Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 409 AM EST Mon Jan 08 2024 Valid 12Z Mon Jan 08 2024 - 12Z Thu Jan 11 2024 ...Southern High Plains through the northern Great Lakes... Days 1-3... A strong winter storm will bring heavy snow and blizzard conditions beginning this morning in the High Plains of southeast CO/northeast NM and then slowly expand northeast into the Upper Midwest by Tuesday morning, and into the Great Lakes Tuesday night, before waning from west to east during Wednesday. Heavy snow rates, significant snow accumulations, and wind gusts of 50mph or more in some areas will create widespread moderate to major impacts through mid-week. Closed mid-level low over the Four Corners to start the period will feature anomalous 500mb heights as low as -4 sigma according to NAEFS, and these anomalies are progged to become even more negative into Tuesday morning as this closed low continues to intensify. This exceptionally deep low will drive pronounced height falls and downstream divergence for impressive ascent shifting across the region, aided by increasingly coupled jet streaks to overlap the most intense upper diffluence with the greatest height falls across the Plains. At the same time, the amplified trough will force intense downstream meridional moisture advection on unidirectional southerly flow out of the Gulf of Mexico, which will surge PWs to +2 sigma according to NAEFS as moist isentropic upglide maximizes in the presence of robust 6-8 g/kg mixing ratios. The accompanying theta-e ridge/WAA wing will lift cyclonically into a TROWAL, and this will help increase instability and likely result in intensifying snowfall rates, and the WPC prototype snow band tool already shows high probabilities for 1-2"/hr snowfall across much of the area. Additionally, the setup has always looked favorable for a potent deformation band developing NW of the cyclone and then translating eastward through Tuesday, and tonight's cross-sections indicate a strong likelihood for this to occur with coincident CI (folded theta-e in the presence of EPV*<0) to support pockets of thundersnow and possibly rates to 3"/hr across parts of astern CO through parts of MO/IA. The exact placement and speed of this band is still uncertain, but impacts will be substantial where it advects due to the combination of these convective snow rates and gusty winds. While immediate impacts may be most intense within this band, the heaviest snowfall will occur where the WAA snow, which in itself could be significant, will transition to the more convective snow as the low pulls away. D1 WPC probabilities for more than 6 inches of snow are above 80% from the Raton Mesa due to a combination of this deformation band and ascent aided via upslope northeast flow, into far NW KS which is where this band may pivot the longest, and through eastern NE where the most intense WAA snow is likely today, followed without a break by the pivoting deformation band tonight Total snowfall in some of these ares could exceed 12 inches. On D2 as the storm accelerates more to the northeast, the highest WPC probabilities for more than 6 inches extend from extreme northern MO towards the Door Peninsula of WI where they exceed 70%, and again, locally more than 12 inches is possible where the WAA snow and the pivoting band occur without a break. This is most likely across parts of southeast Iowa. By D3, the low pulls away but some lake enhanced snow may develop across the U.P. and northern L.P. of MI where WPC probabilities for more than 6 inches reach 30-40%, bringing storm total across that area also up to 8-12 inches. Finally, there is a challenging potential for some over-performing snowfall within robust WAA bands surging across northern IN/IL and into lower MI late tonight into Tuesday. Temperatures will be marginal for heavy snow, but intense ascent could dynamically cool the column and allow for rapid accumulation despite low SLR. This could include the Indianapolis and Chicago areas, where, although total snowfall accumulations are likely to be modest as snow changes to rain by Tuesday morning, there could be a few hours of very heavy snowfall rates accumulating to several inches, despite low WPC probabilities. ..Northwest... Days 1-3... Nearly continuous precipitation into the Pacific Northwest during the next 3 days will manifest via waves of heavier precipitation in between bouts where it is less intense. The result of this will be widespread heavy snowfall in the terrain from the Olympics, through the Cascades, down into the Sierra, and as far east as the Northern Rockies and maybe even to the Tetons and Wasatch. Snow levels during the event will waver considerably, starting low but rising to around 5000ft by Tuesday morning behind a leading warm front, but then crashing almost as quickly back to 1000-3000 ft (lowest north) by Wednesday morning, and continuing to collapse thereafter behind a strong cold