202 FXUS62 KGSP 100228 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 1028 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 1030 PM: The latest update hastens the increase in PoPs across the southern portion of the forecast area as the better tropical moisture fetch is already generating more widespread showers immediately upstream. Although better instability over the eastern Carolinas should keep the best precip rates in those areas overnight, coverage should steady grow in the western Carolinas and northeast Georgia through the early morning hours. Otherwise, the forecast remains on track with deep SE flow to 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. Pops will ramp up to high likely in most areas (w/categorical along the escarpment) toward daybreak in association with the advecting moisture plume. The main concern with this activity will 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 /03Z WEDNESDAY THROUGH SUNDAY/... At KCLT and elsewhere: Shower coverage is starting to rebound from the south late this evening, and coverage will grow overnight as the high precipitable water air returns from the southeast. Mainly VFR cigs late this evening will steadily lower into the MVFR range through the early morning hours, with an extended period of IFR likely as the better showers roll in with the initial tropical moisture plume early Wed. Cigs should be very slow to improve Wed. afternoon as a drier slot works in. The dry slot, however, could also permit isolated to scattered thunderstorms to form in a few places. Expect steady ENE to NE winds most locations east of the mountains, with SE flow at KAVL. Outlook: Periodic restrictions, heavy rainfall, and some gusty winds are expected late Wednesday night through Thursday as tropical cyclone Michael passes over or near 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 area. Confidence Table... 02-08Z 08-14Z 14-20Z 20-00Z KCLT High 100% Med 71% High 84% Med 79% KGSP High 93% High 83% High 88% High 100% KAVL High 90% High 90% Med 79% High 100% KHKY High 84% High 93% High 93% High 87% KGMU High 90% Med 75% High 88% High 100% KAND High 84% Med 68% High 91% High 91% 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