Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 222 PM EST Mon Jan 02 2023 Valid 00Z Tue Jan 03 2023 - 00Z Fri Jan 06 2023 ...Western U.S.... Days 1-3... An active pattern continues across the west as weak shortwaves move onshore within generally zonal flow D1-2, before a more impressive impulse within yet another atmospheric river (AR) comes into the West on D3. For tonight through Wednesday, two modest shortwaves will lift into the West - the first tonight across CA, with a second impulse lifting into OR/WA Tuesday night. These will be accompanied by weak upper diffluence for additional ascent, with snow levels generally hovering around 2000-4000 ft, higher south. Moisture anomalies will be modest through Wednesday with GFS indicating PWs around +0.5 standard deviations, highest across the Southwest, supporting heavy snow on D1 across the Sierra, Mogollon Rim, CO Rockies, and other portions of the Four Corners and southern Great Basin, with forcing and moisture generally waning D2 to produce a reduction in coverage. WPC probabilities D1 are high for more than 6 inches across these areas on D1, with D2 probabilities becoming confined just to the Sierra and Shasta/Trinity/Siskiyou region as the next AR approaches late. The more impressive trough and accompanying AR shifts into the region late D2 and D3 with probabilities for IVT above 500 kg/m/s exceeding 90% moving into CA according to GEFS and ECENS. Snow levels will climb within strengthening WAA downstream of a closed low which will be just offshore at the end of the forecast period, reaching as high as 7000 ft in CA and 4000-6000 ft elsewhere. Ascent driven by WAA will be enhanced by divergence/height falls in the mid-levels, intensifying LFQ diffluence at the edge of the approaching Pacific jet streak, and upslope enhancement on orthogonal flow into the Sierra. Impressive snowfall is likely, with and WPC probabilities for more than 6 inches are already above 90% in the northern CA ranges and the Sierra, where more than 2 feet is likely in the higher terrain. ...New England... Day 3... An impressive closed mid-level low over the Upper Midwest will shed vorticity lobes eastward around it and through the base off the accompanying longwave trough to produce ascent into New England on Thursday. A secondary jet streak blossoming downstream of the primary trough axis will strengthen and advect northward from the Southeast, providing additional synoptic ascent as diffluence improves atop the shedding vorticity energy. This should result in a modest low pressure moving through the eastern Great Lakes, with WAA ahead of it driving widespread precipitation across New England and Upstate New York. Initially this will all be rain, but cold air draining southward from a surface high positioned south of Hudson Bay will keep at least central and northern New England cold enough for precip to change to wintry p-types, with snow, sleet, and freezing rain all possible. The guidance is highly variable and uncertainty remains into how the column will evolve to support snow, sleet or freezing rain, but it is looking more likely that at least impactful wintry precip is likely on Thursday. WPC probabilities for 4 inches or more of snow are as high as 20-30% across interior Maine, and reach as high as 50% for 0.1" of freezing rain in the northern Adirondacks and St. Lawrence River Valley, with lower probabilities extending into much of central New England. ...Central Plains and Upper Midwest... Days 1-2... A shortwave ejecting from the Four Corners tonight will rapidly amplify into an impressive closed low with 700-500mb heights falling to -2 to -3 standard deviations below the climo mean according to the NAEFS ensemble tables. As this low deepens during its northeast trek from eastern CO through the western Great Lakes, an accompanying upper level jet streak will amplify as it surges northward from the Southern Plains downstream of the primary trough axis. This will place favorable cyclonic LFQ diffluence atop the closed low, providing robust ascent to drive a deepening surface low across the region. The strongly meridional flow downstream of this low will advect copious moisture northward, noted by PW anomalies surging into the Upper Midwest that reach +4 standard deviations according to NAEFS, with GFS climatological return probabilities reaching as low as 0.5 into the Upper Midwest on Tuesday. This moisture is reflected by pronounced 290-295K isentropic upglide with mixing ratios of 5-6 g/kg lifting northward into the system, and it is almost certain a major winter storm will bring heavy snow and freezing rain to the Plains and Upper Midwest through Wednesday, with