Subj : HVYSNOW: Probabilistic Heavy Snow And Icing Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Nov 25 2023 09:14:13 FOUS11 KWBC 250914 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 413 AM EST Sat Nov 25 2023 Valid 12Z Sat Nov 25 2023 - 12Z Tue Nov 28 2023 ....South/Central Plains... Day 1... As snow winds down across the Rockies today, the second phase of this event will become the focus across the Central Plains from Kansas into Iowa. As the closed mid-level low over the Four Corners opens and begins to fill, it will eject eastward and may manifest to a neutral tilt as a lobe of vorticity swings through its base across OK and KS tonight. This will occur in tandem with at least peripheral LFQ diffluence from the upper jet streak ejecting to the south of the region. A wave of low pressure developing beneath this vorticity max will move east, resulting in increasing downstream theta-e advection northward, some of which will overrun a leading warm front producing moist isentropic upglide to expand precipitation. At the same time, low/mid level fgen will intensify beneath an axis of favorable deformation, and this will likely produce bands of heavier snowfall within the broader precipitation shield. Atmospheric cross sections suggest a high threat for CSI across KS within any bands, and this is collocated with the best WPC prototype snow band tool probabilities for 1"/hr rates. While the most intense snow rates and heaviest accumulations are likely to be over Kansas, there is still a modest signal for a continuation of a translating band into Missouri and Iowa, where SREF probabilities for DGZ depths exceeding 100mb have increased to 10-30%, but both the signal for CSI and the resultant WPC snow band tool probabilities decrease rapidly into the Upper Midwest. WPC probabilities for 6+ inches of snow are generally 40-60%, highest across central KS, with locally more than 8 inches possible in any linger banding. Farther northeast, WPC probabilities for more than 2 inches are as high as 40% near Kansas City, and 5-10% towards the Quad Cities, Iowa. Along the transition zone/surface baroclinic gradient, some of the aforementioned overrunning precipitation could result in light freezing rain. Again, the regional forecast soundings seem to support more IP than ZR, so amounts will likely be light where freezing rain does occur. Some additional accretion, albeit minor, is also possible later today as the DGZ may begins to dry out while low level saturation persists which could set up very light ZR/ZL over west-central OK late Saturday. WPC probabilities for more than 0.1" of ice peak around 20% in portions of N-Central OK. ....Great Lakes and Northern New England... Days 2-3... A significant lake effect snow (LES) event is likely beginning Sunday as much colder air drops across the still warm Great Lakes. A sharp, but positively tilted, shortwave is expected to deepen as it tracks across Manitoba, extending a trough axis and resultant height falls into the Upper Midwest by Sunday aftn. This feature is the progged to take on a negative tilt by Monday morning as a potent vorticity maxima rotates through its base and into the Northeast, while secondary vorticity energy shifts into the trough from upstream. This will produce significant cyclonic flow across the eastern third of the country, with the coldest air beneath this trough advecting over the Great Lakes on Monday characterized by 850mb temps crashing to -10C to -15C. This will occur across lake surface temperatures that are generally +5 to +12C as reflected by GLERL, producing steep lapse rates through an inversion depth approaching 15 kft, and SBCAPE possibly exceeding 1000 J/kg in some areas. With flow slowly backing from WSW to NNW, this will likely result in expanding areas of lake effect snow in the favored NNW snow belts. Lake effect snow is likely to begin first in the U.P of MI Sunday afternoon behind the accompanying surface cold front, and then spread through the L.P. Sunday night, eventually developing east of Lakes Erie and Ontario by Monday morning. Once LES begins, it may persist through the end of this forecast period, waning only as a brief period of WAA on brief shortwave ridging moves across the region on Tuesday. The heaviest LES is likely D2 across the U.P. and western portions of the L.P. of MI where WPC probabilities for more than 4 inches are above 80%, especially in the Porcupine and Huron mountains, as well as near Traverse City, MI. If bands set up correctly, D2 amounts in these areas could be double this threshold, however. More significant LES is progged to develop late D2 and especially on D3 when DGZs crash concurrently with their depth increasing, while forcing remains quite robust over the lakes. Additionally, flow on Monday could support some effective fetch from an upstream connection providing additional moisture downstream of Lakes Erie and Ontario where snowfall could be quite heavy. Snowfall rates will likely exceed 1"/hr, and WPC probabilities D3 for more than 4 inches are highest along the Chautauqua Ridge and Tug Hill Plateau, where they exceed 80%. Much heavier snowfall totals are possible in these areas, with additional moderate to heavy snowfall accumulations likely in the other favored NW snow belts downstream of all the Great Lakes. There remains some uncertainty into the amount of wind shear and exact wind direction, but in some places this setup appears to be favorable for significant and impactful snow accumulations, especially late Monday through Tuesday. This negatively tilting trough and accompanying vorticity maxima shifting east over the Great Lakes and into New England will help drive surface cyclogenesis the New England coast Monday. The track of this low continues to pivot a bit farther NW/inland, resulting in a marginal thermal structure for any wintry precipitation. However, the column should support at least a period of heavy snow across the terrain of northern NH and northern ME as plentiful moisture shifts inland Monday before the low pulls away Monday night. WPC probabilities for 4+ inches reach 20-40% from the White Mountains of NH through much of the elevated portions of northern ME. ***Key Messages for Thanksgiving Weekend Winter Storm*** --Heavy snow bands in Kansas today An area of moderate snow will expand across Kansas today and tonight with embedded heavier bands containing 1"/hr snowfall rates. This snowfall has a high (70-90%) chance of producing more than 4 inches of snow, with locally higher amounts exceeding 8 inches possible (10-20% chance) in the heavier snow bands. --Hazardous travel due to snow, wind, and icy roads Locally heavy snow combined with gusty winds will cause hazardous travel due to snow covered roads and reduced visibility from central Kansas through northern Missouri. Some light icing could also cause slippery roads across northern Oklahoma, especially on bridges and overpasses. --Cold conditions for the Rockies and Plains Temperatures are expected to be 10-20 degrees below normal through Sunday, with wind chills falling into the single digits or below 0 at times. Weiss $$ = = = 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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .