Area Forecast Discussion
Issued by NWS Greer, SC

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957 FXUS62 KGSP 160235 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 1035 PM EDT Wed Aug 15 2018 .SYNOPSIS... A trough of low pressure will develop west of the Appalachians through Thursday and then persist well into the weekend as a stalled surface front lingers north of the region. Moisture will increase ahead of this system to produce a lengthy period of unsettled weather from Friday onward. A weak cold front will approach slowly from the northwest early next week and likely arrive in the region on Wednesday. && .NEAR TERM /THROUGH THURSDAY/... As of 1030 pm: Another quiet/seasonably mild night is in store, with fog and low stratus likely developing again in the mtn valleys during the early morning hours. However, in light of the lingering mid and high-level clouds seen across the area this evening, it may take a couple of more hours before the fog/stratus begins to expand. Min temps should be right around normal. An upstream trough will amplify over the Midwest on Thursday, but any shortwaves ejecting east from this system should impact mainly the Ohio Valley and central Appalachians. Once again, the best forcing over the southern Appalachians will be weak terrain triggering. Scattered PoPs appear warranted as better moisture returns in the S to SW 850 mb flow. Temperatures should recover at least another degree or two despite the clouds given the warm advection. && .SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/... As of 2:30 PM EDT Wednesday: We will allow the pockets of showers and thunderstorms across our forecast area (FA) Thursday evening, to wane or end with the loss of heating and minimal forcing. The exception will be in the mountains where a SW flow will continue to draw moisture into the western slopes, in a mechanical lift type fashion. It is difficult to pick up the various lobes of energy in a seamless wave train of vort lobes. One appears destined for our area Friday, while a lee side trough settles in. Precipitable water values should be rising to between 1.75 and 2.00 inches. SBCape on the order of 1500-2000 J/KG, although 0-6KM bulk shear will remain aob 20 knots. Overall thinking is for POPS to rise into the likely category west, and medium chance east. The forecast is for the upper level energy, interacting with a moist atmosphere and meso convergent areas to ignite the showers and thunderstorms. The potential for locally heavy rain and a few downburst episodes not out of the question. The positive tilted trough approaching will be gradually deamplifying Friday night and Saturday. Nevertheless, several waves of energy will continue to work through the SW flow aloft, and the stubborn surface wind shift/lee side trough will remain in place. Therefore, although we back off POPS Friday night to some degree, they should not end. The parade of S/W type features will continue into Saturday, as a result with instability on the increase once again, expect even better chances of showers and thunderstorms. We have POPS Saturday reaching categorical in the mountains and high chance or low likely values elsewhere. Note, the idea that the surface front will become nearly stationary, or waffle north and south beyond this part of the forecast cycle, is a bit worrisome for heavy rain potential. At least during this part of the forecast cycle the models are not very interested in heavy rain. Temperatures in this part of the forecast will be knocked back from earlier in the week, with clouds and the precipitation potential. && .LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... As of 2:30 PM EDT Wednesday: Our forecast area (FA) sits under one trough Saturday night and Sunday. As the lead system pulls out there should be a lull sometime Sunday morning. However the next upper level system will be dropping into the Missouri Valley later Sunday, destined to be a weather player in the extended portion of the forecast. With fairly high precipitable water values in place, and the old frontal boundary draped across our region, scattered to numerous showers and thunderstorms should erupt Sunday into Sunday evening, with a bit of a diurnal maximum. Unlike the earlier trough, this feature approaching early next week, and into the middle of the week, appears stronger and has more amplitude associated with it. The quasi stationary boundary may well be shifted back to the north as the southwest flow increases. Judging by the jet structure of a favorable right entrance region of a jet-let, this along with upper level energy support a continuation of the unsettled weather into midweek. POPS are quite high during the period, although there will be breaks or minimums in the long range time period. However, the theme appears to be sound. Temperatures will be tricky if we get back into the warm sector, or clouds thin a bit at times. Persistence in the forecast seems to be the best route to take at this point. && .AVIATION /03Z THURSDAY THROUGH MONDAY/... At KCLT and elsewhere: Very isolated convection, mainly in areas east of KCLT, will diminish over the next couple of hours. Otherwise, the main concern for this period will be the potential for fog/low stratus in the mtn valleys early Thu morning. While KAVL has largely been spared of restrictions the past couple of mornings, increasing dewpoints will result in increasing chances for IFR or lower conditions Thu morning, and 2SM/SCT003 are advertised from 10-13Z. The other main TAF sites should remain VFR. Otherwise, tomorrow afternoon/evening is expected to be slightly more convectively active than today, but still not enough to warrant any Prob30s outside of KAVL. Winds will become calm or light/vrbl this evening, becoming SW at around 5 kts by late morning/early afternoon Thu. Outlook: Increasing coverage of diurnal thunderstorms us expected from Friday through the upcoming weekend and into early next week. Restrictions will be limited mainly to areas in heavy showers and low clouds/fog across the mountain valleys each morning. Confidence Table... 02-08Z 08-14Z 14-20Z 20-00Z KCLT High 100% High 100% High 100% High 100% KGSP High 100% High 100% High 100% High 100% KAVL High 100% Med 62% 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 and ensemble forecasts are available at the following link: && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...HG NEAR TERM...HG/JDL SHORT TERM...TS LONG TERM...TS AVIATION...JDL is the U.S. government's official web portal to all federal, state and local government web resources and services.