front. The primary driver of this extended period of unsettled weather is a series of shortwave impulses dropping southeast from the Pacific and onshore the Northwest before settling into the Great Basin. The first of these is progged to approach the PacNW coast late tonight before surging to the Northern Rockies on Tuesday, followed almost immediately by an event stronger impulse with more intense vorticity streaming into OR/CA by Thursday morning. These impulses will be embedded within otherwise confluent W/NW mid-level flow, and beneath the approaching LFQ of a robust Pacific jet streak impinging on the coast by Wednesday. This suggests that the most impressive deep layer ascent through height falls and upper diffluence will occur within a rapidly moistening column noted by IVT probabilities exceeding 80% for 500 kg/ms. Where the best synoptic lift can be aided by intense upslope flow through orthogonal mean wind, intense precipitation rates are likely. Additionally, regional forecast soundings beneath the amplifying upper trough indicate steep lapse rates up from the surface, supporting some elevated instability to further enhance precipitation rates, and where this is all snow, the WPC prototype snow band tool suggests snowfall could be 2-3"/hr at times, aided by SLR that is likely to be slightly above climatological means. The heaviest precipitation begins this evening, with one surge occurring on Tuesday, followed by a secondary surge on Wednesday, possibly a bit farther south than on Tuesday. For D1, this results in WPC probabilities for more than 6 inches reaching above 80% across most of the WA Cascades and some of the higher terrain of the OR Cascades. Farther east, WPC probabilities for more than 6 inches are 50-70% in the Blue Mountains and Northern Rockies. The first surge on Tuesday will then drive an expansion and intensification of the snowfall, with snow levels plummeting later in the day behind the cold front. WPC probabilities exceeding 80% for more than 12 inches along the Cascades, the Olympics, and Northern Rockies, with high probabilities for more than 6 inches extending into the northern CA ranges, and much of the terrain as far southeast as the Wasatch Front and Tetons. During D3 as the cold front sags farther to the southeast, and a final surge of moisture/ascent pivots onshore, WPC probabilities for more than 1 foot again eclipse 70% in the OR Cascades, and also extend into the northern Sierra. More than 6 inches of additional snow on D3 is likely in the Olympics, WA Cascades, Northern Rockies, and terrain of the Great Basin into the Wasatch. Additionally, with snow levels continuing to crash, light accumulations are possible even into most of the valleys across the Intermountain West as far southeast as the Four Corners. Storm total snowfall in the Cascades and Olympics could exceed 5 feet in some areas, with major to extreme impacts likely even at the passes due to blizzard conditions. Additionally, although confidence is quite low at this time, the CIPS snow squall parameter is highlighting the potential for snow squalls behind the cold front dropping into the inter-mountain West. If these convective snow showers or snow squalls develop, they could produce briefly extremely limited visibility and heavy snowfall rates which could severely impact travel Wednesday into Thursday. It will be worth monitoring how this evolves during the next few forecast cycles. ...Southern Appalachians through New England... Days 1-3... An extremely anomalous mid-level low centered over the Mid-Mississippi Valley Tuesday morning will continue to deepen as it shifts northeast towards the eastern Great Lakes by Wednesday morning. During this time, height anomalies are progged to reach the minimum climatological percentile in the database according to NAEFS, and while the anomalies weaken with time to the northeast, this system will remain exceptionally intense into D3. Downstream from this closed low, height falls and divergence will overlap with a strong coupled jet structure to help deepen an associated surface low as it tracks into the Great Lakes, with an occlusion to a triple point over New England likely thereafter. The result of this evolution will be exceptional moisture transport out of the Gulf of Mexico and up the eastern seaboard, noted by IVT anomalies reaching as high as +8 sigma near the DelMarVa Tuesday night, exceeding the maximum climatological percentile for the rolling 4-week period inherent to the NAEFS tables. The overlap of this extreme moisture and intense synoptic ascent will result in expanding heavy precipitation across the east, but with intense WAA accompanying the moisture surge, much of the area will be too warm for wintry precipitation. There will be two exceptions to this. The first is into the Southern/Central Appalachian foothills, along the Blue Ridge of NC/VA, and into the Allegheny Mountains/Laurel Highlands where the