Area Forecast Discussion
Issued by NWS Greer, SC

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000 FXUS62 KGSP 050831 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 331 AM EST Sat Dec 5 2020 .SYNOPSIS... In the wake of a cold front, light snow and brisk winds will continue this morning in the North Carolina mountains near the Tennessee border. Another fast-moving system could bring light precipitation to the region Monday, and possibly another round of light snow for the mountains. Seasonably cool and dry weather is otherwise expected through Friday. && .NEAR TERM /THROUGH TONIGHT/... As of 320 AM Saturday...the last of the rain showers are currently moving east of the I-77 corridor with lingering showers streaming over the NC/TN border region. Still expecting the rain showers to gradually transition to snow showers over the next few hours as frz levels drop. Still anticipate sub-Advisory level snow accumulations, however some localized areas across the Smokies and Tenn spine could receive up to about 3 inches before the snow tapers off later this morning. Wind gusts up to roughly 40 mph will continue over the higher terrain as well this morning, with the strongest gusts above 4000ft or so. They will gradually taper off thru the aftn as the pressure gradient weakens overhead. Otherwise, the upper trof axis will continue to lift NE and off the Atlantic Coast today/tonight. This will result in a brief, nearly zonal upper pattern towards the end of the period early Sunday before the next round of upper-lvl energy approaches the fcst area from the north and west. At the sfc, the low pressure system will continue to move northward and up the Atlantic/New England Coast as broad high pressure spreads back over the region in its wake. This will make for a sunny and breezy Saturday for most of the CWFA with temps outside the mtns, near normal, if not a couple degrees below. Highs across the higher terrain will be 1 to 2 categories below normal. Clouds and light NW-flow snow showers will linger over the higher terrain and especially along the NC/TN border thru late morning. Any accumulations should remain below Advisory criteria, with the exception of a few isolated spots along the NC/TN border. As we move into the aftn, guidance has bndy layer moisture diminishing across the higher terrain and precip ending. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... As of 200 AM Sat: Near-normal temps and near zero PoPs are expected Sunday as weak high pressure remains over the region. The upper flow will begin to turn cyclonic ahead of a shortwave crossing the middle Ohio Valley. This wave is embedded in a deep eastern CONUS trough; additional shortwaves are positioned upstream and will rotate closer to the CWA by Monday. Still some disagreement among the major models on how they will phase. The NAM, ECMWF, and Canadian GDPS all phase the first wave with the southernmost one, resulting in enough forcing for light precip over a portion of the CWA early Monday; the unphased GFS depiction is dry for us at that time. The cooling should occur from the top down, and the wetter models also depict warmer temps. Thus it appears likely most of the area still will be warm enough to advertise rain, except in the higher elevations. Cold advection will continue through Monday, although most of the guidance depicts a lull in dynamic lift. Thus, precip chances will diminish for a time, except in the NW Flow upslope areas, as a fair amount of low-level moisture will persist as winds turn NW. Temps will remain cool enough across some of the mountains to expect accumulating snow to continue thru the day. Cloud cover and CAA will keep max temps considerably below normal. The final shortwave looks like it will have cut off by the time it crosses the southern Appalachians Monday afternoon or evening; precip chances increase again across the area again as a result, and temp profiles will become increasingly supportive of wintry precip across the lower elevations as time goes on. The challenge is to determine whether the upper low will be associated with enough moisture to carry precip downstream of the mountains, and in that case, if it will be deep enough for ice crystallization. For our Piedmont, it still looks more likely than not that the answer to one of those questions will be "no," although the GFS and ECMWF are both pretty close. The GFS has better moisture but the ECMWF has cooler wet-bulb profiles. Most likely, however, it looks most likely precip will end in the Piedmont before temps cool enough Monday evening. Min temps Tue morning will dip into the 20s at most locations, several degrees below normal. && .LONG TERM /TUESDAY THROUGH FRIDAY/... As of 240 AM Sat: One more trailing shortwave is progged to traverse the area Tuesday as upper trough shifts offshore, but without any moisture to speak of. A broad ridge will replace these features, bringing dry wx and moderating temps to the South Wednesday thru Friday. Maxes will be a couple categories above normal Thu and Fri, in the upper 50s and lower 60s for the mtn valleys and Piedmont. One or more shortwaves will reach the Plains by the end of the work week; GFS and ECMWF both depict cyclogenesis such that a cold front approaches the area Friday night. The CMC GDPS instead shows the sfc low crossing the Great Lakes. Given this disagreement, and this development being very close to the end of the period, precip chances are being kept below the mentionable threshold thru 12z Sat. && .AVIATION /09Z SATURDAY THROUGH WEDNESDAY/... At KCLT and elsewhere: Over the past hour or so, the shower activity has moved east of our taf sites with skies clearing as the sfc low lifts farther NE. As such, most sites have returned to VFR including KAVL where cigs are currently BKN040. Sites should remain VFR for the remainder of the taf period with some patchy fog possible thru the overnight and into the morning. Winds should remain brisk enough to offset the fog threat over most sites, however MVFR to IFR visby may occur before sunrise at KAVL. Otherwise, winds will remain out of the W to NW thru the period with speeds weakening some thru the morning and then picking back up modestly during the aftn outside of the mtns. KAVL will likely continue to see gusts in the 20 to 25kt range thru the morning with gusts tapering off by the aftn. Outlook: VFR conditions expected for most of the weekend. Moisture will return Sunday night with cig restrictions likely on Monday as another low pressure system lifts out of the Gulf and up the Atlantic Coast. Confidence Table... 08-14Z 14-20Z 20-02Z 02-06Z KCLT High 100% High 100% High 100% High 100% KGSP High 100% High 100% High 100% High 100% KAVL High 83% High 100% High 100% High 100% KHKY High 100% High 100% High 100% High 100% KGMU High 100% High 100% High 100% High 100% KAND High 100% High 100% High 100% High 100% The percentage reflects the number of guidance members agreeing with the scheduled TAF issuance flight rule category. Complete hourly experimental aviation forecast consistency tables are available at the following link: www.weather.gov/gsp/aviation && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...Wimberley NEAR TERM...JPT SHORT TERM...Wimberley LONG TERM...Wimberley AVIATION...JPT

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