Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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433 FXUS62 KRAH 281438 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 1035 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 1035 AM Wednesday... Conditions still look favorable for severe storms and heavy rain late this afternoon through at least early Thursday. The quasi- stationary synoptic front was analyzed over the Foothills yet again this morning, having made little progress east without better support from the upper low settling toward the Tenn Valley. The front isn`t expected to move much today either, leaving central NC well within the warm sector, which will be characterized by MLCAPE upwards of 2000 J/KG owing to strong diabatic heating and warm advection from the south. Clusters of storms may contain supercells by late this evening as mid-level winds increase to 40-50kt and near surface flow backs/strengthens in response to the approaching upper low, with increasing threat of a tornado toward the NC/VA border where winds will likely be backed the most. The biggest uncertainty is a focus for convective initiation given just broad isentropic accent around 300K and the lack of strong height falls/DCVA until later tonight. Thus, convection is likely to form on mesoscale boundaries (ie differential heating or localized convergence), which makes it hard to prog a specific area for hazard threats. Mostly southwesterly deep layer flow will cause bands to be oriented southwest to northeast and may train over some unlucky areas. PW is 1.75-2" and isolated storms have produce 2-4" of rain the past couple of days. The flash flood threat will be greatest in urban areas. CAM ensembles still favor the US-1 corridor for multiple rounds, and the threat of severe/flooding looks to be greatest from 21z-03z, given best overlap of peak heating and increasing forcing, though the forcing aspect will likely continue some convection through he overnight hours. 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 /12Z WEDNESDAY THROUGH SUNDAY/... As of 745 AM Wednesday... IFR to MVFR ceilings in place across the western and central terminals are expected to lift to VFR between 15 to 18z. 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, with re-development of showers and thunderstorms expected across the area this afternoon. As upper-level support increases overnight, showers and storms could persist well into the overnight hours. Given continued moist low-level airmass in place, areas of IFR to MVFR stratus will also be possible. 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/BS 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.