Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 521 PM EST Tue Feb 28 2023 Valid 00Z Wed Mar 01 2023 - 00Z Sat Mar 04 2023 ...California through the Four Corners... Days 1-2... Amplified trough digging down the Pacific Northwest coast will move onshore central CA Wednesday aftn as a closed low, and the continue to deepen while shifting across the Great Basin and into the Four Corners by Thursday morning. This closed feature will reach maximum depth near the TX Panhandle late Thursday, noted by 500-700mb heights approaching -4 standard deviations according to the NAEFS ensemble tables. The movement of this potent upper low will be paired with the development of strengthening coupled jet streaks, one digging along the Pacific coast upstream of a strengthening subtropical jet streak extending from near the Baja Peninsula through the Central Plains. The core of each of these jets will likely reach 130-150 kts, with the favorable coupled diffluent region atop the greatest height falls over the Southwest Wednesday. Moisture embedded within this subtropical jet streak will arc eastward, which when combined with confluent flow downstream of the amplified mid-level trough will result in impressive IVT for which both the GEFS and ECENS ensemble probabilities suggest may reach 500 kg/m/s as it moves into southern CA and the Desert Southwest. The overlap of the strong synoptic ascent with this anomalous moisture will result in heavy precipitation D1 and D2 across the region. Snow levels through the event will gradually collapse beneath the upper low, falling from 3000-4000 ft to 500-1000 ft coincident with some of the heavier snow as lapse rates foster better instability. Snowfall rates across the Sierra will likely eclipse 2"/hr at times tonight, and these intense rates are progged to shift southeast beneath the best ascent into the Mogollon Rim and other parts of the Four Corners on Wednesday. While most of the heavy snow is expected to be in the terrain, the increased instability leading to heavy snow rates could allow for modest snowfall accumulations even in the valleys and foothills, especially northeast of the Mogollon Rim where theta-e lapse rates indicate better conditional instability and the potential for some CSI banding. As this entire system pulls away D2, the snow should gradually wind down from southwest to northeast into NM and CO. WPC probabilities D1 for more than 6 inches of snow are above 80% along the entire Sierra Crest, into the Peninsular and Transverse ranges of Southern California, Mt. Charleston, along and north of the Mogollon Rim through the Wasatch, and into the CO Rockies and San Juans. There is likely to be more than 4 feet of snow in the Sierra, with 2+ feet likely in this other terrain on D1. By D2, the heaviest snow shifts eastward, with WPC probabilities for more than 6 inches rising above 80% along the Mogollon Rim and into the White Mountains, with greater than 50% probabilities extending into the San Juans and Sangre de Cristos. Locally more than 2 feet of additional snow is likely in the higher terrain, with high probabilities for more than 2 inches across much of the lower elevations and foothills, and spreading as far east as the Sacramento Mountains. ...Northern Plains... Day 1... An amplifying shortwave entrenched within generally zonal flow across the Northern Plains will close off across the Dakotas Wednesday morning before quickly shearing out to the northeast across the Great Lakes. This feature will move along a sharp baroclinic gradient between a cold high pressure to the north and impressive warm advection arising from the south, with the associated fgen intensifying with time partly due to the low-level WAA, but aided by the ageostrophic response of the LFQ of a subtropical jet streak shifting eastward with time. Moist advection on 285K isentropic ascent will be generally modest, but should push PWs up to around 0.5 standard deviations above the climo mean, with a weak theta-e ridge extending cyclonically into a modest TROWAL Wednesday morning. The overlap of this TROWAL to enhance mid-level instability with strengthening overlapped ascent and increasing fgen supports a broad area of moderate snow, with a band of heavier snow likely where the deformation and fgen overlap. Although the system will be relatively transient, snow rates of up to 1"/hr are likely as shown by the WPC prototype snow band tool, which could result in heavy accumulations in a narrow corridor as the temporal duration of these bands extends due to motion parallel to the accompanying surface low. WPC probabilities for more than 6 inches are 50-80% across southern ND, extreme northern SD, and into parts of western MN on D1. As this storm ejects quickly, snowfall should wane late D1, with only minimal probabilities less than 20% for 6 inches reaching northern WI into early D2. ...Pacific Northwest... Days 2-3... Shortwave ridging across the Northwest will give way to a shortwave trough moving onshore near the British Columbia/Washington State coast Thursday aftn, with a secondary shortwave approaching the Pacific Northwest coast again late D3 /Friday night./ Confluent flow downstream of these shortwaves combined with the approach of a modest upper jet streak will drive anomalous moisture onshore noted by IVT forecasts exceeding 250 kg/m/s Wednesday night into Thursday. This setup will support periods of moderate to heavy snow D2 as 700mb flow begins orthogonal to the Cascades which will drive better ascent through