172 FXUS62 KGSP 101110 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 710 AM EDT Wed Oct 10 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 THURSDAY/... As of 700 AM EDT Wednesday: Main updates are to aviation parameters this morning, and to increase pops. Also from late in this shift, based on 5a advisory for Michael, opted to issue a Tropical Storm Warning for Elbert/Greenwood/Abbeville, and a Watch for the rest of our first tier of southern counties (and York SC). Winds a little iffy but based on *probabilities* felt it prudent to go ahead. Otherwise, the overall pattern should be no surprise to our loyal AFD fans out there...big upper ridge in the western Atlantic, deep trough in the Rockies with a shortwave lifting out into the Plains, surface low in the central Plains with a warm front extending toward the Great Lakes and a cold front stretched south, and of course Hurricane Michael in the Gulf. Surface high in the western Atlantic getting caught up along the Appalachians will continue to ageostrophically adjust down the eastern slopes in a lookalike damming event, but with impacts by way of increasing isentropic upglide just the same. For the overnight period into later this morning, the concern will be the slug of tropical moisture coming inland off the Atlantic around the base of the upper ridge. Upslope and isentropic upglide over the region will continue today as PWs increase through the early morning hours from <2" to 2-2.25" especially across the Upstate. Moisture flux vectors are maximized, heading right into the escarpment, between 09-15z this morning. Updated WPC QPF hits on this potential with increased QPF across the escarpment and NC mountains of 1-1.5" just in the 6hr period from 12z-18z alone (some higher amounts locally of course). CAMs reflect this as well, with pretty good movement across the Piedmont but of course cells getting hung up along the escarpment. With that, have opted to begin the mountain FFA at 12z rather than previous start time of 18z. Later today, behind this initial slug of moisture, we should see a relative lull in the widespread QPF, but with that lull and a bit of drying aloft will come the potential for some isolated convective concerns. Deep-layer shear will not be increasing just yet but the low-level moisture should allow SBCAPEs to increase between 500- 1000J/kg especially across the Upstate in sort of a pulse-to- multicell-cluster-like environment (but a little lower instability than usual). Isolated wind gusts certainly not out of the question with any storms that are able to develop. Enter Michael. The storm will be moving into/across southern GA around or shortly after midnight tonight, and all guidance is bringing it in to SC sometime after sunrise Thursday. GFS is a little slower than the NAM and ECMWF but enough consensus there to be reasonably confident. The storm will then push through SC and into NC during the day. 1) QPF. Per WPC guidance, they have shifted the axis of heaviest rain just a tad to the NW. This is concerning for the CLT Metro area, where now our storm totals are now around 5" (vs. 4" this time yesterday), with slightly higher amounts further SE. Most of this falls in the 12z Thu-00z Fri time period. There is still some uncertainty with the axis of heaviest rain but in looking at the larger picture, certainly seems that as Michael interacts with the baroclinic zone set up by the lookalike-damming, we could get something that looks like a deformation axis setting up with some conditional symmetric instability and frontogenetical forcing (think winter weather) that could enhance some banding-like structures. No change of course to the FFA but we could see some hefty amounts in very short periods of time if this comes to fruition, and urban flash flooding in the CLT Metro is an increasing concern. Another note is that new HRRR guidance (HRRE specifically) is quite concerning across the mountains tonight. Probably a little too high (4-8"!) but is concerning enough to mention as a possibility. 2) Winds. Aforementioned interaction between Michael and that baroclinic zone may act to locally strengthen winds due to the sustained (rather than weakening) pressure gradient along the SE portion of our area. After consultation with surrounding offices, opted to go with TR.W for Elbert/Abbeville/Greenwood and TR.A for the rest of our 1st tier of southern counties. We actually have very little expectation that we would actually SEE tropical-storm-force winds, but probabilities are high enough to warrant the issuance. If nothing else, with increasingly saturated soils, some impacts are likely. Hard to tell how far north the stronger gusts will extend at this time but isolated gusts 20-30kt certainly not out of the question anywhere east of the mountains (and of course at the peaks, but that's not unusual). 