Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 337 PM EDT Mon Apr 03 2023 Valid 00Z Tue Apr 04 2023 - 00Z Fri Apr 07 2023 ...Four Corners through the Upper Midwest... Days 1-3... ...A major late-season blizzard will bring significant snow, wind, and mixed precipitation... Guidance continues to cluster on the strength and track of a rapidly strengthening and major low pressure system which will begin to consolidate in the lee or the CO Rockies this evening. The primary driver of this evolution will be an amplifying mid-level low which will close off and deepen over WY early Tuesday before lifting northeast into the Northern Plains by Wednesday, while driving 500-700mb heights to as much as -4 standard deviations below the climo mean according to the NAEFS ensemble tables. In addition to this deepening mid-level low, impressive jet energy will coalesce atop the central high Plains Tuesday morning through Wednesday as a northern stream jet streak arcs zonally into New England while a trailing northern stream jet streak phases with a subtropical jet over the Southern Plains to produce intense diffluence within the increasingly coupled jet structure. The overlap of these features will allow this surface low to deepen rapidly as it lifts northeast into the Upper Midwest by Wednesday aftn, eventually becoming vertically stacked and occluding eastward to a triple point across the Great Lakes late D2. As the large scale ascent amplifies into a deep longwave trough, downstream meridional moisture transport will intensify as isentropic lift on the 290-295K surfaces surge northward through the nation's Heartland and into the Northern/Central Plains. This will drive PW anomalies to as high as +3 standard deviations across the MS VLY, and while PW anomalies will be lower into the cold sector of this system, mixing ratios exceeding 4g/kg on the 295K surface advecting into the cold side of the system and lifting into a robust mid-level TROWAL will allow for considerable moisture to be wrung out as snow from the High Plains through the western Great Lakes. Initially, this snow will develop just north of the consolidating cyclone as a residual baroclinic gradient draped west to east across WY and into NE/SD is overlapped by the approaching LFQ of the phasing jet streak to the south, to produce enhanced low-level fgen to drive impressive ascent. This will occur within an environment of above normal PWs, an exceptionally deep DGZ noted by high SREF probabilities for > 100mb and regional forecast soundings suggesting up to 300mb of depth, steep lapse rates above the DGZ, and the fgen driving strong omega into the column. The overlap of these features suggest an intense band of snow will translate across the Central High Plains with snow rates of 2-3"/hr likely as noted by the WPC snow band tool late tonight into Tuesday. This will likely be the most intense snowfall of the event. However, as the system begins to shift northward while continuing to deepen, a robust deformation band will setup beneath the overlapping TROWAL, and this will likely result in a pivoting band of heavy snowfall, with only slightly lower rates and SLRs, from western SD northeast through the Arrowhead of MN. A prolonged period of moderate to heavy snow will quickly accumulate here as snowfall rates reach at least 1-2"/hr, possibly higher at times, especially in eastern ND/western MN where cross sections indicate the potential for CSI overnight Tuesday into Wednesday as the band pivots into Canada. During Wednesday, this low will continue to shift northeast and then occlude to the east, bringing a slow end to the heavy snow across Minnesota by Wednesday night. WPC probabilities on D1 for more than 6 inches are high across a broad region of terrain from the Bitterroots and Salmon River ranges of ID southward through the Wasatch of UT, across much of the CO Rockies, San Juans, and Sangre de Cristos, and northward into the Big Horns and Wind Rivers. Locally 1-2 feet is likely in some of these higher terrain features. Farther to the east, a more impressive synoptic swath of heavy snow will develop north of the low, with WPC probabilities indicate a high chance for more than 6 inches from the central High Plains of eastern WY through the Pine Ridge, Black Hills, and as far as southern ND on D1. There are additionally high probabilities for more than 12 inches across much of this area too, with a focus near the Black Hills, Pine Ridge, and Laramie Range. Locally, 2-3 feet is possible on the upwind side of the Black Hills where upslope flow will enhance already impressive ascent and moisture. During D2, the heaviest snow swath will pivot northeastward, with WPC probabilities indicating a greater than 50% chance for 6+ inches from northeast SD northward along the Red River Valley of the North and into western MN, with moderately high probabilities for more than 12 