Area Forecast Discussion
Issued by NWS Greer, SC

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000 FXUS62 KGSP 032335 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 635 PM EST Thu Dec 3 2020 .SYNOPSIS... Low pressure developing across the Deep South tonight will cross our area late tomorrow into early Saturday before developing into a Nor`easter off the Mid-Atlantic coast. An upper air disturbance could bring light precip to the region Monday. Seasonably cool and generally dry weather is otherwise expected for much of the next week. && .NEAR TERM /THROUGH FRIDAY/... 630 PM Update...Nearly steady state conds continue in high cloud cover and hipres dominating the sfc pattern. The far sw/rn NC mtns and NW GA may see a few brief showers and sprinkles over the next few hrs associated with moist sw/ly return flow enhanced by weak topo lift. As of 200 PM Thursday: The main weather interest into Friday is associated with an upper low that was centered over the mid Mississippi Valley. The upper low will slowly swing east trough the Tennessee Valley while gradually phasing with stronger northern stream energy. Continued weak warm air advection and increasing cloud cover ahead of this system tonight will keep most areas above freezing with 30s to near 40 common. Improving upper dynamics and fetch off the Gulf of Mexico will support cyclogenesis along a cold front over the northern Gulf tonight. The low pressure system will race northeast through the day, pushing into our portion of the I-85 corridor early in the evening. Rain ahead of this feature will overspread the area from the southwest through the day, possibly becoming moderate to heavy at times in the afternoon across the mountains. Fortunately, guidance keeps wet-bulb values above freezing by the time precipitation would start so it appear we will be dealing with plain ole rain. Even though the warm sector of the system tries to work into far eastern areas during the afternoon and evening, the most aggressive guidance has minimal instability, so in coordination with neighboring offices have removed thunder mention from the forecast. Warm air advection and increased cloud cover will roughly cancel out for highs again in the 50s to near 60 east of the mountains Friday with mainly 40s across the mountains. && .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/... As of 225 PM Thursday: The aforementioned surface low will continue to rapidly intensify as it departs the area Friday evening. Rain will gradually come to an end east of the mountains during this time as drier air filters in behind the departing cold front, enhanced by downsloping flow off of the mountains. Rainfall totals will generally be less than an inch, with one to locally two inches possible across the escarpment and southwest mountains. A brief northwest flow event may occur on the back-side of this system into the first part of Saturday. However, only marginally cold air will be especially chasing the best moisture out of the area. In fact the moisture may become shallow enough that a period of sleet or freezing rain may mix in at times focused across the highest elevations due to a lack of ice nuclei. An inch or two of snow will be possible above 5000 feet with a dusting possible due around 3500 feet nearest the TN border. Breezy but sub-advisory northwest winds and drying conditions expected to continue through the day Saturday. Weak high pressure will briefly build into the area Saturday night into Sunday. Good radiational cooling conditions will support lows generally in the mid 20s to low 30s. Daytime highs will continue to be mainly in the 50s, although 30s to 40s across the mountains on Saturday. Clouds may begin to increase late in the day Sunday ahead of our next potential system. && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... As of 150 pm Thursday: It appears likely that the complex split flow pattern will become phased across the East either late in the short term or early in the extended...with the timing of this process depending upon which global model one prefers. The timing and details of this evolution will have implications on the sensible weather for our forecast area early next week. The hot-off-the- presses ECMWF looks very similar to the 12Z Canadian in phasing a northern stream wave into the southern stream across the Deep South Sunday night/early Monday. The result of this far-south trajectory is cyclogenesis that occurs almost in the central Gulf, with a track into south Florida and minimal precip impacts for our area. While the GFS upper air evolution looks quite a bit different, with phasing of the two streams occurring later and farther northeast, it too depicts surface development well to the south. The difference lies in the fact that the GFS has a much more robust/clipper-like upper low crossing the forecast area on Monday, resulting in a round of northwest flow showers, some of which could break containment into the foothills and Piedmont considering how far the mid-level speed max is depicted. Having said all that, either of these scenarios support some potential for light precip across the forecast area on Monday, with the GFS scenario being a little more favorable for possible wintry precip. Even so, this looks to be a very low-at-best qpf event, and slight chance pops are about as far was we`re willing to go at this point. Otherwise, the extended period will be dry and seasonably cool, with some moderation in temps likely by Day 7, when global models generally agree in building an upper ridge across the East. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/... At KCLT and elsewhere: Flight conds will deteriorate during the TAF period as a highly moist and dynamical low approaches the area and interacts with an insitu wedge of hipres. Expect areas of showers to begin across the wrn terminals shortly aft daybreak and then become more widespread thru the early afternoon over all sites. Lowering CIGS into the IFR and LIFR range will become widespread by the afternoon and persist thru the period. Some lowering of VSBY into the MVFR/IFR range will also occur within heavier rain bands. The gust potential will be low as sfc mixing remains very shallow while winds generally align se/ly to sw/ly thru the evening. Outlook: SHRA will come to an end Saturday with a return to VFR over most sites by the afternoon. Breezy northwest winds will develop Friday night and continue into Saturday. Confidence Table... 23-05Z 05-11Z 11-17Z 17-18Z KCLT High 100% High 100% High 100% High 89% KGSP High 100% High 100% Med 70% High 100% KAVL High 100% High 100% High 100% High 94% KHKY High 100% High 100% High 100% High 94% KGMU High 100% High 100% High 80% High 100% KAND High 100% High 100% High 80% 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...JDL NEAR TERM...Munroe/SBK SHORT TERM...Munroe LONG TERM...JDL AVIATION...SBK

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