Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 418 AM EDT Tue Apr 04 2023 Valid 12Z Tue Apr 04 2023 - 12Z Fri Apr 07 2023 ...Four Corners through the Upper Midwest... Days 1-2... ...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 major low pressure system now that we are in the early stages of the system's development. The primary driver of this evolution will be an amplifying mid-level low that is closing off over WY early this morning 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 this 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 afternoon, eventually becoming vertically stacked and occluding eastward to a triple point across the Great Lakes on 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. Heavy snow with rates up to 2"/hr is already ongoing early this morning across eastern WY into southwestern SD just north of the consolidating cyclone and within an area of enhanced low-level fgen to drive impressive ascent. In fact, over a foot of snow has already been reported near Hot Springs, SD and Casper, WY with more to come through the day. 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 tonight 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. The eastern gradient between heavy snowfall amounts and minimal accumulations will be very tight as warm air both aloft and at the surface nose northwestward along the track of the surface low. A few miles difference with the eventual low track could make a big difference in regards to snowfall accumulations across central/northern MN and eastern SD. WPC probabilities on D1 for more than 8 inches are high across a broad region of terrain from 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 of storm total snowfall is likely in some of these higher terrain features as the heaviest snowfall diminishes this afternoon. 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 northwest MN 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 and eastern ND. 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 additional 6+ inches from eastern ND to northern MN on the backside of the low pressure system. ...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 today before the ascent shifts eastward and moisture dries out across the region. Periods of moderate to heavy snowfall will continue through the afternoon hours, primarily into the upslope regions of the OR Cascades, with snow levels generally around 500-1000 ft. 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, Olympics, and northern California mountains, where WPC probabilities for more than 6 inches generally reach 20-40%. ...Upper Midwest through New England... Days 1-2... 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 on Wednesday night and moving along the associated warm front into New England. 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 today 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. Deep snowpack in place across northern WI and the U.P. of Michigan may keep surface cold air in place long enough to lead to a few tenths of an inch of ice accretion. WPC probabilities for more than 0.1" of freezing rain reach as high as 60% across northern WI and the U.P. of MI on D1, with splotchy 10-20% probs for at least 0.25". By D2, 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 as CAD develops from a Canadian high pressure and weak coastal low, with sub-freezing layer depths at precip onset 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, with modest ptype and QPF questions. However, confidence is increasing in an impactful mixed-precip event as shown by pWSSI moderate impact probabilities for ice already reaching 40+% in NH/ME, and WPC probabilities are above 80% for 0.1" and 40% 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. Snell/Weiss ...Key Messages for April 3-5 Major Winter Storm... --Widespread Heavy Snow Totals 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 today and Upper Midwest Wednesday. Power outages, wind damage, blowing dust, and extremely critical fire weather conditions are likely.