Subj : HVYSNOW: Probabilistic Heavy Snow And Icing Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jan 03 2025 09:01:02 FOUS11 KWBC 030900 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 400 AM EST Fri Jan 3 2025 Valid 12Z Fri Jan 03 2025 - 12Z Mon Jan 06 2025 ....Western U.S....=20 Days 1-3... Precipitation will continue to spread inland across western Oregon=20 and northwestern California ahead of an occluded low approaching the=20 coast this morning. Deep, moist southwesterly flow ahead of the=20 boundary, along with strong forcing will support an uptick in=20 precipitation, with orographically focused moderate to heavy amounts=20 forecast to develop along the coastal ranges into the southern=20 Cascades and northern Sierra Nevada. However, high snow levels are=20 expected to confine any heavy snow accumulations to the highest=20 peaks early on. Then, as the upper trough and associated cold front=20 begin to move inland, snow levels will begin to lower, expanding the=20 potential for heavy snow along the southern Cascades, the=20 northwestern California ranges, and the northern Sierra later today.=20 Meanwhile, a well-defined warm front, lifting across the northern=20 Cascades, Intermountain West, and Rockies, will be a focus for=20 organized precipitation and locally heavy mountain snow as well=20 today.=20 Meanwhile to the east, moisture focused by low-to-mid level=20 convergence along the western periphery of strong high pressure=20 extending from central Canada into the Northern Plains will support=20 a swath of light to moderate snows extending from central Nebraska=20 northwestward through the High Plains and into the northern Rockies.=20 Upslope flow is expected to help enhance amounts across the central=20 to the northwestern Montana ranges.=20 By early Saturday, the upper trough is expected to bring high=20 elevation snow into the central Rockies as it moves across the Great=20 Basin into the region. This will be followed by a second system,=20 which is forecast to bring additional orographically focused heavier=20 amounts into the Olympics and the northern Cascades Saturday into=20 early Sunday. Conditions will remain unsettled across the Northwest into Monday,=20 as a third shortwave impacts the region. However, a building=20 upstream ridge will preclude any long fetch of deep moisture and the=20 threat for widespread heavy amounts across the region. WPC probabilities indicate that three day totals exceeding 8 inches=20 will be mostly confined to the higher elevations of the Cascades,=20 northern Sierra Nevada, and portions of the Rockies from=20 northwestern Montana to northern Utah and Colorado. Widespread, but=20 lighter amounts are expected across central and southeastern=20 Montana, with high probabilities for accumulations greater than 4=20 inches covering much of the region.=20=20 ....Great Lakes and Northeast...=20 Days 1-2... The strengthening -NAO block over Greenland and the North Atlantic is aiding a mature surface cyclone to stall over Quebec and help produce persistent cold northwest flow between it and a strong surface high over Saskatchewan. This will prompt favorable LES on northwesterly flow along with a few upper level disturbances traversing the Great Lakes over the next few days. The Great Lakes will gradually cool as a result, but the lapse rates will remain=20 steep enough over and downwind of the lakes to where lake-induced=20 instability could top 500 J/kg on occasion. The snow belts of the=20 Michigan U.P., northern portion of Michigan's Mitten, from=20 northeast Ohio on north east along the Chautauqua Ridge, and from=20 the shores of eastern Lake Ontario to the Tug Hill will be most=20 favored for heavy snow through early Sunday AM. Favorable fetch connecting a band from lakes Superior, Huron, and Ontario will help produce the most significant amounts (1-2ft+) in Upstate NY between Syracuse and the Tug Hill. Snow could also be measured in feet=20 from northeast OH, northwest PA, and western NY (south of Buffalo).=20 Snowfall rates in these areas of 1-2"/hr are expected and could come in waves over the next few days. The WSSI shows Major Impacts for these areas with even a small Extreme area (extremely=20 dangerous to impossible travel and widespread closures) in the Tug=20 Hill through Sunday AM.=20 ....Central Appalachians and Mid-Atlantic... Day 1... A compact wave of low pressure moving across the Ohio Valley early this morning will move progressively eastward beneath a modest=20 shortwave racing southeast within the broad trough amplifying=20 across the eastern CONUS. Immediately in the wake of this=20 shortwave, a potent jet streak reaching 150kts will dig rapidly out of the Northern Plains, producing a narrow but intense corridor of overlapping ascent through height falls, PVA, and LFQ diffluence.=20 This will help at least subtly deepen the low and the accompanying=20 baroclinicity/attendant fronts as it moves eastward to off the VA=20 coast Friday night. The system is compact and progressive, so snowfall in general is=20 expected to be low outside of the Central Appalachians. Here,=20 despite the modest forecast depth of the DGZ, appreciable upslope=20 flow will drive ascent favorably into the DGZ, while some moisture=20 enhancement occurs on post-frontal NW flow from the Great Lakes.=20 This suggests an extended period of moderate to heavy snow,=20 especially D1, with the higher elevations receiving the most=20 significant snow accumulations. Event total snow here could reach=20 double- digits in the higher terrain. Finally, although total accumulations are expected to be minimal, some snow showers or convective snow showers moving along the cold front=20 may crest the Appalachians and move across the Mid-Atlantic states Friday afternoon as the vorticity max dives southeastward. The=20 late afternoon timing of this vort max allows for steep lapse rates from the sfc all the way up into the DGZ, which could promote brief snowfall rates above 1"/hr as shown by low probabilities in the WPC snowband tool. Low-level temperatures will be slightly above=20 freezing from Richmond, VA to Philadelphia, PA, but wet-bulb temperatures below 0C could allow any heavier rates to accumulate on roads and elevated surfaces, causing hazardous