overrunning precipitation will initially encounter sub-freezing surface wet-bulb temperatures. This will result in an axis of sleet, transitioning primarily to freezing rain with some light to moderate icing likely. The rapid erosion of the cold high pressure combined with heavy rain rates which tend to runoff more easily than they can accrete ice will limit ice accumulations, but WPC probabilities for 0.1 inches of ice are 10-30% D1 into D2, highest along the VA Blue Ridge, before changing to rain. The other concern will be an area of very heavy snowfall in the highest terrain of from central PA through New England. Snow levels are progged to rise to as high as 4000 ft within the strong WAA/best IVT, so eventually nearly all of the area will turn over to rain below those levels. However, even as low as 2000ft precipitation will likely begin as heavy snow with rates of 1"/hr or more within the pronounced fgen driven by strong WAA before transitioning to rain in the Catskills, Adirondacks, Berkshires, and Greens and Alleghenies. Snowfall accumulation in these ranges should be modest as reflected by WPC probabilities for 4+ inches reaching 30-70% before changing to rain. However, above 4000 ft in the White Mountains of NH/ME, and even some of the lower elevations across northern/central ME, the event may stay primarily snow and accumulate significantly despite below-climo SLR in the warm/moist environment. WPC probabilities for snowfall are above 80% for 6+ inches, with local amounts as high as 1-2 feet in the highest terrain likely. Additionally, this will likely be very heavy and wet snow as reflected by WSSI-P probabilities for major impacts due to snow load eclipsing 50%, which could result in power outages in some areas, especially when combined with the increasing winds. ...Northern Plains... Day 3... A wave of low pressure will skirt along the Canada/North Dakota border late Tuesday into Wednesday, accompanied by modest WAA along a leading warm front. As this advects eastward, weak enhancement of moisture will be wrung out by the accompanying lift within the WAA and downstream of height falls associated with the driving shortwave aloft. This feature is likely to be transient and progress rapidly to the east, but a band of moderate snow is likely from central ND through northern MN. WPC probabilities are 10-20% for 4+ inches of snow. Weiss ***Key Messages for Jan 7-11 Major Winter Storm Southern Rockies through Midwest and Eastern U.S.*** --Hazardous Travel Expected in Areas of Heavy Snow Snow will develop in the Plains on Monday, and advance into the Midwest on Monday Night and Tuesday. Bursts of heavy snow may accumulate up to 2 inches in an hour, and result in hazardous travel. A heavy, wet snow will also develop in the interior Northeast Tuesday Afternoon and Night. --Extremely Dangerous Plains Blizzard A blizzard is expected in the plains of northeast New Mexico, eastern Colorado, the Oklahoma and Texas Panhandles, western Kansas, and southwest Nebraska. Wind gusts as high as 60-70 MPH will create ferocious blizzard conditions with whiteouts. Travel will become extremely dangerous to impossible. If you must travel, pack a winter survival kit as wind chills will plummet below zero. --Significant River, Flash, and Coastal Flooding Widespread and potentially significant river and flash flooding are likely for the central Gulf Coast and Eastern U.S. Monday through early Wednesday. Powerful onshore winds will lead to widespread coastal flooding along the eastern Gulf Coast and much of the East Coast. Significant coastal flooding is likely, especially on the Mid Atlantic coastline, including tidal rivers and bays. --Damaging Winds May Lead to Power Outages Widespread wind gusts in excess of 50 MPH are likely in the eastern Gulf Coast, central Appalachians, much of the East Coast, and New England. Prepare for power outages. ***Key Messages for Jan 8-11 Northwest Winter Storm*** --Major Winter Storm for the Cascades Back-to-back storms will cross the Pacific Northwest Monday night and Tuesday night, with several feet of heavy snow and strong winds across the Cascades. Blizzard conditions are likely. Snow levels rise to around 5000 feet Monday night with an atmospheric river but quickly fall to between 1500 and 2500 feet behind a cold front on Tuesday, leading to considerable impacts for many mountain passes with the second storm. --Heavy Snow Elsewhere in the West Over a foot of snow is likely (70-90%) in the higher elevations of the Northern Rockies on Tuesday and Wednesday. Snow levels falling to below 1000 feet later on Wednesday and early Thursday should lead to some snow accumulations in valleys as well. --Storm Will Move to the Central U.S. The storm system is forecast to dig across the Western U.S. through Thursday and reach the Plains late in the week. People in the Plains and Midwest should prepare for another winter storm and stay tuned for updates.