Area Forecast Discussion
Issued by NWS Greer, SC

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000 FXUS62 KGSP 270018 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 818 PM EDT Wed Apr 26 2017 .SYNOPSIS... Warm and dry high pressure will persist over the region through the evening. A dissipating cold front will cross our region on Thursday, with warm and humid high pressure building back in for Friday and Saturday. Another front approaches from the west on Sunday. && .NEAR TERM /THROUGH THURSDAY/... As of 630 PM EDT Wednesday: IR satellite imagery showing cirrus encroaching upon the cwfa, the onset of the expected top down moistening accompanying the mean transition to a quicker sw flow through a deep layer tonight. Well defined cold front was analyzed near the mid-Mississippi valley at 22z and remains progged to cross into the TN Valley by around daybreak, with a weak/broken line of convection to accompany. The front/line will then move eastward in the high terrain by mid morning, likely weakening due to the nature of the nocturnally stabilized airmass out ahead. After that however, heating will aid destabilization as mid lvl lapse rates steepen, leading guidance to suggest upwards of 1-2k j/kg sbcape atop the region, likely concentrated east of the mtns. Said instability will be maximized along the I85 corridor, however further north toward I40, abundant cape supportive of deep convection will also be present. Given the instability progs, and roughly 40-50kts 0-6km Bulk Shear in place, strong/severe convection is certainly possible with the greatest threats being from damaging winds and large hail, although an isolated tor will not be ruled out. For this reason, SPC has placed nearly the entire region in the marginal risk for severe wx. As for pops, trends will rapidly increase after daybreak, where by midday likely pops prevail atop the Blue Ridge southward into the Upstate. These pop trends will lower to the east with time into the mid/late afternoon, however mainly due to increasing range. Temperatures on Thursday will be a few degrees cooler than today thanks to the increasing sky cover early in the day, yet still slightly above normal. && .SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/... As of 200 PM Wednesday: Convection is expected to taper off diurnally Thursday evening. A weak continental high will build over the area by early Friday, and heights rise steadily through the remainder of the period as the Bermuda high regains strength over the Southeast. A warm front will move north out of the Gulf states and into the CWFA Friday morning as subtropical air returns. Guidance suggests some light precip may develop with this feature, so a small PoP is advertised early in the day. This transitions northward to the Blue Ridge and vicinity by the afternoon as the front lifts further north and as diurnal destabilization occurs. That said, a rather strong subsidence inversion is progged, so convection should remain isolated if not totally suppressed. Warm southerly flow continues Friday night. This does not look capable of generating precip, though some low cloud cover will develop and/or remain trapped beneath the inversion. Accordingly we will advertise increasing clouds along and south of the Blue Ridge overnight. Temps/dewpoints will trend warmer by Sat aftn, and lapse rates continue to be pretty good in the mid to upper levels. While the GFS/EC suggest the capping inversion will no longer be present, and respond with QPF as a result, the NAM is capped and totally dry. It seems suspicious that the cap would erode in the face of a building ridge, but nonetheless I have kept PoP at slight chance across the CWFA. In the event storms do fire, weak shear, dry midlevels, and impressive SBCAPE values indicate the potential for a few pulse storms producing damaging winds. && .LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... As of 230 PM EDT Wednesday...the main concern out in the medium range will be the passage of a strong cold front on Monday and the potential for severe weather. Before we get to that, the upper pattern is expected to continue to amplify on Saturday night and Sunday with an anticyclone parked off the Southeast coast and an upper low deepening as it moves out across the Srn Plains Sunday and then lifts northeast toward the upper Midwest Sunday night. This pattern should be favorable for pulling in moisture from the wrn Atlantic early in the period, and then from the Gulf as the cold front approaches late on Sunday. Altho mainly diurnal convection will weaken across the region with sunset on Saturday evening, expect the improving moisture and mechanical lift to maintain a chance of showers near the Blue Ridge Escarpment overnight, with precip chances gradually expanding from there on Sunday, eventually reaching the likely range in the upslope areas and chance elsewhere. The significant mid/upper forcing should remain well to our west through the day, but the GFS/GEFS suggests the air mass will support CAPE on the order of 1500-2000 J/kg...thus scattered thunderstorms looks like a really good bet for the afternoon/evening. The model also shows just enough shear/helicity to make one think about the possibility of organized storms, but the lack of a trigger or boundary could be the missing ingredient that keeps our storms from getting really interesting while we probably watch a severe weather event unfold to our west. Temps were nudged up just a bit. Which brings us to Monday. Timing will be the key. It is worth noting the operational GFS continues a trend of slowing down the arrival of the cold front, which suggests an improving chance of organized severe storms. At the same time, the model continues to show weakening of the system as it reaches us on Monday, suggesting that our potential will not be as great as the potential along the front to our west on Sunday. That being said, even though the potential will be less on Monday over the western Carolinas, the model still suggests strong shear on the order of 40-50 kt and SRH on the order of 200-300 m2/s2 to go along with CAPE in the 500-1000 J/kg range. This should be sufficient to support the development of organized storms, maybe even mini-supercells. If the system slows down even more, the potential for greater instability goes up and we will have more of a problem. Altho SPC does not indicate a threat right now on D6, it stands to reason that if the environment develops as shown in the models, we will have a severe threat on Monday, at least over our eastern zones. For now, file this one away until we are done with tomorrows potential event. The front should move thru quickly and we dry out from the west by mid-evening. The rest of the fcst is quiet as weak high pressure moves in behind the front on Tuesday with nearly zonal flow aloft. Temps will cool off back to near normal. A weak srn stream system may affect us by late Wednesday, so precip chances climb back into the chance range by the end of the period. && .AVIATION /00Z THURSDAY THROUGH MONDAY/...
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At KCLT and Elsewhere: Save for select NC mtn valleys where pre-dawn fog remains possible, expect VFR conditions to continue tonight as just some higher cloudiness should thicken overnight. Ahead of an approaching cold front, return flow moisture may lead to mvfr clouds encroaching upon the sw fa after daybreak and percolating around the balance of the area throughout the day. Deep convection remains likely along and ahead of the front, with latest model timing threatening terminal locations after 16z. Outlook: More in the way of limited diurnal tstm chances are expected Friday with the potential for thunderstorms increasing progressively over the weekend. Confidence Table... 00-06Z 06-12Z 12-18Z 18-00Z KCLT High 100% High 100% High 100% Med 65% KGSP High 100% High 100% High 95% Med 78% KAVL High 100% Med 61% High 90% Med 70% KHKY High 100% High 100% High 100% Med 76% KGMU High 100% High 100% High 92% Med 78% KAND High 100% High 100% Med 70% Med 78% 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
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&& .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...JPT NEAR TERM...CDG/CSH SHORT TERM...Wimberley LONG TERM...PM AVIATION...CDG/CSH

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