Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 338 PM EDT Tue Apr 04 2023 Valid 00Z Wed Apr 05 2023 - 00Z Sat Apr 08 2023 ...Four Corners through the Upper Midwest... Days 1-2... ...Major late-season blizzard to continue across the Northern Plains and Upper Midwest... A closed low embedded within a deepening longwave trough across the Intermountain West will shift northeast into the Northern Plains tonight and then gradually eject into Ontario on Wednesday while continuing to deepen, noted by NAEFS ensemble tables suggesting 700-500mb heights reaching -3 to -4 standard deviations from the climo mean. Impressive synoptic lift downstream of this feature due to height falls and PVA will overlap efficiently with an impressive coupled jet structure to help deepen a surface low as it tracks northeast from the lee of the Rockies through the Upper Midwest and eventually into Ontario Wednesday night. At this time it will also become vertically stacked and occlude eastward to a triple point across the Great Lakes, resulting in weak secondary low pressure development moving eastward by Thursday. Downstream of this surface low, moisture advection will continue to intensify on low/mid level flow becoming increasingly robust out of the south, surging Gulf of Mexico moisture northward. This will manifest as intense 290-295K isentropic ascent with impressive mixing ratios of 4g/kg wrapping cyclonically into a TROWAL around the low pressure, especially late Tuesday into Wednesday. PW anomalies associated with this moisture plume are progged to approach +3 standard deviations, suggesting there will be plentiful moisture for a major snow event in regions that remain cold enough for all snow, generally NW of the 850mb low track, despite it being early April. SLRs will be challenging during this event as Baxter climatology suggests SLRs will generally be around 10:1. However, regional forecast soundings indicate SLRs should be above to well above climo thanks to a colder than typical thermal structure combined with an extremely deep DGZ (high probabilities from the SREF of > 100mb of depth) in some areas, and strong ascent where an impressive area of deformation interacts with the TROWAL. This will likely produce impressive snow rates of 1-2"/hr, potentially up to 3"/hr at times as noted by the WPC snow band tool, especially where some CSI may develop across eastern ND/western MN. Where any banding can persist as the system translates northeast, it is possible some April snowfall records may be approached or exceeded during this event. The majority o the remaining snow will occur on D1 before the system exits into Ontario, Canada. WPC probabilities suggest continued heavy snow in the Wasatch and along the Wasatch front of UT where the probability for more than 6 inches remains above 30%, continuing the enhanced avalanche threat locally due to the incredible snowfall that has piled up there all winter. The primary snow swath though will be from northern SD through northern MN where WPC probabilities indicate a greater than 70% chance for 6+ inches of snow, and locally as much as 18 inches is expected, especially in eastern ND/western MN where the best CSI/banding potential exists. ...Pacific Northwest... Days 2-3... A closed mid-level low dropping out of the Gulf of Alaska will weaken through late week as it approaches the Pacific Northwest coast, but will likely shift onshore as a lobe of vorticity with modest amplitude as it takes on a subtle negative tilt on Friday. This feature will be accompanied by weak but persistent upper level diffluence within the LFQ of an approaching and digging jet streak, resulting in increasing large scale ascent across the Northwest Wednesday night into Friday. Downstream of the aforementioned closed low, mid-level flow between 700-500mb will initially become zonal and point orthogonally into the region, but will gradually back to be more SWly on Thursday, driving enhanced warm and moist advection. This will drive a modest atmospheric river (AR) onshore, with CW3E probabilities for IVT exceeding 250 kg/m-s reaching 95%, with some embedded low probabilities for 500 kg/m-s reaching the Cascades late D2. While this overlap of moisture and ascent will result in expanding and intensifying precipitation across the region, the accompanying WAA will help surge snow levels to above 4000-5000 ft late Thursday into Friday, such that heavy snow accumulations should generally be above the major pass levels. While WPC probabilities for more than 6 inches are very low on D2, they increase rapidly during D3, but remain confined generally to the higher terrain above pass level. D3 WPC probabilities for more than 6 inches reach as high as 30% in the Olympics, WA Cascades, and parts of the Shasta/Trinity region of northern CA. ...Upper Midwest through New England... Days 1-2... An amplified closed low characterized by 700-500mb heights dropping to -4 standard