Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 454 AM EST Wed Jan 10 2024 Valid 12Z Wed Jan 10 2024 - 12Z Sat Jan 13 2024 ...Great Lakes and Northeast... Days 1-2... Strong surface low will be positioned near eastern lower Michigan to start the period, but will translate rapidly northeast with time through D1. This storm is quite wrapped up owing to an anomalous closed mid-level low directly overhead, and a strong secondary vorticity maxima rotating northeast into New England producing a strongly negative tilt. The vertical stack of the primary low will allow a secondary low to develop along the occlusion to the east, driving the most intense moisture advection into New England and then Canada by the end of D1. This moisture transport will remain exceptional, with IVT anomalies from NAEFS reaching +6 sigma over ME early on D1 before pivoting off into Canada the latter half of the day. This suggests the heaviest precipitation will be in northern New England, primarily in the higher terrain of ME, as strong accompanying WAA warms snow levels above 4000 ft, but WPC probabilities for additional snowfall exceeding 4 inches are only 20-30%, and only around Mt. Katahdin and Mt. Washington. As the low pulls away to the northeast, impressive CAA will develop on increasing N/NW winds which will support increasing coverage of lake effect snow (LES) downstream of the Great Lakes. Overall, LES appears modest in most areas as shortwave ridging returns quickly into D2. However, the most impressive LES is likely downstream of Lake Ontario into the Tug Hill Plateau where, despite this secondary shortwave approaching to locally back the mid-level flow to more W/SW, this will still support cool air moving across the long fetch of Lake Ontario to support heavy LES. WPC probabilities D1 and D2 exceed 80% for 4 inches both days. ...Central Plains to the interior Northeast... Days 2-3... A shortwave diving into the Pacific Northwest Thursday will race southeast across the Great Basin to reach the Southern Plains on Friday. This will drive a deepening longwave trough across the Intermountain West, with this trough pivoting eastward and elongating with time. As the primary vorticity maxima associated with the potent shortwave rounds the base of this trough and tilts negatively before closing off later Friday, the central part of the country will become entrenched within a potent and persistent trough to support yet another winter storm arcing from the Plains into the Northeast beginning Thursday night. The surface low is likely to develop Thursday evening in the lee of the Southern Rockies in response to PVA/height falls downstream of the potent shortwave, and within an area of increasing upper diffluence as a jet streak intensifies downstream of the longwave trough axis. Moisture will begin to return out of the Gulf on moistening 290-295K isentropic upglide, which will combine with SE 700-500mb flow around the trough to surge PWs to +1 to +2 sigma according to NAEFS, highest across the lower MS VLY D2 and into the western OH VLY D3. This moisture will be acted upon by strengthening synoptic deep layer lift as the upper trough tilts negatively, and a coupled jet structure develops overhead and efficiently overlaps the greatest height falls. This suggests that the surface low will strengthen rapidly as it shifts northeast towards the Great Lakes by D3, with a large shield of precipitation expanding along its track. There remains considerable uncertainty into the exact track of this storm both in the deterministic models and the ensemble clusters, some of which is likely due to the impacts of the preceding storm and where it lays its residual baroclinic gradient. However, regardless of the exact track of this system, it is likely that along and north of the track, especially beneath what should become an intense TROWAL, heavy snowfall will occur. The synoptic evolution should support heavy snow bands both within the leading WAA, and again within any deformation axis that could develop on the NW side of the mature cyclone, especially by D3. It is too early to determine exactly where those will setup, but this is likely to be a strong and large-scale system with heavy snow spreading from portions of the Central Plains through the Great Lakes. WPC probabilities D2 for more than 4 inches of snow are 40-70% in a narrow corridor focused around southeast Nebraska, before expanding considerably into D3 reaching above 50% from central Missouri through much of the Great Lakes region. Locally much higher totals are likely, reflected by WPC probabilities for more than 12 inches on D3 exceeding 40% from near Chicago and Milwaukee through much of the L.P. of MI. Additionally, later on D3, as the primary low occludes to a secondary triple point to its east, the best moisture flux should shift into New England where once again robust WAA should result in heavy snowfall in the higher elevations of Northern New England. WPC probabilities for more than 4 inches of snow are 50-70% in the higher terrain of the Adirondacks, Greens, and Whites. Regardless of exactly where this system tracks, impacts are likely to be substantial as reflected by the WSSI-P indicating already a large area of 30-50% probabilities for moderate impacts due to snow rate, snow amount, and blowing snow. ...The West... Days 1-3... An extremely active pattern continues across the Pacific Northwest and into the Northern Rockies/Great Basin through late week as a series of shortwaves drop onshore from the Pacific. The first of these systems is moving