Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 409 AM EST Thu Jan 26 2023 Valid 12Z Thu Jan 26 2023 - 12Z Sun Jan 29 2023 ...Lower Great Lakes, Central Appalachians, and Interior Northeast... Day 1... Interior and coastal low pressure phases over New England this morning as the upper trough takes on a negative tilt with rapid deepening as the low quickly lifts north through New Brunswick this afternoon. Strong warm air advection east of the low track limits heavy snow after 12Z to far northern Maine where there are high Day 1 probabilities for over 4 inches. Strong NW flow will develop behind this low. Although the 850mb-sfc delta-T is modest, strong omega into the subtly deepening inversion should yield periods of heavy LES in the favored N/NW snow belts downstream of Lakes Michigan, Erie, and Ontario into tonight where there are high WPC probabilities for more than 4 inches. Additionally, the NW flow will result in favorable upslope snow development across the Central Appalachians of WV, with a secondary maximum possible along the spine of the Green Mtns in VT. WPC probabilities for more than 4 inches are moderate for both of these areas. Behind the cold front, secondary shortwave energy embedded within the amplifying longwave trough across the east will likely produce scattered snow showers and snow squalls this afternoon over Lower Michigan and northeast Ohio then tonight over 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. ...Northern Rockies...Northern Plains...and Northwest... Days 1-3... Sprawling low pressure centered just north of Hudson Bay will likely remain in place through the middle of next week with shortwave energy rounding the gyre breaking down the ridge currently over the PacNW and focusing Pacific moisture over the Northern Rockies Friday through Saturday before directing Arctic-sourced air south. A leading shortwave trough swings southeast across the Canadian Prairies today, driving a clipper-type low pressure over the Dakotas where a burst of snow will occur ahead of its associated warm front, with a few wet inches likely. However, the trailing cold front will begin to sink south over the northern Plains tonight and bank against eastern slopes of the Northern Rockies through the Black Hills on Friday, pushing farther south into the North-Central Rockies Saturday. A second shortwave trough will dig south from British Columbia over the Pacific Northwest tonight into Friday followed by a much stronger, positively-tilted trough tracking down the PacNW Coast Saturday. The second wave will pivot east ahead of the front Friday, tightening the low-level baroclinic gradient. The associated surface low will move southeast along the front, aiding in ascent, which will become even more impressive as upslope flow intensifies on NE flow in the post-frontal regime thanks to a 1050mb surface high shifting south down the Canadian Rockies Saturday. Additionally, mid-level fgen will increase along the front, and the placement of the most intensely sloped fgen appears to correlate well with the deepening DGZ as the column cools, suggesting snowfall will become intense into the terrain with snowfall rates of 1-2"/hr likely, as snow levels fall quickly to the surface Saturday with the cold front. There is high confidence in this becoming an extreme snowfall event into the terrain, with multiple feet of snow likely in some areas. WPC probabilities for more than 8 inches are high both D1.5 and D2.5 over all MT Ranges, northern ID ranges, and northwest WY ranges as well as the Medicine Bow over southern WY to the Park Range of CO. Locally, more than 3 feet of snow is likely in the higher terrain. Moderate accumulations are also possible in the lower elevations and valleys as the intense dynamics overcome more marginal thermals initially, with the column cooling impressively Friday night/Saturday as the Arctic air moves in. The coastal upper low Saturday will direct some Pacific moisture over the Cascades with snow levels dropping below 2000ft Saturday and Day 3 snow probabilities moderate for 6"+ for the WA/OR Cascades/Olympics. ...North-Central Plains and Midwest... Days 2-3... A strong baroclinic zone along a cold front Friday night over the north-central Plains allows bands of heavy snow to shift east along the SD/Neb border. The lee-side low over CO and strong high pressure from the Canadian Rockies reinforces frontogenesis. A deep DGZ with a cross-hair signature of moisture and lift from the low-level fgen should produce high SLRs and efficient snowfall accumulation with bands oriented in the direction of motion. Day 2.5 snow probabilities for 6"+ are still 40-60% with the focus shifted south a bit into northern Neb then extending east into northern IA. The frontogenesis weakens a bit farther east as a trough over the southern Midwest directs bands to push east over northern IL through southern MI. Day 3 snow probabilities are moderate for 4"+ from eastern IA across Chicago and through southern MI. Jackson Key Messages for Jan 25-26 Winter Storm... --Heavy snow over New England will quickly lift north into Canada this morning. --Snow squalls should cause hazardous driving conditions this afternoon over eastern portions of the Midwest this afternoon and western Pennsylvania and New York tonight. --Snow-covered roads and reduced visibility due to blowing snow will make travel difficult across impacted areas.