Area Forecast Discussion
Issued by NWS Greer, SC

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000 FXUS62 KGSP 041537 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 1037 AM 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/...
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As of 1035 AM Friday...the leading edge of the conveyor belt of precip was gradually working its way eastward into the mtns, while the area east of the Blue Ridge was still free of precip, or at least anything more than a few passing sprinkles. Will amend the precip probs for today to better describe the arrival time of the main precip band. Overall, this will not change the 12-hr chance though. The SPC has general thunder over the southeastern part of the fcst area, and there is some chance of elevated storms in that area, so some thunder will be added mainly to the Lakelands and lower Piedmont for the afternoon and early evening. Temps look ok now. 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.
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&& .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 /16Z FRIDAY THROUGH TUESDAY/...
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At KCLT and elsewhere: Conditions expected to deteriorate thru the 12z TAF period as a moist and dynamical low develops over the fcst area and interacts with an in-situ wedge of high pressure. Showers will become more widespread across the fcst area thru the afternoon/evening with CIGS lowering into the IFR and possibly LIFR range. Some lowering of VSBY into the MVFR/IFR range will also likely occur within heavier rain bands. The gust potential will be fairly low-end as sfc mixing appears shallow with winds generally aligning SELY to SWLY thru most of today. They will eventually veer around to WLY and increase in speed early Sat. The main exception will be KAVL, where winds will take on a more NWLY direction with stronger gusts in the 20 to 26kt range. Outlook: VFR conditions should return over most sites by late Sat morning. Breezy WLY to NWLY winds will continue thru late Saturday. Another round of restrictions is likely early next week as another low pressure system lifts out of the Gulf and up the Atlantic Coast. Confidence Table... 15-21Z 21-03Z 03-09Z 09-12Z KCLT High 83% Med 78% High 86% High 100% KGSP High 95% High 90% High 91% High 100% KAVL High 85% Med 70% Med 76% High 87% KHKY Med 76% High 86% High 85% High 100% KGMU High 100% High 85% High 90% High 100% KAND High 90% Med 66% 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
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&& .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...Wimberley NEAR TERM...JPT/PM SHORT TERM...Wimberley LONG TERM...Wimberley AVIATION...JPT

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