Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 435 AM EDT Fri Mar 17 2023 Valid 12Z Fri Mar 17 2023 - 12Z Mon Mar 20 2023 ...Great Lakes & Interior Northeast... Days 1-3... An anomalously strong closed low will pivot across the Great Lakes today through Saturday before ejecting into the Canadian Maritimes on Sunday, becoming an impressive trough draped as far south as the TN Valley and into the Mid-Atlantic. The most intense ascent associated with the height falls downstream of this trough will be collocated with coupled jet streaks Friday, with the accompanying upper diffluence helping to subtly deepen a surface low moving through Ontario D1. This surface low is progged to occlude late D1 into D2, shifting towards secondary development north of Maine along a triple point Saturday, the result of which will be a cutoff of the best moisture transport to push the heaviest precipitation east of the area. However, a trailing surface trough will pivot around the occluding low to provide additional low-level convergence for ascent, which will trail one or possibly two surface cold fronts also moving across the area as weak shortwaves rotate around the amplified gyre centered over Ontario. While synoptic ascent will shift east and then wane during D1 providing just enough lift within the accompanying moisture plume to produce heavy snow over northern Maine D1, lake effect snow (LES) will become widespread and impressive late Friday around Lake Superior, and then Saturday into Sunday across the remaining Great Lakes. The driver of this LES event will be a combination of height falls and PVA within the closed trough overhead, with impressive CAA atop the above-seasonal normal lake temperatures according to GLERL to produce sfc-850mb delta-T values of 20-25C, driving inversion heights up over 10,000 ft. This should support an extremely favorable environment for LES in the favored W/NW snow belts beginning Friday across the U.P. of MI, shifting southeast on Saturday to encompass the western L.P. of MI and then downstream of Lake Erie and Ontario before waning slowly on Sunday. Effective fetch from upstream lakes combined with single band setups along Lakes Erie and Ontario could result in impressive snowfall rates of 2+"/hr, with 1-2"/hr likely in other bands off Lakes Michigan and Superior. During the 2-day event, WPC probabilities for 4 or more inches across northern ME are as high as 10%, but are much more significant for the LES. For D1, WPC probabilities for more than 6 inches are above 70% along the south shore of Lake Superior, where locally more than 1 foot of snow is possible across the Keweenaw Peninsula. During D2, the heavy LES shifts east, with WPC probabilities for more than 6 inches reaching 40-60% near Traverse City, MI and into the Tug Hill Plateau. Finally, as the secondary cold front and then subsequent surface trough ejects eastward Saturday into Sunday, this could be accompanied by convective snow showers or scattered snow squalls along and behind the convergence axes. The snow squall parameter gets quite high Saturday across the Great Lakes and northern Ohio Valley, before shifting eastward into the interior Northeast and northern New England Saturday night into Sunday. Some uncertainty into how widespread the snow squall threat will be with this front as 0-2km fgen appears modest for ascent, but there is an overlap of 50-100 J/kg of SBCAPE and some pockets of progged negative theta-2 lapse rates within the 0-2km layer. Will need to monitor this evolution closely as any snow squalls could produce rapidly changing and dangerous travel conditions due to gusty winds and intense snowfall rates. ...Central Plains... Day 1... A cold front dropping southward through the Plains this aftn/eve will have the potential to produce snow squalls along the boundary. Impressive 925mb fgen will slope upward into the DGZ, which while subjectively shallow, will provide strong ascent to produce snow growth this aftn. This front will also help to drive steep low level lapse rates to support SBCAPE approaching 200 J/kg, which when combined with the enhanced fgen and negative 0-2km theta-e lapse rates will produce strong omega to support convective snow showers, with squalls also possible noted by a high snow squall parameter. While total snowfall accumulations will likely be minimal, NW flow oriented parallel to the sinking boundary suggests that where any of these squalls or heavy snow showers can train to the southeast, an inch or two of snow is possible as reflected by large spread in the WSE plumes. Regardless of accumulations, briefly heavy snow rates in excess of 1"/hr combined with strong winds will create rapidly changing travel conditions and reduced visibility. ...Southern Rockies & West Texas... Days 1-2... A series of shortwave troughs digging eastward from the Great Basin will slowly migrate across the Four Corners and Southern Rockies through Saturday, interacting with a cold front which will drop southward through the Southern Plains and then bank into the terrain of NM late Saturday into Sunday becoming stalled. This will drive a slow increase in ascent across the Southern Rockies as height falls and concurrent downstream divergent flow overlap with low-level frontogenesis and increasing upslope flow from the E/NE behind the front. This increasing ascent will act upon a moistening column noted by PW anomalies reaching as high as +1 standard deviation above the climo mean across NM, driven by both Pacific jet energy aloft and return flow out of the Gulf of Mexico and up the Rio Grande Valley around the post-frontal high pressure dropping into the Southern Plains. This evolution should result in two periods of moderate to locally heavy snow - the first generally focused across the San Juans and Sangre de Cristo Mountains, with the second farther south into the Big Bend region of Texas. While there is high confidence in moderate to heavy snow placement for the lead shortwave, differences in the northern extent of moisture, timing of the cold front and associated cold air/intensity of upslope flow, and position of the shortwave, produce lower confidence D2. For D1, WPC probabilities for more than 4 inches are generally 30-50% across the San Juans, Sangre de Cristos, and much of the higher elevations of central NM. Then on D2 the heaviest snow axis shifts southward reflected by WPC probabilities for 4+ inches which have increased for the Big Bend area including the Davis Mountains and Stockton Plateau, reaching as high as 30-40%. ...Pacific Coast States... Day 3... After a much needed break the next couple days, a modest shortwave will subtly amplify the mid-level flow Pacific to provide a favorable overlap of moisture and ascent to spread precipitation back onshore the Pacific Coast. This shortwave is progged to lift into CA Sunday morning with modest height falls and a subtle backing of 850-700mb flow to the SW. This will be accompanied by the approach of an extensive and NW to SE oriented Pacific jet streak reaching 110-130 kts, placing favorable LFQ diffluence into the West collocated with the best mid-level height falls. This will have the dual effect of providing large scale ascent to the region, while also increasing moisture noted by modest probabilities according to CW3E of IVT exceeding 250 kg/m-s by late Sunday. Snow levels within the most intense WAA spreading into CA will rise to around 6000 ft, but will otherwise be generally 4000-5000 ft, and WPC probabilities for more than 6 inches of snow are above 60% along much of the Sierra, and 20-30% in the Shasta/Trinity region of northern CA, and 10-30% farther north along the OR Cascades. ...Key Messages for March 16-18 Winter Storm... --Lake effect snow will increase in coverage across the Great Lakes today and Saturday in the wake of a storm system moving into Ontario, Canada. --Heavy lake effect snow will likely develop south of Lake Superior today, then spread across all of the more typical W/NW snow belts to the east of Lakes Michigan, Erie, and Ontario through Sunday. --Heavy snow rates exceeding 1"/hr are likely within these lake effect snow bands, producing low visibility, hazardous travel, and rapidly changing road conditions. The greatest snowfall is expected around Lake Superior where local amounts of 1-2 feet of accumulation are forecast. --A reinforcing cold front may cause snow squalls to develop from the eastern Great Lakes through the interior Northeast Saturday and Sunday. Briefly heavy snow rates and gusty winds within any snow squalls will produce hazardous travel. Weiss