inches in the middle of this swath. Additional heavy snow is also likely to continue in the Wasatch and along the Wasatch Front of UT on D2. By D3 the heavy snow exits into Canada leaving only additional light accumulations across eastern ND and northern MN. ...Pacific Northwest... Days 1 & 3... A weakening jet streak and lobe of mid-level vorticity on the upwind side of a deepening trough will continue to spread modest moisture and ascent into the Cascades tonight into Tuesday before the ascent shifts eastward and moisture dries out across the region. Periods of moderate to heavy snowfall will continue through Tuesday aftn, primarily into the upslope regions of the OR Cascades, with snow levels generally around 500-1000 ft, and this will likely accumulate to more than 6 inches along much of the Crest, with locally more than 12 inches possible. With snow levels relatively low, moderate snowfall accumulations are likely even at some of the Passes including Willamette and Santiam. After a break with drier weather on D2, a trough digging out of the Gulf of Alaska will pivot southeast towards British Columbia, resulting in increasing SW mid-level flow and accompanying WAA into the Pacific Northwest coast. This WAA will help drive snow levels rapidly upward, reaching 4000-5000 ft Wednesday night and Thursday, but coincident with PW anomalies surging to +1 to +2 standard deviations. This will result in increasingly heavy precipitation spreading onshore the region, with snow once again developing across the Cascades and Olympics where WPC probabilities for more than 6 inches reach 20-40%. ...Upper Midwest through New England... Days 2-3... An impressive closed mid-level low emerging from the Central Rockies will deepen to -4 standard deviations at 700mb according to the NAEFS ensemble tables while lifting northeast through the Northern Plains and into the Great Lakes by Wednesday evening. This low will combine with coupled jet streaks, one exiting zonally into New England while a secondary subtropical jet streak intensifies across the Southern Plains, to drive a strengthening surface low across the Plains. As features become vertically stacked Wednesday morning, this low will occlude to a triple point over the Great Lakes, with secondary wave development occurring D3, and then moving along the associated warm front into New England by the end of the forecast period. With broad southerly flow downstream of this evolution, warm and moist advection will intensify and spread a swath of moisture along and north of the warm front from MN/WI/MI late Tuesday into New England Wednesday night and Thursday. While the pronounced WAA will eventually turn all of this precipitation over to rain, a period of transitional precip from snow to sleet to freezing rain is likely in a west to east oriented arc along the warm front. The duration of any individual p-type is likely to be minimal across the Upper Midwest, but WPC probabilities for more than 0.1" of freezing rain reach as high as 30% from Duluth, MN eastward across northern WI and the U.P. of MI on D2. Late D2 and into D3, moderate to potentially heavy sleet and freezing rain will spread across northern New England from the St. Lawrence Valley through the White Mountains of NH and much of Maine. This latter part of the event could become substantial and a CAD develops from a Canadian high pressure, and sub-freezing layer depths at precip onset are above the 75th percentile from McCray, 2019 for long duration freezing rain events. This indicates that many areas of NH/ME may start at sleet, with significant sleet accumulations possible for parts of ME, before a long duration freezing rain event develops. Uncertainty continues into exactly how this will evolve, especially since we are currently still outside the high-res forecast range. However, confidence is increasing in an impactful mixed-precip event as shown by pWSSI probabilities for ice already reaching 30+% in NH/ME, and WPC probabilities are above 80% for 0.1" and 30% for 0.25" in parts of ME. Depending on how this evolves, ice accretions (and sleet) could be greater than this, and also may extend farther south almost to the Maine coastline. Weiss ...Key Messages for April 3-5 Major Winter Storm... --Widespread Heavy Snow Totaling Over One Foot A major winter storm will produce over a foot of snow from the Central Rockies through the Northern Plains, challenging some April snow records in the Dakotas and northwest Minnesota. The highest totals may locally exceed two feet. 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 Tuesday and Wednesday, causing dangerous to impossible driving conditions and considerable disruption to daily life. --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 tonight, and Upper Midwest Wednesday. Power outages, wind damage, and blowing dust, even outside of thunderstorm activity, are likely.