travel despite=20 minimal accumulations, during the Friday aftn/eve commute. Additionally, where low-level instability is the greatest, especially overlapping the higher moisture content focused along the MD/PA line, isolated snow squalls are also possible. ....Northern High Plains... Days 1-2... The forecast remains on track and very similar to the previous issuance as an amplified upper level trough moving into the=20 Pacific Northwest this evening will spread downstream ascent=20 through increasing divergence into the Northern High Plains by=20 Saturday morning. This feature is progged to split into dual=20 vorticity lobes, with the primary one swinging into the Central=20 Rockies, while a secondary impulse lifts into Alberta/Saskatchewan. Downstream of this trough, weak shortwave ridging will initially=20 be placed over the region, but this will gradually become displaced by the approaching trough, leading to increasingly strong deep-=20 layer ascent. This forcing will act upon a saturating column as=20 290K isentropic ascent maximizes in a narrow corridor from NW to SE from the foothills of central MT east into the High Plains of SD,=20 which will effectively overlap with a stripe of robust 700-600mb=20 fgen. The overall setup is not ideal for extremely heavy snow, but=20 an expanding footprint of snowfall is likely, and rates will be=20 enhanced by a cold column with very fluffy SLRs expected.=20 Additionally, the guidance has become more excited about snowfall=20 potential as reflected by an EFI of 0.8 to 0.9, primarily due to an extended period of moderate to at times heavy snow. WPC=20 probabilities have increased as well, and now feature a 70-90%=20 chance for 4+ inches across central MT, with locally up to 10=20 inches possible in some of the higher terrain, and a 30-50 chance=20 for 4+ inches across far NW SD.=20 ....Central Plains through the Mid-Atlantic... Days 2-3... Guidance remains aligned on a significant winter storm to impact a large area spanning from the Central Plains across the Ohio Valley and to the Mid-Atlantic Saturday night through Monday. While there continues to be considerable temporal and some latitudinal spread=20 in the models, the overall evolution and footprint is quite=20 similar, with impactful snow, sleet, and freezing rain expected. Starting Saturday afternoon, a deepening shortwave will eject from the Central Rockies and move into the Southern Plains by Sunday=20 morning while taking on a negative tilt. At this point there is=20 already considerable differences in the timing and intensity of the upper low, with the EC/UKMET and accompanying ensemble the=20 fastest, the GFS/GEFS making up the slow/strong end of the=20 envelope, and the CMC/CMCE in the middle. That being said, the answer likely lies somewhere in between but could lean towards either camp. The ensemble sensitivity analysis from 12z 1/2 would suggest the differences begin this afternoon as the trough moves ashore the West Coast, so these differences should be brought closer together in a few more forecast cycles. The WAA regime in place ahead of the storm without a strong surface high to the=20 north- northeast would support some of the more northern solutions, but the upper- level configuration in the western Atlantic limits=20 this potential. Regardless, significant spread in timing and=20 intensity results in lower than typical confidence for D3. Despite uncertainty in timing and placement, confidence remains in an impactful winter storm. Regardless of the exact placement, the=20 setup is favorable for a widespread wintry precipitation from the Central Plains/Mid-Mississippi Valley by the end of D2 through the Ohio Valley D3 and eventually Mid-Atlantic D3-D4. As the upper low tilts negatively over the central U.S. and then closes off, a=20 subtropical jet streak will sharpen and rotate around the base of=20 this trough leading to coupled downstream ascent. At the same time, increasing downstream moist advection on low-level flow from the=20 Gulf of Mexico will rapidly saturate the column as isentropic=20 ascent along the 295K surface lifts northward and then pivots=20 cyclonically into a TROWAL around the developing cyclone.=20 As this low intensifies, in conjunction with the strengthening=20 WAA, the setup seems to conceptually support a laterally=20 translating band of heavy precipitation from west to east. With=20 DGZ depth probabilities rising to above 30% for 100mb of depth, and a an overlap of folding theta-e with -EPV in cross sections supporting CSI/CI, an impressive band with snowfall rates at least 1-2"/hr appears likely. Additionally, a secondary axis of=20 deformation could lengthen the heavy snow in some areas with=20 more impressive snowfall rates. There is still uncertainty as to where the transition zone between heavy freezing rain, sleet, and snow will occur but the EFI is=20 highlighting values exceeding 0.9 with an SoT of 2 from eastern KS through southern IN, and the WSSI-P is suggesting a greater than=20 70% chance for moderate impacts in this same geographic window. Given the strong WAA and deep low-level cold airmass in place, sleet could become a dominant ptype from parts of eastern KS through southern IN/northern KY, with over an inch of sleet accumulation possible for some areas. In addition to impacts, although confidence in exact placement is uncertain, WPC probabilities current reflect a high risk (>70%) for 8+ inches near the NE/KS border through northern MO and into central IL, with a stripe of high probabilities for 6+ inches=20 extending east along I-70 through IN and west-central OH. On the=20 south side of the system, WPC probabilities indicate high chances (70-90%) for at least 0.1" ice accretion in a stripe from eastern=20 KS through southern MO and into southern IL, southern IN and much of KY. 0.25" ice probs of 40-60% are found from southern MO to KY. This system has prompted the issuance of key messages which are=20 linked below. Pereira/Snell ....Winter Storm Key Messages are in effect. Please see current=20 Key Messages below... https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!5H5hnZtkp-rhOKjNWO-8xlb6XuPlMQYf5zBVWlPiut_jM= 3Voua0gBjlHmCCAm4LHwDzSYF2hgcUK7B6VKkxupQIWpz0$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .