Area Forecast Discussion
Issued by NWS Greer, SC

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000 FXUS62 KGSP 041804 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 104 PM EST Fri Dec 4 2020 .SYNOPSIS... Low pressure will cross our area this afternoon into early Saturday before developing into a Nor`easter off the Mid-Atlantic coast, bringing rain to most areas and light snow to the mountains. Another fast-moving system could bring light precipitation to the region Monday. Seasonably cool and generally dry weather is otherwise expected for much of the next week. && .NEAR TERM /THROUGH TONIGHT/... As of 1247 PM Friday...the leading edge of the precip band has broken up somewhat over the intervening few hours, but expect as the forcing increases rapidly late today, it will blossom and fill back in, so the trend toward very high precip probs still looks good. Have made a few adjustments on the back end this evening as the upper/sfc low deepen rapidly as they exit the fcst area. Will re-evaluate thunder chances across the CLT metro area in light of the updated SPC Day 1 moving the wrn edge of the Marginal perilously close. Still like the idea of a small thunder chance over the Lakelands/lower Piedmont for elevated storms. There is increasing concerns over a period of strong wind gusts this evening, which will be evaluated for the afternoon package as well. Otherwise, an upper low centered over the mid-Mississippi Valley will slowly swing east thru the Tennessee Valley while gradually phasing with stronger northern stream energy. Improving upper-lvl dynamics and fetch off the Gulf of Mexico will support cyclogenesis along a cold front over the northern Gulf. The low pressure system will race NE today, with the center of the low expected to move right over the CWFA this evening. Rain ahead of this feature will overspread the area from the SW thru the day, possibly becoming moderate to heavy at times this aftn/evening across the NC Mountains and Piedmont. Temps should remain too warm to support anything but rain, and the majority of guidance has minimal instability at best, so thunder remains unlikely. As the low lifts NE of the CWFA early Saturday, the gradient wind increases out of the W and NW, and we can expect some light NW-flow snow showers over the NC Mtns. The higher peaks could see some wind gusts approaching Advisory criteria, but these speeds should be relegated to elevations of roughly 5000ft and above. In addition, snow accums are not expected to exceed an inch for most of the impacted areas, although higher amounts are possible over the northern mtns and along the TN border thru 18z Saturday. && .SHORT TERM /SATURDAY THROUGH MONDAY NIGHT/... As of 200 AM Fri: Moisture profiles in the NW upslope areas near the Tenn border suggest that any snow showers will continue for a portion of the day Saturday, tapering off as the moisture is depleted. Winds will continue to be brisk through the day at those higher elevations. A relatively weak sfc high will fill in thereafter; the upper flow becomes quasi-zonal if not slightly anticyclonic on Sunday. It gets a little complicated late Sunday into Monday as one or more shortwaves rotate around the departed low to our north, and into the Southeast. The major models remain in some disagreement on how these waves phase, but the orientation of the upper jet would seem to support good dynamic lift across the CWA even if the phasing isn`t perfect; this looks likely to result in the shortwaves congealing into an upper low that reaches its deepest state over the CWA around midday Monday. A remaining appreciable difference is in how much moisture is brought in ahead of the southernmost shortwave, which originates in the ArkLaTex region. The ECMWF and Canadian GDPS are wetter in this sense, suggesting deep moisture is replenished sooner and beats the cold air into the area, so to speak. The GFS is a bit drier and colder to start, but a bit wetter with the upslope precip on the backside of the system, so it ends up a bit snowier overall. It also hints at a touch of Piedmont snow as the low departs. The NAM does not yet encompass the whole event but appears to feature a moisture trend more like the EC/CMC camp while also being colder. The NAM/GFS differences however may be attributable to their higher resolution. Went with a blend of PoP and temp trend guidance that favors mainly slight-chance values due to the low confidence, with a more EC/CMC-like timing. This keeps almost all of the snow mention over the mountains and foothills; it looks like another event where the same forecast trends that warrant higher PoPs likely will warrant generally warmer temps. Even with the EC/CMC solutions having better moisture initially, the continental airmass preceding the system doesn`t allow for really good moisture fetch and the event is a pretty quick hit. Thus QPF, and the resulting snow accums, are currently small. Temps initially will run just a little below normal. Above-normal mins are expected Sunday night under increasing moisture, and significantly below-normal temps Monday and Monday night with the ongoing precip giving way to CAA. && .LONG TERM /TUESDAY THROUGH THURSDAY/... As of 310 AM Fri: Sfc high pressure looks to dominate the pattern over the Southeast from Tuesday through early next Friday, suggesting a relatively quiet period. The initial East Coast upper trough will shift offshore late Wednesday. The Euro does show another trailing shortwave diving toward the Gulf around this time, but it depicts cyclogenesis too far south and east to affect us; this depiction may be something to watch as that could change, but for now not worthy of any PoP. Temps moderate back above normal by Day 7. && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/... At KCLT and elsewhere: Busy for the next 12 hours as a very dynamic upper low forces the deepening of a surface low moving across the western Carolinas this evening. Conditions will generally be MVFR due to a low cloud ceiling this afternoon ahead of a band of showers moving in from the west. The shower activity is expected to expand and fill in as the strong mid/upper forcing comes in to play this afternoon. The convection-allowing models are in good agreement with the timing of this band of mainly showers moving east across the fcst area between roughly 21Z and 03Z, with some shower activity lingering over wrn NC into the early morning hours. Will employ a 4-hr TEMPO group to account for the higher intensity of precip and low clouds bringing conditions down to IFR at least temporarily. We lack sfc-based instability, which will limit thunder chances, and the stable shallow layer of air near the sfc will also limit our wind gust potential. In this situation, the winds will be howling from the S/SW this evening, but from roughly 3k feet and higher. Note this is not low enough to incorporate in the TAF as a low level wind shear, which by definition is in the lowest 2k feet. However, it will be very close, and that strong wind shear will exist somewhere in the 2k to 5k foot layer, with SW winds on the order of 40-50kt for a period ramping up around 21Z and then dropping off rapidly with the back end of the precip after 01Z. We should remain MVFR for a few hours this evening. East of the mtns, a rapid clearing will take place followed by a wind shift to WNW or NW behind the cold front. Right now this is timed at 06Z at KCLT, but could occur earlier. Meanwhile, at KAVL, the low cloud ceiling may persist through the early morning hours as low clouds blow up the valley, bringing a few showers. All sites should be VFR from roughly the start of morning ops onward. Winds will be gusty from the NW over the mtn valleys, but gusts are not as likely outside the mtns. Outlook: VFR for most of the weekend. Moisture will return Sunday night with a ceiling restriction likely on Monday as another low pressure system lifts out of the Gulf and up the Atlantic Coast. Confidence Table... 18-24Z 00-06Z 06-12Z 12-18Z KCLT High 83% High 80% High 100% High 100% KGSP Med 79% High 89% High 100% High 100% KAVL High 87% High 82% Med 77% High 95% KHKY High 88% High 90% High 100% High 100% KGMU High 82% High 90% High 100% High 100% KAND Med 75% High 87% 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/PM SHORT TERM...Wimberley LONG TERM...Wimberley AVIATION...PM

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