Area Forecast Discussion
Issued by NWS Greer, SC

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000 FXUS62 KGSP 280548 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 1248 AM EST Sun Feb 28 2021 .SYNOPSIS... A front stalled south of our region will gradually lift northward as a warm front through Sunday. A cold front will approach the southern Appalachians from the northwest Sunday night and cross the area on Monday. Dry high pressure moves from the Great Lakes to the mid Atlantic coast Monday night into Tuesday before unsettled weather returns from the west Tuesday night into Wednesday. A stronger storm system could arrive by Friday. && .NEAR TERM /THROUGH TODAY/...
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1230 AM Update...A llvl ridge has built in from the NE and trapped high moisture at the sfc from previous rainfall. Thus, dense fog is persisting across the NC zones east of the mtns this morning. A southern expansion of dense fog is possible thru the morning hrs as winds weaken and a DFA is in place thru 13z to account for this potential. As of 940 PM: The subtropical ridge near the area will maintain WAA thru tonight, which will amplify as sfc cyclogenesis occurs over the Midwest. This will lead to abundant cloud cover once again, with areas of fog, some dense across much of the area outside of the mountains tonight. Dense fog has been patchy and short lived, but could become widespread and constant enough for a Dense Fog Advisory overnight. A small PoP exists overnight over much of the mountains and northern NC Piedmont via the same mechanisms; upslope forcing along the southern Escarpment and adjacent mtn ridges will further enhance chances there. Lows will be 15 to 20 degrees above normal with some locations seeing steady or even slightly rising temps. The fog and low stratus will only slowly erode Sunday morning, with fog dissipating by late morning but clouds lingering possibly into early afternoon. After scattering, moisture higher aloft is expected to maintain mostly cloudy conditions across the board. With the wedge finally dissolving in the morning, that should allow deeper mixing; midlevel winds will be fairly strong in the gradient between the Midwest low and the Atlantic ridge. This should result in a breezy/gusty afternoon, with temps reaching well into the 70s in many locations, a few degrees shy of records at our climate sites. RH will not be low enough for this to pose any fire danger concerns.
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&& .SHORT TERM /TONIGHT THROUGH TUESDAY/... As of 205 pm Saturday: Frontal zone will be approaching the forecast area at the start of the period, then will sag southeast across the area throughout Monday. A rather vigorous precip band is expected to accompany the front across the TN Valley into the southern Appalachians, with likely-to-categorical pops forecast across the NC mtns by daybreak Monday, with high pops overspreading the remainder of the forecast area throughout the morning. Sort term model guidance continues to reveal a strong signal toward weakening of the precip region as it move across our area, as low level convergence diminishes due to disruption of the flow pattern by the high terrain, and as deeper forcing remains north of the forecast area. In fact, by early afternoon, it appears likely that there won`t be a whole lot left of the frontal band as it`s pushing east of the area. All this being the case, any excessive rainfall concerns Sun night/Monday should be confined to the areas closer to the TN border, and even there the threat appears marginal at best. Guidance continues to depict scant instability (i.e., sbCAPE < 100 J/kg) in the warm sector ahead of the front. However, the weakening forcing profiles do not support much of a high shear/low CAPE severe storm threat, nor do the shear profiles, which are rather inadequate given the paltry buoyancy. Otherwise, high temps will generally occur during the morning across the mtns, with temps steady (valleys) or slowly falling (peaks/ridge tops) throughout the day. As usual, downslope warming will offset any late morning/afternoon CAA across the Piedmont/foothills, allowing max temps to again warm to above normal...by about 5 degrees or so. The remainder of the short term will see a return to normal...if not a slightly below normal temps and dry weather. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... As of 220 pm Saturday: A split flow regime will dominate the pattern across much of the Conus, with an active southern stream likely bringing two rounds of precipitation during the period. Fortunately, there is more of a consensus among deterministic global model guidance regarding these two events, particularly an event that is expected to unfold in the Tue night/Wed time frame, when a compact, but rather potent upper low is forecast to move across the TN and OH Valleys into the Appalachians. The GFS continues to be weaker/more suppressed with this feature, but much less so than in yesterday`s runs, giving us the confidence to bump pops into the likely/categorical range for much of the forecast area after midnight Tue night, with pops quickly tapering off from the west Wed morning. Current indications are that sufficient cool air will be left in place by retreating surface high to create a wintry precip concern across the mtns and perhaps the I-40 corridor in the foothills/Piedmont. However, the absence of any signal of a strong cold air damming event/mech to lock in cold air suggests such an event would likely be of the nuisance variety. Guidance is also in better agreement regarding potential impacts of a southern stream system later in the week, although differences remain regarding the details. Nevertheless, the GFS and ECMWF/Canadian camps identified yesterday are definitely trending toward one another, with a slightly more potent/wetter system depicted in the ECMWF/Canadian, and a much less potent/not nearly as wet and more phased flow solution per the GFS. As such, pops are warranted Friday through early Saturday, but these generally remain only in the 30-40% range. Temps are forecast to remain around, if not slightly below climo through the period, although a brief return to above-normal conditions may return Thursday ahead of the late week storm system. && .AVIATION /06Z SUNDAY THROUGH THURSDAY/...
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At KCLT: Dense fog and VLIFR conds will persist at KCLT this morning as a shallow llvl ridge has setup within highly moist sfc layer. Not anticipating much improvement until after sunrise into mid morning when the BL warms, but there shud be a quick transition to MVFR or VFR arnd 16z when gusty conds and increasing llvl mixing commences. Winds will remain locked in sw/ly ahead of an approaching wrn bndry and the gusty conds will reduce aft 22z-23z. Elsewhere: KHKY will have the best chance of locking in dense fog and VLIFR restrictions this morning as other sites remain a little better mixed and will likely see VSBYs in the IFR to LIFR range arnd daybreak, except at KAVL where down-valley flow and lack of a shallow inversion may keep conds in the VFR to MVFR range. Low-end sw/ly gusts are likely this afternoon and will diminish after 23z. KAVL will see the best chance of precip thru 06z with a moist frontal bndry approaching the TN/NC line. Thus, a PROB30 with -shra and possible IFR restrictions are mentioned aft 02z thru the period. Outlook: Periodic flight restrictions and chances for precip are expected to linger Mon into Mon night. Drier high pressure will build back into the area Tuesday. Confidence Table... 05-11Z 11-17Z 17-23Z 23-00Z KCLT High 100% High 90% Med 66% High 80% KGSP High 89% High 90% Med 78% High 95% KAVL High 100% High 85% Med 66% High 95% KHKY High 94% High 80% Med 66% High 90% KGMU High 100% High 90% Med 72% High 90% KAND High 89% High 90% Med 78% High 80% 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
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&& .GSP WATCHES/WARNINGS/ADVISORIES...
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GA...None. NC...Dense Fog Advisory until 8 AM EST this morning for NCZ035>037- 056-057-068>072-082-502-504-506-508-510. SC...Dense Fog Advisory until 8 AM EST this morning for SCZ001>010.
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&& $$ SYNOPSIS...DEO NEAR TERM...RWH/SBK/Wimberley SHORT TERM...JDL LONG TERM...JDL AVIATION...SBK

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