Area Forecast Discussion
Issued by NWS Greer, SC

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000 FXUS62 KGSP 232353 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 753 PM EDT Mon Apr 23 2018 .SYNOPSIS... A slowly moving low pressure system will continue to bring moist flow in from the south into Tuesday. The atmosphere will generally dry out Wednesday before another moist area of low pressure crosses the region Thursday through Friday. && .NEAR TERM /THROUGH TUESDAY/... As of 750 PM EDT Monday: Heaviest rainfall has shifted east of the area as the warm conveyor belt has moved east. Still, weaker moist upglide remains over the area with patchy rain moving through. Analysis shows wedge still in place across the area with instability to our west and south. this is keeping the thunderstorms over GA from maintaining themselves as the move toward the area. Looks like the chance of convection will remain limited as best overnight. There will still be pockets of heavier rain moving across the area, so will leave flood watch in place, but any flooding will be from slow rises. Winds aren`t especially strong across the higher elevations, but have had reports of some trees down due to the combination of wind and wet soils. Will leave the wind advisory in place as well. Temps are generally on track with only some minor adjustments. Otherwise, what appears to be in-situ type cold air damming has set up across the area, which is reflected in the solid surface NEly flow in the wedge of high pressure and a deck of low cloud across the entire area. A wedge of low-level dry air has worked into the eastern part of the forecast area, but cloud cover has prevented insolation all day and scouring of the wedge at this point in the afternoon seems unlikely, especially considering the substantial isentropic upglide and strong northeasterly flow at the surface. This is reflected in model CAPE fields, that have a safe boundary between our area and any instability that has been able to develop to our south. Strong SEly 850mb winds are still in place over the FA, and with gusts to 45kt+ still being reported at the highest peaks, high elevation wind advisory for the NC mountains is still well-placed. The advisory was extended to go until 600Z (2am) tomorrow morning due to suspicion that gusts may not drop off quite as quickly as previously thought, as models have been hanging on to stout 850mb flow past midnight. The best pressure gradient will begin to relax after midnight, but will do so relatively slowly, keeping gusts elevated across much of the forecast area overnight. A triple-point low is in the early stages of development over south/central Georgia, and will continue to undergo cyclogenesis through the evening and tonight as it moves towards the northeast. This low should help to scatter out the wedge early tomorrow morning as it moves through our forecast area, though with the upper low and its attendant moisture making little progress eastward by tomorrow, chances for showers and even potentially a thunderstorm in our far southern and eastern zones will remain through the end of the near term period. && .SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/... As of 220 PM EDT Monday: the short term fcst picks up at 00z on Wednesday with a closed upper low opening back up over the region as a broader area of trofiness persists across much of the Eastern CONUS. By late Wed/early Thurs, the above upper trof lifts NE of the area as another upper trof digs down across the ArkLaTex region. This trof axis is expected to be approaching the CWFA as the period ends late Thurs. At the sfc, the large low that has been affecting the region is expected to be centered right over the Carolinas or a bit north of there. By early Thurs, the low will lift northward and towards New England and deepen in the process. In the low`s wake, broad high pressure will gradually move over the area from the west as an area of deeper moisture lingers to our west. For the remainder of the period on Thurs, the sfc high appears to weaken as another low tries to spin up over the Southeast. The models are not very clear wrt the pattern evolution for the later part of Thurs, with the GFS keeping drier air over the fcst area compared to the other operational models. The NAM spins up another sfc low along the Northern Gulf Coast on Thurs, with the other models hinting that a low will develop over that area. As for the sensible fcst, PoPs will taper off on Wed with only a small amount of QPF expected over the CWFA. PoPs increase again on Thurs as the deeper moisture mentioned above moves over the area and another sfc low tries to develop to our southwest. Temps will be right around climatology on Wed with highs dropping about a category below normal for Thurs. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... of 230 PM Monday: The extended forecast starts at 00z Friday with a progressive full-latitude trof pushing thru the Plains and Midwest. This trof will be picking up a southern stream wave over the central Gulf states, lifting that feature across the CWFA early Friday. The GFS has generally been the strongest with this wave and even inducing a compact, but moist, sfc low over the Carolinas. The last few runs of the EC had stubbornly stayed weaker and drier for Friday, but now the 12z run has come in a lot like the 12z GFS. The latest SuperBlend has a shotgun 30-40% CHC of rain on Friday, and this looks like a good compromise. However, if the GFS is right, PoPs will likely need to be higher. Either way, QPF shouldn`t be enough to cause any flooding problems. The main trof will then push into the area Friday night, with the southern end trying to break off somewhat and slow down over the Carolinas. Again, the GFS is the most bullish on this, and shows continued showery activity thru the day on Saturday. Meanwhile, the EC is fairly dry with the passage of this wave. I have some lingering PoPs into Saturday. Temps both days will feature a depressed diurnal curve with slightly above normal min temps and slightly below normal max temps. The model guidance is generally in good agreement on the longwave upper trough finally shifting east, as an upper ridge moves in across the MS Valley to the Great Lakes Sunday thru Monday. Dry weather and a warming trend back to slightly above normal temps is expected by the end of the extended fcst period. && .AVIATION /00Z TUESDAY THROUGH SATURDAY/... At KCLT and elsewhere: Cigs and vsby generally varying between MVFR and IFR this evening as patches of moderate to heavy rain move north across the area. Expect cigs to fall to LIFR overnight with IFR cigs as rain diminishes in intensity but good coverage continues. Gusty ENE wind continues as well, SE at KAVL. Expect some improvement across the west after daybreak and at KCLT for the afternoon. However, MVFR cigs may linger through the day with IFR in showers or TSRA at KCLT. Gusts should diminish during the morning but direction remaining NE for all but KAVL where they go N and KAND where they go W. Outlook: Moisture lingers along with unsettled weather on Wednesday and Thursday as a trough of low pressure persists across the eastern part of the country. Confidence Table... 00-06Z 06-12Z 12-18Z 18-00Z KCLT High 84% High 88% High 86% Med 78% KGSP Med 74% High 86% High 87% Med 74% KAVL High 81% High 94% High 94% High 87% KHKY High 82% High 89% High 92% High 87% KGMU Med 64% High 92% High 88% Med 70% KAND Low 56% High 87% Med 77% Med 70% The percentage reflects the number of guidance members agreeing with the scheduled TAF issuance flight rule category. Complete hourly experimental aviation forecast consistency tables and ensemble forecasts are available at the following link: www.weather.gov/gsp/aviation && .HYDROLOGY... Although precipitation rates do not appear sufficient in any 3 to 6 hour windows to produce flash flooding, duration flooding remains possible, especially in the most upslope preferred locations and over the adjacent foothills. Storm totals of 3 to 4 inches are still expected, with localized 4 to 6 inch amounts in the most upslope preferred areas. Flooding of the most common flood-prone and low-lying areas may steadily develop from the southwest today, with main stem flooding of the upper French Broad River and associated tributaries quite possible. Flooding of main stems and streams in the Upstate may also develop, especially along the Broad and Saluda rivers, with crests not occurring until Tuesday and perhaps early Wednesday. Additional significant rises may develop along the Catawba River chain in western NC during and immediately after the heavy rainfall. Water level forecasts for these rivers are accessible at http:/water.weather.gov/ahps2/index.php?wfo=gsp. && .GSP WATCHES/WARNINGS/ADVISORIES... GA...Flood Watch through Tuesday morning for GAZ010-017-018-026-028- 029. NC...Flood Watch through Tuesday morning for NCZ033-049-050-053-059- 062>065-501>510. Wind Advisory until 2 AM EDT Tuesday for NCZ033-048>053-058- 059-062>064. SC...Flood Watch through Tuesday morning for SCZ001>007-010. && $$ SYNOPSIS...SBK NEAR TERM...Carroll/RWH SHORT TERM...JPT LONG TERM...ARK AVIATION...RWH HYDROLOGY...

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