upslope, with additional light to moderate snow persisting into D3 as modest confluent flow and some increasing WAA ahead of the next shortwave re-asserts itself across the region. The heaviest snow should remain in the higher terrain, as shown by WPC probabilities for more than 6 inches exceeding 70% in the Olympics and WA Cascades D2, stretching into the OR Cascades, Bitterroots, Blue Mountains, and Tetons by D3. However, snow levels D2 will be generally 1000-1500 ft, and then only around 500-1000 ft D3, so despite lighter snow likely on Friday, the area mountain passes will likely continue to experience at least modest impacts due to snow through the last 48 hours of the forecast period, and high WPC probabilities for significant snow exceeding 4 inches exist at passes from Willamette to Stevens along the Cascades, and into Lookout/Lolo Passes near the Northern Rockies. ...Central Plains, Upper Midwest into the Great Lakes and Northeast... Days 2-3... A strong closed mid-level low emerging from the Four Corners with 500-700mb heights around -4 standard deviations below the climo mean Thursday evening will eject northeast into the Southern Plains by Friday morning and then move rapidly northeast towards the lower Great Lakes by the end of the forecast period. This amplified trough will be accompanied by increasingly coupled jets as a strengthening southern stream jet streak downstream of the trough axis sharpens and arcs poleward across the lower MS VLY, while a northern stream more zonally oriented jet streak, with core strength of 190 kts, arcs eastward over New England on Friday. The paired diffluence between these two combined with the impressive height falls and downstream divergence associated with the mid-level low will result in a rapidly deepening surface low as it moves from OK late Thursday towards the Great Lakes by Friday evening. Intense meridional moist advection will develop downstream of this trough axis, surging moisture from the Gulf of Mexico northward into the OH VLY Friday morning with PW anomalies progged to exceed +3 standard deviations. This moisture will lift northward on increasingly intense isentropic ascent lifting over the southern edge of a Canadian high pressure moving across Ontario, and the associated theta-e ridge will likely rotate into a TROWAL during Friday. The result of all of this is likely to be a potent winter storm with an expanding precipitation shield, resulting in increasing coverage of heavy snow, sleet, and freezing rain. Confidence by D3 is low in placement and timing of heaviest precipitation due to considerable spread among the operational global models, but on the NW side of this low, an axis of deformation and increasing fgen, especially during rapid intensification which allows for strong ageostrophic low-level wind response to enhance CAA, should yield an axis of rain changing to heavy snow from the Central Plains through the Upper Midwest. Exactly where this band sets up and how much snow will occur is very uncertain, but it is possible significant accumulations are possible, and current WPC probabilities feature a 30-60% for 4+ inches from western KS through the Chicago metro area late D2 through D3, with some locally higher amounts possible, highly dependent on how this system evolves. Farther downstream within the more intense WAA ahead of the low pressure, a swath of heavy snow is more certain, although confidence still remains modest in placement and timing. However, WPC probabilities for more than 4 inches are above 50% for much of lower Michigan and into Upstate NY. South of the heavy snow, WPC probabilities for 0.1"+ of freezing rain are 10-30% for southern lower Michigan and into the higher terrain of western PA on D3. Weiss ...Key Messages for Winter Storms pushing through the Southwest... --A winter storm system will continue to impact the Sierra Nevada and Southern California ranges through Wednesday, before pushing over the Southwest U.S. Wednesday into Thursday. --Snowfall rates of 2 to 3 inches per hour is expected for the Sierra Nevada and Southern California ranges through Wednesday. Blizzard conditions will make for dangerous to impossible travel. --Heavy snow with rates of 1 to 2 inches per hour pushes across the high terrain of the southern Great Basin and southern Rockies through Thursday, where hazardous travel conditions are likely. There is the potential for avalanches in some mountain ranges. --If you plan to travel through any mountain passes in California or the Southwest through Wednesday, be prepared for rapidly changing conditions and have winter driving supplies. ...Key Messages for Plains to Northeast Winter Storm... --Confidence continues to increase that a potent winter storm will move from the Southern Plains Thursday to the Great Lakes Friday and then the Northeast on Saturday. --A swath of heavy snow will likely accompany this storm, and it is probable that significant accumulations will occur north of the track of this low pressure. --The combination of heavy snow and strong winds may result in hazardous travel as well as impacts to infrastructure late this week into the weekend. --A corridor of significant mixed precipitation, including sleet and freezing rain, is possible south of the heaviest snow. --Uncertainty remains with the timing and location of the storm track, which determines where the axis of heaviest precipitation and most significant impacts will occur. Keep up to date with the latest forecasts as this storm evolves.