today's guidance increasing available QPF and resultant snow and ice even further. As the low consolidates and deepens across the Central Plains tonight, impressive WAA will spread an expanding shield of precipitation northward from Kansas/Nebraska into Iowa/Minnesota/South Dakota. This will manifest as snow across the colder regions, rain to the south, and a mix of sleet and freezing rain where the warm nose moves across the cold surface temperatures. As this WAA continues to shift northward coincident with the advance of the theta-e ridge, an impressive TROWAL will wrap cyclonically within the WCB around the low, helping to increase instability which will already be surging northward noted by MUCAPE > 100 J/kg. This will help to drive theta-e lapse rates below 0C/km coincident with an axis of impressive -SEPV, suggesting an impressive band of heavy snow with thunder developing beneath the best deformation. While a band of snowfall is also likely on the leading edge of WAA translating northward, the heaviest snow is progged within this pivoting band which is likely to move across northern NE, southeast SD, and far southwest MN overnight Monday into Tuesday. Soundings in the vicinity of this band indicate an extremely favorable setup for heavy snow rates noted by fgen driving intense omega into a deepening DGZ, producing a cross-hair signature, and aided by instability aloft. Both the HREF probabilities and WPC snow band tool indicate a likelihood for 2-3"/hr snowfall rates, and where upright convection can assist, would not be surprised to see 3-4"/hr rates in localized areas. The guidance has trended a bit SW with this band today, and the heaviest snowfall is progged from north-central NE through far southwest MN. Here, WPC probabilities for 8+ inches are above 80%, and it is likely several locations will come in above 18 inches where this impressive band pivots the longest. Surrounding this heavy snow axis, widespread 4+ inch snowfall probabilities above 30% exist from the NE Plains of Colorado through Minneapolis/Twin-Cities and into the western U.P. of MI where a translating band of heavy snow is likely as well. There has also been a noted increase in wind within area soundings and supported by CIPS analogs. While the winds do not look quite strong enough to fracture the dendrites to lower SLR and reduce accums, it could result in significant blowing and drifting of snowfall to enhance travel impacts despite modest SLR in the extremely moist environment. While WPC probabilities D2 are modest overall, they have increased along the Bufalo Ridge, southern MN, and far NW IA where a secondary deformation axis may develop. Confidence is low on this evolution, but WPC probabilities for more than 4 inches are mow 30-50%, and locally much higher snow is possible if this band develops as shown by the HRRR/NAM due to higher SLRs. South/southeast of the heaviest snow, an axis of heavy freezing rain is also extremely likely, although some uncertainty remains into just how efficiently ice can accrete. The first part of D1, 00Z-12Z Tuesday, should feature extremely efficient accretion of freezing rain with modest precipitation falling atop cold surface temperatures in the mid to upper 20s, combined with increasing wind and at least modest dry advection from the northeast to offset latest heat release of freezing. As the WAA intensifies it will be accompanied by increasing instability as well, resulting in heavier precipitation rates which will offset accretion through runoff and reducing the ability of dry advection to maintain a favorable environment for freezing rain. Still, a significant freezing rain event is likely and WPC probabilities for more than 0.25" are 30-40% from far northeast NE through much of northern IA and into parts of WI and MN. Weiss ...Key Messages for January 1-3 Winter Storm... --A large swath of moderate to very heavy snow, significant freezing rain, and some sleet is expected across the Plains through Tuesday, continuing across the Upper Midwest through Wednesday. --Intense snow rates of 1-3"/hr (accompanied by thunder) are quite likely from parts of Nebraska and South Dakota into Minnesota early Tuesday. Snowfall will rapidly accumulate to more than 12 inches over this region. --Gusty winds around 30 mph will produce areas of blowing and drifting snow throughout the day on Tuesday. This will result in snow-covered roads, reduced visibility, and difficult to impossible travel. --Localized significant ice accumulations exceeding 0.25" are likely from portions of eastern Nebraska through southern Minnesota. This may lead to dangerous travel, scattered power outages, and tree damage.