Area Forecast Discussion
Issued by NWS Greer, SC

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000 FXUS62 KGSP 121054 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 654 AM EDT Wed Aug 12 2020 .SYNOPSIS... Daily showers and thunderstorms will steadily become more numerous as the week progresses. High temperatures of a couple degrees above normal through Wednesday will become near to below normal by the end of the week with the deeper moisture and considerable cloud cover. && .NEAR TERM /THROUGH TONIGHT/...
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As of 630 AM EDT Wednesday: All shower activity died off a couple of hours ago with a plume of lingering high clouds still evident on IR satellite. Otherwise, winds remain calm over the majority of our sites with some mountain valley fog/low stratus currently being observed at the usual locations. Fog/low stratus should burn off by 14z or so. Otherwise, mid to upper heights will recover modestly over the fcst area today as heights fall to our west over the Mississippi River Valley ahead of a weak cutoff low. With low-lvl flow expected to veer back around to SELY across our lower terrain but favor a more NLY direction over the mtns, this should produce some low-level convergence and enhanced upslope component. This will likely produce at least a slight increase in diurnal convective coverage this aftn/ evening across the mountains and adjacent foothills. In addition, the CAMs have been suggesting that sct convection could linger thru late tonight and into the overnight as weak low-lvl convergence persists across the CWFA. As has been the case the past few days, fcst profiles depict modest lapse rates and seasonably weak shear. Thus, a few thunderstorms could become severe today/tonight, however heavy downpours and the potential for isolated flash flooding remains the primary concern. High temps are expected to top out just above normal.
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&& .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... As of 320 am Wednesday: The main driver of the weather across our area...indeed across much of the Southeast during the late week will be a upper vorticity center over the western Ohio Valley and associated frontal zone extending from the lower Miss Valley into the Mid-Atlantic. As the vorticity center drifts slowly east through the period, forcing and moisture will increase across much of the region, with precipitable water values likely reaching...if not exceeding 2" across much of our area during the Thu/Fri time frame. Although the better forcing will still be west of the area through much of Thursday, ingredients should support high coverage of mainly diurnal showers and storms. The main threat Thu into Thu night...as it will be for much of at least the next 5 days...will revolve around the potential for locally excessive rainfall. On Thu, this threat will be largely driven by the high number of showers/storms, anticipated warm rain processes/efficient rainfall producers, and slow cell movement, as forecast soundings depict average LCL-EL mean wind of < 5 kts. However, as time goes on, a more organized excessive rainfall threat could emerge Thu night through Fri night, when guidance is in generally good agreement in lifting a surface wave west of the Appalachians, which would tend to focus low level forcing and relatively strong SE upslope flow into southwest NC. While most guidance sources reflect this "big picture" scenario, differences in the intensity and atmospheric response to the key features is significant among the various solutions, so an organized hydro threat is by no means a slam dunk, but the signal is there. Otherwise, high pops...generally 60-80% are carried through the period. Although the highest pops are tied to the afternoon/evening peak of the diurnal heating cycle, convection will be possible, if not likely well beyond this window. Max temps will be near to slightly below climo and min temps above climo through the period. && .LONG TERM /SATURDAY THROUGH TUESDAY/... As of 340 am Wednesday: Very unsettled weather will persist into at least the early part of the medium range, as the upper vorticity center of the short term period, which is forecast to be more or less evolved into a bonafide trough by the start of the medium range, will still be to our west, supporting continued tropical moisture and respectable forcing over the forecast area. This will support continued high pops...generally 70-80% on Saturday, with a continued excessive rainfall threat, which is likely to increase with each day as antecedent conditions steadily deteriorate. However, as the weekend progresses, the GFS and ECMWF are somewhat converging toward a consensus in progressing the trough across the region and shoving the moisture plume east by Monday. Thus, the forecast will feature a gradual downward trend in pops for Sun-Tue, also with a trend toward more of a typical diurnal convective cycle. However, the details of this trend toward less active weather remain very much up in the air. Temps will begin the period below normal, but gradually trend back toward climo with time. && .AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
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At KCLT and Elsewhere: All shower activity died off a couple of hours ago with a plume of lingering high clouds still evident on IR satellite. Winds should be light to calm thru mid-morning with more of a NELY wind component will set up later this morning and a SELY direction prevailing for the afternoon/evening. Otherwise, I anticipate VFR cumulus bases from sfc heating today, with another round of sct to numerous TSRA this afternoon/evening, possibly lingering into the overnight. This was handled with PROB30s at each site, except for KCLT, where the slightly higher PoPs warranted a TEMPO for TSRA from 19 to 23z. Outlook: Above normal coverage of diurnal convection is expected thru the upcoming weekend. Morning fog/low stratus restrictions will be possible each morning, especially over the usual mountain valleys. Confidence Table... 11-17Z 17-23Z 23-05Z 05-06Z KCLT High 100% High 100% High 100% High 90% KGSP High 100% High 100% High 100% High 100% KAVL Med 65% High 100% High 100% Med 65% KHKY High 100% High 95% High 100% High 100% KGMU High 100% High 100% High 100% High 100% KAND High 90% 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
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&& .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...DEO NEAR TERM...JPT SHORT TERM...JDL LONG TERM...JDL AVIATION...JPT

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