Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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961 FXUS62 KRAH 280943 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 543 AM EDT WED SEP 28 2016 .SYNOPSIS... A potent upper level low over the Great Lakes will drift southward through the Ohio valley today, stall over the Cumberland Plateau in eastern Tennessee and Kentucky on Thursday and Friday, then retrograde north through the Ohio valley and lower Great Lakes this weekend. && .NEAR TERM /TODAY AND TONIGHT/... As of 310 AM Wednesday... As the mid/upper level cyclone drifts south towards the TN Valley, models indicate a weak surface wave will develop along the quasi- stationary frontal zone in place across Western NC. Daytime heating and result moderate destabilization within the seasonably moist PWATS 1.5-1.7" east of the frontal zone will support the re- development of showers and storm across the area this afternoon and evening, with increasing upper level diffluence and the arrival of better H5 height falls after 00z yielding the potential for convective regeneration and a south to north training band of convection overnight. Central NC remains in a marginal risk for severe storms this afternoon and evening, as the approach of the mid level speed maximum into the Southern Appalachians will increase 0-6km shear values of 30 to 40kts, highest north and west of the Triangle. If we indeed destabilize as strongly as the NAM suggest (2500- 3000J/Kg of MLCAPE), wouldn`t be surprise to see SPC upgrade the marginal risk to a slight risk as some of the stronger updrafts could exhibit rotation and bring the threat of wind damage, hail and isolated tornado. Storm intensity should decrease between 03 to 06z, as the BL cools/stabilizes from loss of heating. Additionally, as the mid/upper level flow becomes parallel to the southerly low-level inflow, convective training would bring the threat of heavy rainfall of 2 to 3 inches or higher with a threat of flash flooding in a few locations. With no airmass change, highs today will be very close to what we saw on Tuesday ranging from upper 70s north to mid 80s south. Lows65 to 70. && SHORT TERM /THURSDAY AND THURSDAY NIGHT/... As of 425 AM Wednesday... The deep cut-off low over the Southern Appalachians and the quasi- stationary frontal zone over Western NC will continue to support scattered to numerous showers and storms across the area Thursday and into Thursday night. Models generally agree that the plume of above normal 1.5-1.7" PWATs will get shunted slightly eastward as the dry slot advances eastward into the western and possibly Central Piedmont by late Thursday/Thursday night. This should eventually lead to an eastward shift in the higher convective rain chances, centered along and east of Highway 1. While favorable deep layer shear of 35 to 40 kts will be sufficient to support severe storms, ongoing/lingering convection across the area from daybreak on could greatly hamper destabilization. Will need to monitor the severe aspect over the next day. Additionally, the potential for some locations to see multi-days of heavy rainfall will result in a continued threat for isolated flooding. Highs Thursday similar to the last 2 days, potentially slightly cooler across the Sandhills and southern coastal plain given more robust rain chances. Highs in the mid/upper 70s NW to lower 80s south. Drier air working its way into the western piedmont could allow for slightly cooler overnight lows in the lower 60s, with mid to upper 60s elsewhere. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... As of 430 AM Wednesday... Overview: High confidence in dry/pleasant conditions over the weekend and early next week gives way to below normal confidence mid/late next week as a highly meridional synoptic pattern evolves over the Continental US and a potentially significant tropical cyclone (yet to develop) could threaten portions of the Southeast coast. A potent upper level low over the Great Lakes will drift southward through the Ohio valley today, stall over the Cumberland Plateau in eastern Tennessee and Kentucky on Thursday and Friday, then retrograde north through the Ohio valley and lower Great Lakes this weekend. An attendant cold front will track east of the Appalachians on Thursday, then stall over eastern portions of the Carolinas Friday into the weekend, gradually washing out late this weekend and early next week. Forecast confidence is above average at the beginning of the long term period (Fri-Sun), but decreases markedly thereafter (Mon-Wed), primarily w/regard to the evolution of the stalled/retrograding upper level low. The 00Z GFS indicates that the upper low will track northward through New England Sun night, at which point it is absorbed by the northern stream jet (Mon) and tracks rapidly NE through the Canadian Maritimes into the northern Atlantic (Tue), with subsidence in the wake of this feature aiding amplification of a deep upper level ridge along the Eastern Seaboard (Wed). The 00Z ECMWF is a little slower and a little further west when retrograding the upper low northward over the weekend. As a consequence, the upper low fails to rendezvous with the northern stream jet (Mon-Tue) and cuts-off along the New England coast (Tue- Wed), precluding significant amplification of an upper level ridge along the Eastern Seaboard as depicted by the GFS. If the robust tropical wave currently approaching the Windward Islands develops into a significant tropical cyclone over the Carribean late this weekend/early next week (as long range guidance continues to suggest), then the evolution of the aforementioned upper level low will likely become a critical piece of the puzzle in determining it`s track and potential impact on the Southeast US coast mid/late next week. -Vincent && .AVIATION /06Z WEDNESDAY THROUGH SUNDAY/... As of 125 AM Wednesday... Weakening area of thunderstorms across the Sandhills may impact the FAY terminal over the next hour or two, where it could produce a wind gusts of 30-35 mph and sub-VFR visibility restrictions in heavy rain. Otherwise, waning instability should lead to the dissipation of ongoing isolated to widely scattered convection across central NC. Moist low-level airmass in place east of frontal zone stalled over the NC mtns and foothills could lead to areas of stratus overnight, along with some patchy fog at KINT and KGSO where drier air aloft has allowed some partial clearing. Stratus should lift to VFR between 13 to 17z with re-development of showers and thunderstorms across the area this afternoon. Looking beyond the 24 TAF Period: The combination of a quasi- stationary front across the Carolinas and a cut-off upper level low settling over the Tennessee Valley will keep unsettled weather in the form of scattered to numerous showers and storms through Thursday. Conditions are expected to improve Friday through the weekend as drier air finally spreads in from the west. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Vincent NEAR TERM...CBL SHORT TERM...CBL LONG TERM...Vincent AVIATION...CBL is the U.S. government's official web portal to all federal, state and local government web resources and services.