816 FXUS62 KGSP 091758 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 158 PM EDT Tue Oct 9 2018 .SYNOPSIS... A persistent 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 through the weekend. Another cold front reaches our region Sunday night. && .NEAR TERM /THROUGH TONIGHT/... As of 155 pm: Persistent upper ridge that has been dominating the Southeast and Mid-Atlantic for quite some time now will continue to be forced farther east into the Western Atlantic today as the Rockies upper low begins to lift toward the northern High Plains, inducing surface cyclogenesis across TX/OK. As this trough lifts and pushes its attendant front to the east, Hurricane Michael will be working his way north across the Gulf, but will also be pushed east with the wavetrain. For this afternoon, primary concern will be a slug of tropical moisture working northwest from the Bahamas toward the Southern Appalachians. Deep southeasterly flow orthogonal to the terrain will allow for continued upslope flow into the the escarpment through the period. As moisture depth gradually increases (resulting in PWs increasing from ~1.5" this morning to over 2" by the end of the period), shower chances will be maximized along the Blue Ridge through the evening, with 20-30 percent advertised for much of the remainder of the area. Extensive cloud cover will limit the instability to less than 1000 J/kg this afternoon, thus thunderstorms are unlikely. However, the increasingly moist air mass should allow for expanding shower coverage, even away from the escarpment throughout the afternoon. Otherwise, the gradual max temp cooling trend will continue, with temps expected to peak at less than 5 degrees above normal. Deep southeast flow continues into Tuesday, with a stronger signal for more persistent/widespread low cloud cover (and also a good signal that rather dense cirrus could overspread the area during the day) suggesting significantly lower heating potential. As such, max temps will be forecast 5-10 degrees cooler than today's readings, or just a couple of degrees warmer than normal. Similar to today, pops ramp up to 30-50% across much of the area by afternoon, with a small sliver of likelies persisting near the eastern escarpment. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... As of 245 AM Tue: Deep tropical moisture will advect over the area Wednesday as a quasi-warm front moves northward with the development of low pressure in the Plains. Meanwhile Hurricane Michael will make landfall and continue to move generally northeastward. With a relatively strong southeasterly fetch into the Blue Ridge Escarpment already set up at daybreak Wed, categorical PoPs are already in the fcst there, and expand to the entire Piedmont through afternoon as vertical profiles saturate. PWATs are progged to rise above 2 inches during this time and deep warm-rain processes will be efficient at producing heavy rain. However, stout flow and some wind shear will be present which should keep cells moving. A lull in the activity also is expected later in the day. Overall, QPF for Wednesday is not especially concerning for most of the area. There remains potential for repeated initiation along the southeast-facing Blue Ridge Escarpment, though, and this first round of rainfall may at least saturate soils and increase the flash flood risk once the rains from Michael get underway early Thursday. Regarding Michael's track, NHC has not made much effective change in the track cone thru the Carolinas with the latest advisory. The GFS and EC are in much closer agreement for our area than they were yesterday at this time, and depict the circulation tracking just southeast of our CWFA border. After having trended westward, the EC has shifted slightly east again on the 00z run. The Canadian and HWRF continue to be on the west side of the model envelope, though the HWRF has trended much closer to the official track. The forecast forward speed is still sufficient to keep the most notable wind impacts to about an 18-hour window, basically Wednesday night and Thursday morning. Followed WPC QPF closely throughout the period but made minor adjustments to enhance upsloping before Michael backs the flow to northerly Thursday morning. Not counting the Canadian, model consensus depicts the heaviest rain as falling close to the eye, over the lower Piedmont. Since the heaviest rain prior to Michael is expected along the Blue Ridge, this is a relatively positive sign hydrologically. At this time we will forego any flood headlines, but continue to mention heavy rain in the HWO. Precip looks to taper off quickly late Thursday as even the slower model depictions are dry in our east by 06z Fri. Cooler air will arrive by Friday morning, with morning lows in the 50s. The track still looks likely to be southeast of the area and so we remain on the unfavorable side for TC tornadoes. That said, NAM and GFS both depict 0-3km shear of 20-30 kt and 0-3 helicity upwards of 200 m2/s2 do overlap with sufficient CAPE to suggest some threat may exist Wed night and early Thu. We currently cannot write off the threat especially if there is any westward jog in the fcst cone. SPC has highlighted a Marginal Risk on the Day 3 severe outlook on our southeastern fringe. && .LONG TERM /FRIDAY THROUGH MONDAY/... As of 150 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-end chance range thru Mon with low QPF values. With instability increasing a bit Mon afternoon...there may be isol general tstms developing across the ern and srn zones. Max temps will rise a bit Mon as the llvl flow veers s/ly allowing highs to reach normal levels. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/... At KCLT and elsewhere: Periodic MVFR cigs will be possible for the first hour or two of the period. Otherwise, patchy showers/-RA will continue to pass over the area through the afternoon. Based upon radar trends, the most likely sites to experience -SHRA will be the western NC sites, and tempos are included at those sites. 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... 18-24Z 00-06Z 06-12Z 12-18Z KCLT High 89% High 94% Med 66% High 90% KGSP High 85% High 87% High 83% High 98% KAVL High 87% High 83% High 90% High 90% KHKY High 89% High 90% Med 76% High 90% KGMU High 88% High 87% High 83% High 95% KAND High 89% High 85% Med 76% High 90% 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...None. NC...None. SC...None. && $$ SYNOPSIS...DEO NEAR TERM...JDL/TDP SHORT TERM...Wimberley LONG TERM...DEO AVIATION...JDL