Area Forecast Discussion
Issued by NWS Greer, SC

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000 FXUS62 KGSP 031144 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 644 AM EST Thu Dec 3 2020 .SYNOPSIS... A moderating high pressure area remains over the area today. A low pressure system will cross the area Saturday. A return to cool and generally dry weather is expected next week. && .NEAR TERM /THROUGH TONIGHT/... As of 645 AM Thursday: Still on track with the current forecast trend through the near-term. Some morning fog has started showing up over Lake Hartwell and surrounding lakes. This not expected to spew out past the bodies of water. High clouds from the west have started to enter the far western zones, so a slightly faster trend in bringing in clouds were updated in the grids. Expect the mid- to upper-level cloud deck to increase through the day. Temperatures should bottom out into the mid to upper 20s by sunrise for most locations with very good radiational cooling and light winds. Otherwise, another pleasant day will be in store for the area on Thursday as the sfc high axis pushes across SC during the morning hours which will back low-level winds from a NW-ly to a S-ly component by the afternoon as the inversion mixes out. A closed upper low has settled over the Southern Plains recently and will gradually push eastward, which is our next weather producer. Shortwave ridging today will allow the area to moderate in temperatures for Thursday and Thursday night, but quickly slides across the region with SW-ly flow aloft building in ahead of the upper low. Mid- to upper-level clouds and moisture advection really stack the deck Thursday night into Friday. The associated frontal boundary and precip with the upper low won`t impact the cwa during the near-term and more information on how everything pans out will be discussed in the short-term. Highs for Thursday will be in the 50s for most with a couple of spots in the mountains struggling to get out of the 40s. Lows will be in the 30s to low 40s in the southern tier. Chance PoPs will be back in the forecast for the SW facing slopes and Smokies after 06Z Friday. && .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/... As of 245 AM Thursday: Good agreement with the guidance on the short term, the main difference is the ECMWF remains faster with the movement of the low pressure system. Therefore, have discounted the timing of the ECMWF and have gone with a slower solution. An upper low moves out of the Southern Plains into the MS Valley Friday. The low moves to the Southern Appalachians Friday night and to the Delmarva Saturday. Surface low cyclogenesis takes place to our west with the low moving into the TN valley Friday. The low then moves by just to our north Friday night bringing a cold front into the area. Guidance now shows the warm sector air mass moving into at least a portion of the CWFA from the south with some sbCAPE developing over or near the I-77 corridor Friday night into early Saturday. With very strong shear and helicity over the area, any location where sbCAPE can be tapped would have a chance for high shear/low CAPE severe convection. As always, the chance of this is low and highly dependent on where/if any sbCAPE develops. Even with only a low severe threat, breezy to windy conditions are expected across the area as the front moves through with a tight pressure gradient and increasing low level winds. The quick movement of this system and lack of strong convection keeps overall QPF on the moderate side, generally less than an inch outside of the mountains. Across the mountains, 1 to 2 inches are possible producing a low end isolated flood threat, especially if there area stronger convective elements and some training. Similar caveats apply to the flood threat as the severe threat. As the cyclone becomes increasingly wrapped up as it moves away from the area, cold advection and lingering moisture from the TN Valley into the southern Appalachians should result in at least a brief period of northwest flow snow showers on Saturday, with minor accums possible across the higher elevations near the TN border. CAA and NW winds are not especially strong or prolonged, so for now, it looks to be a minor event at the highest elevations. Highs will be near normal Friday then fall to around 5 degrees below normal for Saturday across the mountains but remain near normal elsewhere. Lows 5 to 10 degrees above normal Friday night fall to around 5 degrees below normal Saturday night. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... As of 320 AM Thursday: A complex/split upper air flow pattern will persist through the medium range, with troughing generally predominant over the East. As such, temps are forecast to be a little below normal until Wednesday when heights rise as the trough axis moves east bringing temps back to normal. There is agreement that an upper low/clipper type system crosses the area in the Monday to Tuesday time frame. In fact the trough digs far enough south that some precip breaks containment into the Piedmont and foothills. In collaboration with neighboring offices, we have introduced slight chance PoP for the mountains and CLT metro area. There is disagreement on QPF and thermal profiles, but should this happen, a rain/snow mix would be possible. A dry forecast is expected for the periods before and after this system. && .AVIATION /12Z THURSDAY THROUGH MONDAY/... At KCLT and elsewhere: VFR conditions will be in store through the TAF period. Light winds area variable for now with the exception of KAVL that still holds a NW-ly component. As the inversion mixes out in the morning winds will back to a SW to S-ly component by lunchtime for most sites. Fog isn`t likely across any of the TAF sites, but mountain valleys and lakes will likely receive some kind of reduction in vsby due to isolated fog development. Sfc high will be in control for half of the TAF period, but mid- to upper-level clouds have already started to roll in and will continue to increase into the nighttime period as the sfc high pushes offshore. Restrictions of any kind are not expected until late Thursday night into Friday. Outlook: VFR conditions will prevail until the next system approaches the area late tonight into Friday. VFR conditions should prevail once again over the weekend. Confidence Table... 11-17Z 17-23Z 23-05Z 05-06Z KCLT High 100% High 100% High 100% High 100% KGSP High 100% High 100% High 100% High 100% KAVL High 100% 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...RWH NEAR TERM...CAC SHORT TERM...RWH LONG TERM...RWH AVIATION...CAC

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