3) Tornado potential. SPC brought the marginal risk in to our area for Day1, mainly for the overnight period. Deep-layer shear will begin increasing after 00z (the NAM is a little more aggressive with this) and will briefly be collocated with afternoon convective instability (albeit weak, <1000J/kg). Better shear will move in overnight ahead of Michael and naturally instability will decrease through the overnight, but even in the middle of the night (06-09z) looks like we could have a combination of 40-50kt shear with 300- 600J/kg SBCAPE and, as would be expected, increasing low level helicity. The tornado threat is low, especially with our area being on the left side of the track, but ahead of the storm it is certainly non-zero and will continue to advertise a "limited" threat especially through the overnight hours, decreasing through the day on Thursday. That's pretty long, but just to recap the two bigger take-home points: Flash flooding potential remains #1 concern and is actually increasing with latest guidance, and have issued GSP's first-ever tropical wind products (in collaboration with surrounding offices and NHC). && .SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/... As of 230 AM Wed: Michael will depart quickly to the east Thursday night. Most model guidance depicts the precip ending in our far eastern zones by midnight if not sooner. Breezy northwest winds will continue into early Friday as cool and dry high pressure begins to build east. Min temps will be near normal in the mountains; the Piedmont will remain several degrees above normal. Maxes will be about normal on Friday. Though high pressure will continue to dominate the sfc pattern across the Southeast, a fast-moving shortwave will traverse the Ohio Valley Friday night. This looks unlikely to produce any mountain precip, but a period of increased cloud cover may result. The sfc high will center over the region Saturday and return flow will start to develop, though max temps will fall back a category or so below normal. && .LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/... As of 150 AM Wednesday: Starting Saturday evening with cool and dry high pressure centered near West Virginia as it transits to the east. Former TC Sergio which crossed the Baja Peninsula is forecast to be over Texas and Oklahoma Saturday evening. As the next upper trough drops down over the northern plains Saturday night and Sunday, Sergio gets caught up in the flow ahead of this trough. Remnants of Sergio moves across the Tennessee and Ohio Valleys going into the start of the week. The cold front advances toward our area on Sunday and Sunday night as the previous high moves well out over the Atlantic. The GFS is more progressive as on Sunday evening the front extends from the Texas Coast to the eastern Great Lakes while the ECMWF has the front from Dallas to St Louis. The GFS was trending wetter with the south end of Sergio brushing the NC mountains, but the latest run is drier. The EC has Sergio passing over the Ohio Valley Monday while we get very little precip. The more progressive GFS has the front pushing through late Monday with dry high pressure transitioning from west to east north of our region through Tuesday night. The EC delays the high's arrival until Wednesday with a lingering front over our area from Monday until Wednesday. At this point, not making any drastic changes to the forecast until we have a better idea for this time period. Cooler Temperatures are the certainty with Mins a few degrees below normal Saturday night then near normal into first part of the week. Max temps near normal. && .AVIATION /11Z WEDNESDAY THROUGH SUNDAY/... At KCLT and elsewhere: Pretty solid IFR this morning and little improvement expected during the day, though pockets of MVFR certainly not out of the question this afternoon. Occasional -SHRA will continue, and have TEMPO TSRA or VCTS for the Upstate TAFs and KCLT (KAVL and KHKY not confident enough to include in TAFs at this time). Cigs drop again overnight tonight with IFR to LIFR prevailing in more widespread SHRA ahead of Michael. Winds will start out ENE for all but KAVL (where SE down-valley winds will continue) and remain ENE for Upstate TAFs, though KCLT/KHKY should see slight veering more to ESE or even SE by early afternoon (might trend back E late in the period but hard to pin down those details just yet). Speeds close to 10kt today will increase slowly late in the period, with some low-end gusts possible for Upstate TAFs and KCLT Thursday morning continuing into the end of the period. Outlook: Periodic restrictions, heavy rainfall, and some gusty winds are expected 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... 11-17Z 17-23Z 23-05Z 05-06Z KCLT Med 71% High 100% High 100% High 85% KGSP High 85% High 100% Med 75% Med 71% KAVL High 93% High 87% High 94% High 85% KHKY High 100% High 94% Med 75% High 100% KGMU High 85% High 100% Med 75% High 85% KAND Med 78% High 87% High 94% High 85% 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 tonight through Thursday evening for GAZ028-029. Tropical Storm Warning for GAZ029. NC...Flash Flood Watch 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. Tropical Storm Watch for NCZ082. SC...Flash Flood Watch from late tonight through Thursday evening for SCZ002-003-005>007-010>013-019. Flash Flood Watch from Thursday morning through Thursday evening for SCZ008-009-014. Tropical Storm Warning for SCZ011-019. Tropical Storm Watch for SCZ009-012>014. && $$ SYNOPSIS...DEO NEAR TERM...TDP SHORT TERM...Wimberley LONG TERM...DEO AVIATION...TDP