Area Forecast Discussion
Issued by NWS Greer, SC

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000 FXUS62 KGSP 271948 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 248 PM EST Sat Feb 27 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 SUNDAY/...
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As of 210 PM: The CWA remains under a decaying CAD setup, with parent high pretty much out of the picture and low-level winds veering such that low-level upglide is pretty much history. However, tell that to the low cloud trapped over the area beneath subsidence inversion, which has kept most of the CWA under low stratus or stratocu so far today, which has kept a number of automated sites from rising out of the 40s. Strong insolation and a remarkably warm subtropical airmass are gradually doing their thing, gradually thinning/narrowing the cloud layer, but we effectively remain under the cooler wedge airmass at the sfc. Obs at the sunny southern Upstate sites have now surged into the 60s--though much more stubborn to warm compared to areas just a little further south on the other side of the wedge boundary. The NC mtn valley sites are in the upper 60s to near 70 with the additional aid of downsloping SW winds. It`s difficult to accurately say how much of the cloud cover will be able to erode before sunset, but some of it likely will remain in place. The max temp fcst this aftn and potentially overnight min fcst will depend considerably on that, which limits our confidence thru tonight. Meanwhile, a convergence zone over the upper TN Valley and near the Appalachians to our north will maintain a small chance of light precip for our northern mtns and vicinity for the remainder of the aftn, probably as sprinkles or weakly convective showers; thunder no longer expected. While the areas that see the low cloud deck scatter should have a few hours of mostly clear sky, the subtropical ridge will maintain WAA thru tonight, which will amplify as sfc cyclogenesis occurs over the Midwest. This is expected to lead to abundant cloud cover once again, likely joined by fog in the NC Piedmont and possibly the eastern part of the Upstate, where the shallow fragment of the wedge should remain. Dense fog is not out of the question given the pattern and guidance consensus. 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. The low stratus looks likely to erode late Sunday morning, though 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 /SUNDAY NIGHT THROUGH TUESDAY/...
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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.
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&& .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
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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.
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&& .AVIATION /20Z SATURDAY THROUGH THURSDAY/...
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At KCLT and elsewhere: Low stratus/stratocu deck associated with decaying CAD regime will make for a tricky forecast this aftn at all sites except KAVL. The initially widespread LIFR deck has begun to scatter/lift in some spots but has remained essentially unchanged since daybreak elsewhere. LAMP guidance is having trouble keeping up with the trends, it seems. Have leaned pessimistic in the first few hrs, but gradual improvement does look likely this aftn across the SC sites and KCLT. Erosion of the deck should allow VFR to persist into evening. A weak front will stall across the area late today, being reactivated as a warm front tonight by low pressure over the mid-MS Valley. Combined with a moist PBL, that looks likely to bring back IFR-LIFR cigs and fog (possibly dense) by early morning to most of the area. This likely won`t scatter/lift until nearly midday Sunday. Winds will be fairly light and mainly southwesterly today/tonight; gusts will develop Sunday, especially following the erosion of the low clouds, with deep mixing in the presence of fairly strong midlevel winds in the warm sector of the aforementioned low. Precip chances are too low to mention this period. Outlook: Flight restrictions and chances for precip are expected to return Sunday and linger into early next week. Drier hipres will build back into the area by Tuesday. Confidence Table... 19-01Z 01-07Z 07-13Z 13-18Z KCLT High 82% High 87% Low 59% Med 71% KGSP High 88% High 94% Med 77% Med 77% KAVL Med 72% High 100% High 87% High 93% KHKY Med 79% Med 76% Med 62% Med 66% KGMU High 87% High 95% High 81% High 80% KAND High 85% High 95% High 83% High 87% 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... GA...None. NC...None. SC...None. && $$ SYNOPSIS...DEO NEAR TERM...Wimberley SHORT TERM...JDL LONG TERM...JDL AVIATION...Wimberley

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