471 FXUS62 KGSP 092115 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 515 PM EDT Tue Oct 9 2018 .SYNOPSIS... A strong upper level ridge will remain over the region and maintain above normal temperatures through Thursday. Moisture levels will increase during this time as tropical cyclone Michael tracks northeast and reaches our forecast area on Thursday. Canadian high pressure will build in Friday and bring seasonably cooler and drier air before a moist cold front reaches the region Sunday night. && .NEAR TERM /THROUGH WEDNESDAY/... As of 515 PM: PoPs were raised slightly for early evening in lower piedmont sections in line with radar trends. Otherwise, deep SE flow will remain across the area through the period, with a plume of deep tropical moisture (PWATs around 2", possibly higher) expected to overspread the forecast area overnight. Currently, area radar imagery indicates isolated to scattered showers over the area, especially from the NC Blue Ridge into the Piedmont. Shower chances will remain maximized (around 50%) into the evening along the Blue Ridge, with 20-30 percent advertised for much of the remainder of the area. Pops will ramp up to 50-70% in most areas (w/categorical along the escarpment) toward daybreak in association with the advecting moisture plume. The main concern with this activity will obviously be locally heavy rainfall, as showers should be very efficient rainfall producers in the extremely high PWAT air. Min temps will remain well above climo. Likely to categorical pops overspread the forecast area throughout the morning, with locally heavy rainfall remaining the primary threat. Although shower chances will persist through the day, a lull of a few hours in the widespread rainfall is anticipated Wed afternoon, as the narrow subsident region north through northeast of Tropical Cyclone Michael pushes through the area. While locally heavy rainfall will be possible in all areas, the most likely region for widespread heavy rain and at least localized hydro issues through the near term will obviously be along the eastern Blue Ridge escarpment, where 1 to 2 inches, with locally higher amounts are forecast through the day tomorrow. A Flash Flood Watch will be issued beginning tomorrow afternoon for these zones. Interestingly, there are major discrepancies in the model instability forecast within aforementioned subsident/mid-level drying region tomorrow afternoon, with the GFS depicting 2000-2500 J/Kg of sbCAPE east of I-85, while the NAM is more in the 500 neighborhood. If the GFS were to verify, there would be the potential for discrete convection to develop across the Piedmont tomorrow afternoon, with perhaps a severe convective threat. Frankly, the GFS just seems way overdone with its instability, so we do not anticipate this threat will develop, especially since shear values are forecast to remain modest at best through the period. && .SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY/... As of 245 PM Tuesday: Not a whole lot of change from earlier thinking as to the track of Michael across the Carolinas on Thursday, as the model guidance and the NHC forecast has shown fairly good continuity. The official track takes the center of Matthew across the SC Midlands during the daylight on Thursday, but the heavy rain threat will occur across most of the area east of the Blue Ridge at least thru Thursday afternoon. The storm is expected to be fast-moving with a quick shot of heavy rain and very high precip rates, mainly in a 6-12 hour period, making flash flooding our main concern. The Wednesday night period will be a transition of sorts between the very moist southeasterly upslope flow in advance of the storm as it makes its way across parts of Georgia. There could be a brief lull in the action during this transition, altho not sure it would really matter much in the grand scheme of things. By late night/pre-dawn on Thursday, the main precip shield associated will Matthew will be spreading northeast across the western Carolinas. The model guidance suggests bringing the categorical precip prob in the upslope areas near the escarpment outward across the entire fcst area by daybreak on Thursday. Heavy rain should spread over the Lakelands after midnight, so a Flash Flood Watch will begin at midnight /04Z Thursday/ for parts of the Upstate and northeast Georgia. The track of the center does not favor heavy rain over the Little TN valley/Pigeon R Valley/upper Savannah basin, so those areas will be left out for now. On Thursday morning, the heavy rain should move in across metro Charlotte and the rest of the wrn Piedmont, so the Flash Flood Watch will begin at 6 am /10Z Thursday/. Some of the model guidance is showing a good signal for very heavy rain on Thursday across metro Charlotte. Given the experience last month with Florence, the highest threat for flash flooding might exist in that area, so we will be keeping a close eye on that. As for the tornado threat, although the right front quad does not move into the fcst area, we will not rule out the tornado threat wrapping into the I-77 corridor starting mid-morning on Thursday and continuing into the afternoon. The storm will be moving northeast quickly, so the back edge of the precip will cross the fcst area SW to NE in the afternoon and evening on Thursday. The best wind gust potential might actually exist in the pressure gradient on the west side of the system late in the day. We will keep a very high precip prob over the wrn Piedmont into the Thurs night period. Some upslope precip on the back end is possible along the TN border, so a chance was kept there. The Flash Flood Watch expiration was handled from 00Z to 04Z Friday. The precip should be long gone by daybreak Friday. Still expecting a quick dry out on Friday in the wake of the storm as high pressure builds in from the Plains. The air mass should be noticeably drier and high temps will be a category or two cooler...back closer to normal...in the afternoon. && .LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/... As of 200 PM Tuesday: A dry ulvl trof will cross the area Sat night and initiate a synoptic pattern change thru the ext range. Upper heights will rise in nearly zonal flow which will support the movement of a broad and dry Canadian high across the ern CONUS during the weekend. This high will bring a definitive airmass change to the FA as sfc td/s are maintained in the L50s while max and min temps drop a cat or so below normal. Basically...a very nice weekend is in store as soundings show a dry column within deep-layered subs allowing for only fair-wx Cu and some passing Ci. By late Sun...a cold front will approach the area and produce sct rain showers...mainly across the mtns. The moisture available with this system is a little suspect as the models indicate the possibility of enhancement from the remnants of TC Sergio...otherwise the front would be mostly dry. In any case...PoPs will remain in the low-mid chance range thru Mon with low QPF values. With instability increasing a bit Mon afternoon...there could be isol general tstms developing across the ern and srn zones with the better surface heating. Max temps will rise a bit Mon as the llvl flow veers s/ly allowing highs to reach normal levels. && .AVIATION /21Z TUESDAY THROUGH SUNDAY/... At KCLT and elsewhere: Periodic MVFR cigs will be possible through early evening with any heavier showers. The most likely sites to experience -SHRA will be the western NC sites, and tempos are included there. Otherwise, conditions should generally remain VFR through at least mid-evening, before cigs begin to return to MVFR, eventually winding up at IFR/LIFR by daybreak. After an evening lull, shower chances will increase after midnight, with most terminals receiving categorical -SHRA and MVFR visby by no later than mid-morning. Cigs should be slow to improve on Wed, likely not reaching MVFR until afternoon. Outlook: Periodic restrictions, heavy rainfall, and some gusty winds are expected late Wednesday through Thursday as tropical cyclone Michael passes over the region. Expect improving conditions after Michael exits the area for the end of the week, but restrictions will be possible again early next week as a front approaches the region. Confidence Table... 21-03z 03-09Z 09-15Z 15-18Z KCLT High 100% High 87% Low 52% High 98% KGSP High 98% Med 70% High 90% High 92% KAVL Med 77% High 90% High 88% High 92% KHKY High 83% High 88% Med 75% High 91% KGMU High 100% Med 74% High 90% High 90% KAND High 100% Med 70% Med 76% High 84% 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 && .GSP WATCHES/WARNINGS/ADVISORIES... GA...Flash Flood Watch from late Wednesday night through Thursday evening for GAZ028-029. NC...Flash Flood Watch from Wednesday afternoon through Thursday evening for NCZ033-049-050-053-064-065-501>510. Flash Flood Watch from Thursday morning through Thursday evening for NCZ035>037-056-057-068>072-082. SC...Flash Flood Watch from late Wednesday night through Thursday evening for SCZ002-003-005>007-010>013-019. Flash Flood Watch from Thursday morning through Thursday evening for SCZ008-009-014. && $$ SYNOPSIS...SBK NEAR TERM...HG/JDL SHORT TERM...PM LONG TERM...DEO/SBK AVIATION...HG/JDL