Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 280044 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 832 PM EDT TUE SEP 27 2016 .SYNOPSIS... A quasi-stationary frontal zone will meander over the area through late in the work week, as a deep upper level low settles over the Tennessee Valley. && .NEAR TERM /THROUGH TONIGHT/...
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As of 845 PM Tuesday While the threat for isolated strong/severe storms has decreased with loss of heating, scattered showers and thunderstorms with locally heavy downpours will remain a decent probability across roughly the west half of central NC through midnight, with coverage expected to gradually decrease afterwards. A very moist air mass for this time of year (PW values 120-150 percent of normal) coupled with lift supplied by an upper jet crossing the northern mid atlantic and weak low level convergence along a slow moving cold front will maintain the threat for scattered showers/storms through the rest of the evening. this forcing is maximized west of highway 1 through 06Z, then projected to weaken through 12Z Wednesday. Will need to monitor for training showers/storms as high PW values indicate the atmosphere supportive if efficient rain producing storms, which may lead to localized flood threat. Across the coastal plain, forcing not as robust though atmosphere just as moist. While showers/storms will not be as numerous compared to the Piedmont, locally heavy rainfall will still occur with the stronger showers/storms. Overnight temps in the 65-70 degree range expected.
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&& .SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/... As of 330 PM Tuesday... Depending on the amount of ongoing showers early wednesday morning, Wednesday afternoon could be a rather active. As the stacked low over the Great Lakes digs south toward the TN Valley, increased forcing for accent and the stalled frontal zone over west-central NC will trigger convection, though the timing of greatest coverage is still a little uncertain given that the best height falls and cooling aloft won`t arrive until later Wednesday afternoon/evening, possibly toward the end of peak heating. Deep shear, while strongest to the northwest, will increase to 30-40kt, especially as the low-level flow backs in response to the approaching height falls, which may support supercell activity within a band of convection that appears favored from US-1 east, based on the available CAMs. All of central NC is in a Marginal Risk (per SPC) and this could be upgraded if instability appears to be sufficiently strong. The other concern for Wednesday evening is isolated flash flooding given system parallel southwesterly mean flow and PW near 2 inches. At the moment, Flash Flood Guidance for 3+ hours is well in excess of 2 inches. The GEFS and EC Ensemble means are roughly 1-2 inches for the central and northern Piedmont areas, so urban areas would likely run the greatest risk. However, confidence in the most favored area is not high enough to consider a watch at this time. Even after an evening round of convection "occurs" the ECMWF indicates another round of precip overnight, which isn`t out of the question given cyclonic flow aloft and no airmass change. it`s hard to put much detail into that portion of the forecast until the mesoscale aspects of Wednesday evening play out, but POPs will remain high overnight. Highs Wednesday should be in the mid 70s to lower 80s, with overnight lows in the mid and upper 70s. && .LONG TERM /THURSDAY THROUGH MONDAY/... As of 355 PM Tuesday... A closed mid-upper low now over the Great Lakes will settle SWD and become increasingly separated/cut-off from the NRN stream flow across Canada, and ultimately reach the Lower OH/NRN TN Valley by early Thu. The models agree that the low will linger there through Fri, then lift NNEWD across the Great Lakes through the weekend. Meanwhile, a SRN stream jet will help carve a lingering trough axis across the SERN U.S. and ERN GOM, SWD to the vicinity of the Yucatan, during the same time. The ultimate placement and strength of this latter feature may play a role in the eventual track and strength of the tropical wave now several hundred miles E of Barbados. At the surface, a secondary cold front associated with the aforementioned closed low will have moved into the Appalachians by early Thu, then drift slowly EWD and merge, over central NC Thu afternoon, with the preceding frontal zone now stretching along the NC Blue Ridge. The front, and large-scale forcing and ERN fringe of colder temperatures/steep lapse rates accompanying the mid-upper low, will serve as a continued focus for a good chance of showers and storms on Thu. The merged frontal zone --and associated WRN bound of instability/convergence/chance of showers and storms-- will then pivot slowly NEWD through the NE Piedmont and Coastal Plain Thu night-Fri, before reaching and dissipating along the coast late Fri- Sat. A lee trough and associated light SLY flow will linger over the Carolinas this weekend, then yield to high pressure forecast to build across the NERN quarter of the CONUS, beneath NRN stream ridging aloft, through early-mid next week. Temperatures are expected to be near to slightly above average throughout the period. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/... As of 831 PM Tuesday... Scattered showers and storms will become less numerous over the next several hours. With a moist low level air mass, IFR stratus looks likely at KINT/KGSO, while sub-VFR conditions with some patchy fog are possible elsewhere in the early morning hours. Stationary front will remain west of the area Wednesday, and showers and thunderstorms will develop again across the area Wednesday 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...SMITH NEAR TERM...WSS SHORT TERM...SMITH LONG TERM...MWS AVIATION...30/SMITH is the U.S. government's official web portal to all federal, state and local government web resources and services.