deviations according to NAEFS ensemble tables will lift across the Northern Plains and then into Ontario Thursday, coincident with an impressive coupled jet structure that will overlap the best mid-level height falls and PVA with increased diffluence to drive robust deep layer ascent across the Upper Midwest. An accompanying surface low will become vertically stacked during D1, and then occlude to a triple point over the Great Lakes, with a secondary wave of low pressure developing and then lifting northeast along a warm front into New England on Thursday. Strong meridional moisture transport out of the Gulf of Mexico will intensify driving anomalous PWs northward, noted by anomalies reaching +3 standard deviations across the Great Lakes Wednesday and New England on Thursday. During this evolution, a cold Canadian high pressure will gradually retreat, but should maintain at least a modest CAD, especially across New England, keeping surface temperatures below freezing even as the strong WAA pushes a warm nose > 0C northward. This will result in a difficult p-type forecast from northern WI and the U.P. of MI eastward into northern New England and Upstate NY. For tonight into Wednesday, mixed precipitation will spread across WI and the U.P. of MI, with a transition from sleet, to freezing rain, to rain, expected. The guidance differs considerable in the evolution of the low-level thermal structure, and while the NAM is colder, and generally handles these events better than other guidance, appears to be overdoing the ability of the cold air to hang on across this area due a lack of strong cold/dry advection in a region of strong warm advection. Additionally, periodic heavy rates should somewhat limit accretion efficiency, but still an impactful sleet/freezing rain event is likely noted by WPC probabilities which indicate a 50-70% chance for 0.1" of ice from near Duluth, MN eastward through the eastern U.P. of MI, with locally more than 0.25" possible. Later Wednesday and into Thursday /late D1 and D2/ a more significant mixed-precip event will unfold across northern New England, especially from the St. Lawrence Valley eastward through the White Mountains of NH and much of central/northern ME. Robust moisture transport in a region of intense ascent will result in widespread precipitation spreading across the region north of the warm front, while the cold high gradually shifts eastward and maintains a more impressive CAD into New England. Here too, expect the CAD will erode relatively quickly due to strong WAA, but precip falling into the CAD will likely result in some intensification of this wedge before erosion, lengthening the duration of mixed precip. Regional soundings indicate the warm nose will be quite strong, as high as +9C, which is above the 75th% for warm nose temperatures from McCray, 2019, but is accompanied by a cold depth of 1500-2000m which is also above the 75th%, suggesting the potential for significant sleet. For this area, anticipate the slightly colder NAM solution is better, and will hedge in that direction which suggests impactful sleet and freezing rain across the area, which is also reflected by medium-high probabilities for moderate impacts due to ice in the PWSSI. WPC probabilities for more than 0.1" of freezing rain have increased, especially for northern NH and north-central ME, where they are now above 90% for 0.1" of ice, and as high as 60% for 0.25". Locally more than 1/3 inch of ice is likely. Additionally, sleet accumulation to 0.5" or more is possible in parts of northern ME. Weiss ...Key Messages for April 3-5 Major Winter Storm... --Additional heavy snow exceeding 1 foot This major winter storm will continue to produce heavy snow rates of 1-2"/hr at times which will create more than 1 foot of additional snow in parts of the Northern Plains and Upper Midwest. Storm total snowfall may reach 3 feet in isolated locations. Avalanches and significant impacts from snow load are possible across portions of the Rockies. --Blizzard Conditions in the Northern Plains Strong winds and heavy snow will create whiteout conditions and significant drifting snow through Wednesday, causing dangerous to impossible driving conditions and considerable disruption to daily life. --Freezing rain across the Great Lakes Periods of freezing rain will create icy conditions for portions of Wisconsin and the Upper Peninsula of Michigan. Isolated power outages and slippery travel is likely. --Life-Threatening Cold for Anyone Stranded Wind chills will fall below zero during the blizzard in the Northern Plains, which would be life-threatening to anyone stranded outside. --Damaging Winds From the Southwest to the Midwest Widespread gusts in excess of 50 MPH will spread from the Southwest and Four Corners to the High Plains today and Upper Midwest Wednesday. Power outages, wind damage, blowing dust, and extremely critical fire weather conditions are likely.