and Detroit, where WPC probabilities for 4 inches are 20-40%. Weiss