Area Forecast Discussion
Issued by NWS Greer, SC

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769 FXUS62 KGSP 241154 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 654 AM EST Fri Jan 24 2020 .SYNOPSIS... Moist flow over a cool wedge of high pressure will make for a chilly, rainy day today. A cold front will cross the area tonight bringing in drier air. Seasonally cool and dry conditions will persist over the weekend. && .NEAR TERM /THROUGH TONIGHT/... As of 645 AM: High pressure centered over northern Quebec is ridging down the Eastern Seaboard this morning. Water vapor imagery shows a deep cutoff low over the Ozarks, which is expected to slowly wobble eastward into the lower Ohio Valley today. A broad plume of moisture is being advected north out of the Gulf in advance of this low. Sfc temps wet-bulbed down very close to freezing across much of the foothills overnight, allowing precip to fall as a cold rain in almost all areas. Aloft a strong warm nose suggested freezing rain was likely where temps fell below freezing, but only a few ridgetop obs actually dropped that low. A few automated obs in lower elevations have occasionally reported UP which suggests sleet pellets mixing in from time to time. The warm nose should only strengthen the rest of the morning, and temps warm slightly. No impacts from wintry precip are expected aside from those few ridges which could see some slight ice accumulation. 850mb winds of 40-50 kt will continue with the southerly flow into the occluding system, and this jet will gradually traverse the mountains and Escarpment through early afternoon. A Wind Advisory covers the impacts of gusts from this flow, which will not mix down too remarkably well within the WAA. Significant enhancement to rainfall rates however will occur which means isolated spots along the south-facing Escarpment, and in the Balsams, are likely to see more than 2 inches rain for a storm total. Minor flooding is possible in these areas. Along and south of the wedge boundary, sfc-based instability will develop within the warm sector of the system. While the major deterministic models show this instability staying south and east of our CWA, the 00z HREF ensemble max SBCAPE rises above 100 J/kg in the eastern SC and southern NC Piedmont zones, so it is looking like a close call. The strong midlevel flow results in 0-3 km shear values up to 50 kt immediately ahead of the cold front as it moves into those areas around or soon after sunset. Coupled with a low LCL forecast, when SPC issued the new Day 1 Outlook they saw fit to place a sliver of our far southeast CWA into the Marginal Risk area for the associated tornado risk. NW-flow snowfall is expected to develop late tonight in the wake of the cold front, along the Tenn border. No significant accumulation is forecast with unfavorably warm temp profiles in the layer of lift, and winds taking their time to become orthogonal. East of the mountains, the drier incoming airmass will bring PoPs to near zero after midnight, though some of the hi-res WRFs depict stratus and fog developing as the cold air moves over saturated ground, beneath a subsidence inversion. Some cloud cover thus is fcst to stick around thru daybreak in parts of the Piedmont. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... As of 130 AM Friday: Deep layered forcing continues to weaken early during the short range. The mean sfc-h5 flow becomes w/ly to nw/ly across the FA with meager amts of moisture affecting the wrn upslope regions of the NC mtns. Dropped soundings across the spine indicate a stg warm nose and very shallow llvl moisture...yet the entire sounding will be below freezing...so no p/type concerns outside of snow possibly transitioning to rain across the nrn NC mtns late while available llvl moisture decreases. The llvl flow across the central NC mtns including the Smokies will generally be w/ly which will limit topo lift and subsequent snowfall amts thru early Sun. For now...have generally a half inch or less and mainly confined to the border region. The winds will be defined nw/ly across the nrn NC mtns so even with lowering moisture...expect around an inch of snow mainly the higher elevations like Beech Mtn. Overall the event looks to remain sub-advisory yet slick roads will be possible both Sat/Sun mornings. A reinforcing cP airmass continues to mix in Sat/Sun which will keep max and min temps right arnd normal. By Sun night...a srn stream low will advance toward the FA area. The models have sped this system up and there/s decent agreement with light precip reaching the wrn zones after 00z. The srn mtns look to saturate up to h5 while sfc temps drop below freezing. So...have introduced accum snow aft 03z thru the period with up to an inch anticipated across the srn/cent mtns and less so up the spine thru daybreak. && .LONG TERM /MONDAY THROUGH THURSDAY/... As of 235 AM Friday: Continued snowfall across the NC mtns will yield little additional accum before the sfc layer rises abv freezing by late morning. Expect arnd a half inch or so generally abv 3.5 Kft. The models develop a more robust cP high building into the region Mon afternoon...so expect dry conds earlier in the period. Made some sigfnt changes to PoPs over the mid-week period. The latest guidance has come in much drier wrt the evolution of a srn stream system. The GFS and the ECMWF have suppressed a moist sfc low further south than the previous runs while the 00z CMC is a curious outlier...which continues to bring the low across the nrn GOM with a good deal of moisture and diurnal snow over the NC mtns Wed and Thu. Thus with some uncertainty...have left slight chance PoPs in with non/accumulating -snsh across the higher terrain. Max temps will likely rise a bit abv normal each day as good insol and a w/ly downslope component offsets a reinforcing polar airmass. && .AVIATION /12Z FRIDAY THROUGH TUESDAY/... At KCLT and elsewhere: Warm advective flow over entrenched in-situ CAD will mean LIFR will be the norm today, possibly lifting to IFR later today. The heaviest precip is expected in the late morning. LLWS criteria may be met at times across the Piedmont from late morning to late afternoon, which has prompted a mention at KCLT. A rumble of thunder is possible in the same timeframe across the area, but chance of TS is too small to include. Furthermore a very small chance of a storm producing damaging winds exists around sunset. Cold fropa will bring precip to an end from west to east this aftn and evening. A strong inversion is depicted as lingering into Saturday morning above the PBL, which will allow cigs to persist until close to daybreak. Cold advection over wet ground may also generate fog. Outlook: Dry conditions will persist Saturday through early next week. Confidence Table... 12-18Z 18-24Z 00-06Z 06-12Z KCLT High 87% High 84% Med 64% Med 69% KGSP High 83% Med 78% Med 65% High 87% KAVL Med 75% Med 69% High 85% High 87% KHKY Med 77% High 83% Med 64% Med 60% KGMU High 81% Med 73% High 81% High 94% KAND High 84% Med 66% High 87% High 94% The percentage reflects the number of guidance members agreeing with the scheduled TAF issuance flight rule category. Complete hourly experimental aviation forecast consistency tables and ensemble forecasts are available at the following link: www.weather.gov/gsp/aviation && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...Wind Advisory above 3500 feet until 4 PM EST this afternoon for NCZ033-048>052-058-059. SC...None. && $$ SYNOPSIS...08 NEAR TERM...08 SHORT TERM...SBK LONG TERM...SBK AVIATION...08

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