Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 413 AM EST Sat Feb 19 2022 Valid 12Z Sat Feb 19 2022 - 12Z Tue Feb 22 2022 ...Eastern Great Lakes and Northeast... Day 1... A potent shortwave racing eastward within broad cyclonic flow across the east will drive a clipper type wave of low pressure and associated arctic front across the Great Lakes and Northeast today. Modest WAA ahead of this system will produce light snow across the region, but the more impressive snow is likely to be in developing LES as CAA commences in earnest behind the front. A deepening inversion and lowering DGZ with unidirectional flow across Lake Ontario should produce efficient and heavy LES, focused into the Tug Hill Plateau where upslope enhancement is also likely. WPC probabilities D1 are greater than 40% for 6+ inches, and locally 10" is possible. LES is also likely downwind of Lake Erie, but with much less accumulation due to Lake Erie being more than 90% ice covered according to GLERL. There, WPC probabilities for 4+ inches are less than 10%. Additionally, there remains an impressive signal for snow squalls along and immediately behind this strong cold front. The high-res guidance continue to show good consensus for a line of convective snow squalls along the front, with smaller broken segments/cells behind the main line. Confidence is increasing that many areas will experience a squall this aftn, as instability climbs towards 150 J/kg coincident with intense low-level fgen. Snowfall rates will likely eclipse 1"/hr, although the limited duration of these rates should keep accumulations to 1" or less. However, dangerous travel is likely as these snowfall rates combine with strong winds which may reach 50mph to produce near white-out conditions at times. ...Pacific Northwest through the Northern Rockies... Days 1-3... A slowly amplifying but broad mid-level trough across the Intermountain West will keep spokes of vorticity rotating around from the north, producing a long duration winter weather event across the Pacific Northwest, with precipitation slowly spreading southward by D3. Confluent mid-level flow, periodic Pacific jet streaks, a modest wave of low pressure, and rounds of WAA ahead of a cold front will converge to produce widespread precipitation across the Northwest and Northern Rockies all 3 days of the forecast period. Snow levels initially will be around 3000 ft, but will steadily fall each day behind a cold front Sunday, and then more rapidly behind a secondary cold front Monday, such that by D3 snow levels across WA/OR/ID/MT will be near sea level. These falling snow levels combined with rounds of precipitation will lead to heavy snow accumulations in the higher terrain, but also the potential for light accumulations even into the lowlands of WA/OR and elevated valleys of ID/MT through the period. Additionally, steepening lapse rates beneath the amplifying upper trough combined with the synoptic deep layer ascent and at least modest fgen along the sinking baroclinic gradient could produce cellular convective snow showers or snow squalls, which is reflected by widespread but scattered low probabilities for 1"/hr snowfall rates from the recent HREF. On D1, the heaviest snowfall is likely in the WA Cascades where WPC probabilities are high for 8+ inches, and locally up to 2 feet is possible. Lesser accumulations are progged in the Northern Rockies, but WPC probabilities for more than 8 inches are as high as 40% near Glacier NP. As the trough deepens D2 and the cold front sinks southward, heavy snow will spread more significantly into the OR Cascades, Absarokas, and other ranges of southern MT/northern WY where upslope flow will enhance the deep layer ascent. WPC probabilities for 8+ inches on D2 are above 80% in the OR Cascades with locally up to 15 inches possible. Elsewhere across the Northern Rockies and into the MT/WY ranges, WPC probabilities for 8 inches or more are 20-40%. By D3, a more impressive lobe of vorticity will further amplify the trough, while the cold front continues to dive southward bringing a rapid decline in snow levels. The heaviest snowfall D3 is again likely in the OR Cascades which will have the best overlap of height falls/jet diffluence and moisture, and WPC probabilities for at least 8 inches are above 50%, and 2-day snowfall in the OR Cascades could exceed 2 feet. With the snow levels crashing, light snow accumulations of 1+ inches are likely in many of the elevated valleys and lowlands of MT/ID/WA/OR, with even some light snowfall accumulation becoming more likely in Seattle, WA and Portland, OR on Monday night. ...Central Rockies through the Upper Midwest... Days 2-3... An amplifying longwave trough across the Intermountain West will be repeatedly re-energized by lobes of vorticity swinging southward through its base from the west. This will leave downstream mid-level divergence and WAA for ascent, aided by increasing upper diffluence within the RRQ of a departing polar jet streak D2, and then a complex evolution of jet structures leading to modest coupling of energy D3. Together, this will produce a cyclogenesis in the lee of the CO Rockies Monday morning, and this low is then likely to track northeast along a surface cold front through D3 while slowly intensifying. The interaction of these jet streaks with the surface low, cold front, and mid-level trough will produce waves of snowfall Sunday and Monday. During Sunday /D2/, the polar jet streak arcing into New England will leave the favorable RRQ for ascent across the Northern Plains. This will combine with the surface cold front sinking southward to produce a robust corridor of fgen which will interact with increasing moisture on WAA of 30-40 kts at 850mb to produce an expanding stripe of snowfall from eastern MT through the ND/SD border. While this jet streak will be the primary driver of ascent D2, the developing low pressure in the CO Rockies D3 combined with increasing LFQ of a secondary jet streak will enhance both WAA and the synoptic ascent to expand the precipitation and increase the intensity of the snowfall across the Northern Plains into the Upper Midwest. Forecast soundings across the Northern Plains/Upper Midwest D3 show a deepening DGZ which the SREF suggests has a >50% chance of exceeding 100mb, combined with a very cold column to support high SLRs. Moist 285K isentropic ascent increases into the DGZ at the same time the fgen becomes most robust, and there is likely to be a stripe of heavy snow, with some embedded banding of snow rates of greater than 1"/hr. WPC probabilities within this region exceed 50% for 4 inches on D2 over northern SD, and then rise to more than 80% from eastern SD into central MN D3, including the Twin Cities area. Locally, more than 10" of snowfall is possible as shown by the WSE and NBM 90th percentile. Further to the south across the Central Rockies including the Front Range, San Juans, and Laramie Mountains, heavy snow is likely to develop Monday, and may persist for several days beyond this forecast period. The surface low will organize just east of the terrain before exiting to the east late D3, and easterly flow north of this low combined with NE flow behind the cold front which will bank into the terrain could produce impressive upslope lift. This upslope will combine with broad divergence aloft and height falls ahead of the mean trough to produce heavy snowfall which will likely exceed 6 inches in much of the terrain. For D3, there remains some uncertainty how much snow will impact the High Plains to the east, but WPC probabilities feature a moderate risk for at least 2 inches along the I-25 corridor and into the Palmer Divide. Weiss Key Messages for the Feb 18-19 Snow Squalls: - A powerful arctic cold front will race across the Northeast today, exiting the New England coast tonight. - Along and behind the front, narrow bands of snow squalls will move rapidly from northwest to southeast. These bands will be accompanied by heavy snowfall rates of 1"/hr or more, gusty winds which may exceed 50 mph, and extremely limited visibility. - The combination of brief heavy snow and wind will likely create dangerous travel conditions as the front moves through. - Temperatures will drop quickly behind the front leading to a rapid refreeze of wet roadways.