Area Forecast Discussion
Issued by NWS Greer, SC

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842 FXUS62 KGSP 201101 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 701 AM EDT Tue Aug 20 2019 .SYNOPSIS... An area of low pressure and elevated moisture will persist across the western Carolinas and northeast Georgia today. A moist cold front will approach the region Thursday and become stationary over the area on Friday. The front will gradually settle south of the region through the weekend as high pressure builds in from the north and maintains cooler than normal temperatures. Moisture could linger behind the front. && .NEAR TERM /THROUGH TONIGHT/... As of 700 AM: The radar echoes and sat imagery suggest we have a weak MCV over the Midlands that is drifting N/NE this morning. This wave is initialized reasonably well on the latest CAMs, and they do have a cluster of convection increasing in coverage as it tracks thru the Charlotte area later this aftn thru this evening. Have adjusted PoPs upward for the eastern third of the CWFA to reflect the trends. The associated cloudiness may result in max temps being even lower than we currently have in there, but I have left them as is for now. As of 245 AM Tuesday: Still plenty of convection lingering across central GA to the Central Savannah River Area early this morning, with some of that convective debris cloudiness drifting north into the FA. The western edge of these clouds have cleared east of the central and western NC mountains, revealing dense valley fog in the Little TN Valley. As the clouds start to thin out over the next few hours, a light SELY low-level flow will help produce some stratus across portions of the Piedmont. Confidence on how extensive the clouds will be is still low, but I do expect at least a fair amount of low clouds around daybreak or shortly thereafter. The clouds will delay diurnal heating early, but guidance still agrees on another round of scattered convection this aftn into early evening. Forecast soundings show a lot more mid lvl moisture than yesterday, and with max temps a category or two cooler, max cape should be more in the 1000-2000 J/kg range instead of the 2000-3000 J/kg yesterday. So I expect less of a severe threat today. A few storms could still be strong, especially in the mountains, where they will be a little removed from the deeper mid-lvl moisture and should have more sunshine this morning. Tonight, a weak low will slowly drift NE over the FA and bring the low-level flow around of the SW overnight. This flow, while losing most of the upslope component, will have a little upglide with it, helping produce some low stratus across portions of the piedmont. A few isolated showers may linger in the upglide flow as well. Temps will be slightly above normal under the cloud cover. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... As of 230 am EDT Tuesday: Relatively zonal upper-level flow is expected atop the region on Wednesday, but weak lobes of vorticity embedded in this flow pattern could provide triggering over the mountains during peak afternoon heating. Mid-level lapse rates should spike back up to the 6.5 to 7 deg C/km range, and the convection allowing models feature activity developing along the Blue Ridge mountains and running quickly eastward across the foothills/piedmont through the evening hours. Warm and moist profiles look to be a limiting factor for much severe weather, but very gusty winds will certainly be possible with any downbursts. Heights will then fall sharply from the upper Midwest through the Ohio Valley Wednesday night through Thursday night. This will permit a cold front to lay over north of the region and promote steadily increasing deep moisture over our area south of the frontal zone. Profiles exhibit precipitable water values over two inches in many areas, but positive energy in the profiles looks a bit higher than on Wed., with an associated uptick in storm strength likely on Thursday afternoon. Min temps remain two categories above climo through the period, with maxes about one category above. && .LONG TERM /FRIDAY THROUGH MONDAY/... As of 245 am EDT Tuesday: The best dynamic forcing associated with a deep closed low over eastern Quebec should pass north of the forecast area Friday through Saturday, but with a trough axis crossing the southern Appalachians early in the period. An associated cold front will lay over into the region, along with continued abundant moisture. Numerous to widespread showers and thunderstorms will likely develop in this moist airmass as the frontal zone and upper level forcing arrive on Friday. The frontal zone draped across the area Friday night will gradually settle south of the region through the weekend. The GFS remains more aggressive than the ECMWF in pushing the boundary southward, but all solutions keep some measure of moisture lingering north of the frontal zone to promote clouds and precipitation and associated cold air damming conditions. Although upper heights will generally build again from the west through the weekend, some bagginess is indicated in the height field west of the mountains. This could reinforce intermittent, weak upglide and help to establish/maintain damming the rest of the weekend. Indeed, easterly flow and continued moisture could permit CAD-like conditions to linger into Monday. Temperatures have been trimmed slightly for the Sat-Mon period, but probably not enough if true damming develops. Will keep above climo PoPs as well for mainly light rain showers. Any thunder should be confined to the far southwest mountains. && .AVIATION /11Z TUESDAY THROUGH SATURDAY/... At KCLT and elsewhere: Well, the thick debris cloudiness held on overnight across the Piedmont, which really limited any fog or stratus. The mountain valleys did fog up, but are starting to mix a little and should scatter out by late morning. KAVL has already gone VFR in last ob, but may go LIFR again briefly during the first hour of the TAF. Once we start seeing convective cu popping, bases may start out in the MVFR level, but may be patchy and quickly lift to VFR by midday. The big story is a weak disturbance drifting over central SC attm. This wave will cross the KCLT area this aftn into early evening, and guidance generally agrees on a cluster of shra and tsra forming with it. Will go with tempo for 19-23z. The rest of the TAF sites will be on the periphery of the wave, and should see typical diurnal convection during the usual peak heating. So going with VCTS and PROB30 for those sites. Guidance still not clear on fog and stratus development tonight, but will trend with some cig and vsby restrictions late, mainly in the NC sites (as confidence is lower in the Upstate). Outlook: The potential for afternoon/evening showers and thunderstorms is expected to increase later this week, as a cold front approaches from the NW into Thursday. Morning fog and low stratus will be possible each day across the mountain valleys. Otherwise, expect VFR. Confidence Table... 11-17Z 17-23Z 23-05Z 05-06Z KCLT High 100% High 100% High 100% Med 72% KGSP High 100% Low 55% High 95% High 94% KAVL Low 55% High 100% High 100% High 94% KHKY High 100% High 100% High 100% High 89% KGMU High 100% Med 65% High 100% High 94% KAND Low 55% Med 70% High 95% 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...ARK SHORT TERM...HG LONG TERM...HG AVIATION...ARK is the U.S. government's official web portal to all federal, state and local government web resources and services.