onshore this morning and will drive a surface low/associated cold front through the Pacific Northwest and as far southeast as the Central Rockies by tonight.This is a potent shortwave but gets sheared out into significantly confluent mid-level flow as the longwave trough amplifies. This shortwave will dive all the way into the southern Great Basin by D2, and while it will weaken within the positively tilted trough, and encounter only modest available moisture, heavy precipitation is likely to extend as far southeast as the CO Rockies, with snow generally occurring above 1500 ft. WPC probabilities for snow exceeding 6 inches eclipse 80% along the Cascades and down into the Sierra, as well as into the Blue Mountains, higher terrain of the Great Basin, and into the Wasatch. Locally, more than 2 feet of snow is possible in the highest terrain of the Cascades and Sierra, and with snow levels crashing late D1, light snow accumulations are possible across many of the lower valleys as well. As that first trough digs across the Four Corners and intensifies into the next central U.S. low, the pattern begins to shift as the primary trough amplifies over Alberta and then digs southward, driving a surface cold front into a more NW to SE orientation as it drops into WA/ID/MT on Thursday. South of this trough, mid-level flow becomes increasingly pinched with W/NW flow impinging again into the Cascades and then spilling southeast across much of the West as the front sags southward. Although IVT will remain modest at less than 250 kg/ms according to NAEFS,this increasing moisture being acted upon by intensifying fgen along the cold front and amplifying upper diffluence ahead of an approaching jet streak should result in widespread snowfall once again spreading across the Northwest. As the pattern continues to evolve into D3, another shortwave approaching the Pacific Northwest coast will drive more impressive mid-level confluence, and the overlap of this pinched flow with the eastward translation of the upper jet will drive IVT to above 500 kg/ms, resulting in much more significant available moisture for precipitation. At the same time the cold front will be sharpening and dropping farther south, driving better ascent and lowering snow levels to the surface as far south as a roughly Portland, OR to Cheyenne, WY line. This strong ascent, steep lapse rates beneath the cold pool, lowering snow levels, and ample moisture, will produce heavy snow accumulations, spreading southeast with time. On D2 WPC probabilities for more than 6 inches exceed 80% in the Cascades, Blue Mountains, Salmon River/Sawtooth region, and the Northern Rockies, with 2+ feet probable in the higher terrain. During D3 the greatest WPC probabilities shift south slightly, focusing in the OR Cascades and east across the northern Great Basin into the Tetons and Wasatch. Again on D3, local totals exceeding 2 feet are likely. Additionally, especially by D3, with snow levels collapsing towards the surface, moderate accumulations are becoming more likely in the valleys and lowlands, with Seattle, WA and Portland, OR both featuring 30-40% probabilities for at least 2 inches of snowfall. Finally, the strong cold front moving across Oregon this morning will continue to shift rapidly southeast across the Great Basin and into the Central Rockies by Thursday morning. This could spread convective snow showers and snow squalls across the area, with the CIPS snow squall parameter showing values above +1 spreading as far south as the Four Corners states. Evaluation of the fields which drive the snow squall parameter indicate the threat has increased this morning as the coverage of overlapping sufficient 0-2km RH/925mb fgen/SBCAPE>100 J/kg to support intense squalls is more widespread, especially along and immediately behind the front. Snow squalls should still generally be scattered, but more numerous convective snow showers are also likely which could cause brief but significant travel disruptions due to gusty winds and heavy snow rates. Weiss ***Key Messages for Jan. 8-13 Major Winter Storm*** --Continuing Blizzard Impacts in the Northwest Periods of heavy snow will continue through this afternoon in the Northwest. An additional 6-12 inches of snow will bring storm total accumulations to several feet in the higher elevations of the Cascades and Olympics. Gusts to at least 60 MPH will create blizzard conditions, continuing considerable travel impacts for many mountain passes. --Heavy Snow in the Sierra Heavy snow rates of 1-2"/hr will cause significant snowfall accumulations likely (70-90%) exceeding 12 inches in the Sierra today. Strong winds of 40-50 MPH during the heaviest snow will produce difficult travel. --Snow squalls across the Great Basin and Four Corners Snow squalls are likely behind a cold front through tonight. These intense bursts of snow pose a significant danger to motorists as rates of 1-2"/hr combine with strong wind gusts to produce rapid changes in visibility, occasional whiteouts, and possible flash freezes on roadways. --Significant Central U.S. winter storm begins Thursday night Confidence has increased that this system will emerge into the Plains Thursday and strengthen into a potent winter storm by Friday. Uncertainty continues in the track and intensity of this system, but a swath of heavy snow is likely from the Central Plains through the Great Lakes where the probability of exceeding 4 inches is above 70% from eastern Nebraska through lower Michigan. Considerable blowing and drifting of this snow in strong winds also possible.