Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 230058 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 858 PM EDT Tue Aug 22 2017 .SYNOPSIS... A pre-frontal surface trough extending from the mid-Atlantic south into the Carolinas will persist into Wednesday. A strong cold front will move across central NC Wednesday afternoon and Wednesday night. High pressure will build into the region late Thursday and especially Friday and Saturday. && .NEAR TERM /TONIGHT/... As of 750 PM Tuesday... Isolated showers have had a tough time developing late this afternoon, even along the surface trough over the Piedmont. Convection over SC has been weakening and decreasing in coverage as well along the Piedmont trough there. It appears that the chance of any additional convection is 10 percent or less. We will update the forecast now that we have sunset and the CINH will only be increasing as the evening rolls on. Upstream convection moving toward the Appalachians is expected to diminish before crossing the mountain barrier later tonight. Another warm and humid night is expected with lows in the 70s, upper 60s in the normally cool spots in the northern and central Piedmont. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
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As of 330 PM Tuesday... A broad, 590m west to east ridge at 500 hPa extending from the northern Gulf of Mexico into the southwestern Atlantic on Wednesday morning, relaxes as troughing develops across the eastern Great lakes and the Northeast. Modest height falls are noted across central NC on Wednesday afternoon and evening as an initial shearing short wave trough shifts east during the day with a second stronger short wave approaching the region from the northwest on Wednesday night. A prefrontal surface trough located across the Piedmont will shift east during the day as the main cold front moves across the mountains during the afternoon and into central NC Wednesday night. These features when combined with a weak to moderately unstable air mass will result in the development of scattered to possibly numerous showers and thunderstorms. The various features are disjointed and will likely result in the development of a couple of bands/broken lines of storms across central NC during the afternoon, one associated with the pre- frontal trough and lead short wave and the other with the cold front and more pronounced trough at 850/700 hPa with these clusters likely merging into a singular line toward the Coastal Plain during the evening. SPC currently has most of central NC outlooked in a marginal severe weather risk. Destablization is perhaps not as impressive as previously progged with afternoon MLCAPE values generally ranging from 800 to 1800 J/Kg. The instability is muted by warm mid levels and weak mid-level lapse rates. Bulk shear values range from 15 to 25kts with a fairly unidirectional southwest to westerly flow which would support line clusters. The end result would be a primary severe weather threat from straight-line damaging wind gusts. Will retain current wording in the HWO with the great severe threat likely to happen between 2pm and midnight. Highs on Wednesday should range between 88 and 94. Expect the cold advection to lag a bit so probably not a lot of cooling on Wednesday night when lows will range between 67 and 74. -Blaes
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&& .LONG TERM /THURSDAY THROUGH TUESDAY/... As of 335 PM Tuesday... At the base of a vortex over Quebec, a shortwave trough in NW flow aloft will amplify across the OH valley and nrn Middle Atlantic states on Thu, then lift up the Northeast coast through Fri, followed by a lingering trough aloft over the Northeast and Middle Atlantic through the weekend. A positive height anomaly/ridge aloft initially over central Canada will meanwhile migrate ewd, to ern Canada and the Northeast US through early-mid next week. Relatively strong (1025-1030 mb) Canadian high pressure will precede the upper ridge and extend south across most of the Ern US throughout the period. Lastly, the models continue to indicate an area of disturbed weather initially over the Bahamas will strengthen as it drifts slowly newd, roughly parallel to and offshore the Sern US coast. While a significant wly component to the flow aloft should keep whatever becomes of this system too far east to directly impact central NC, the MSL pressure gradient between this system and the aforementioned Canadian surface high would result in breezy conditions for central NC, particularly the ern half, both Mon and Tue. The amplification of the aforementioned shortwave trough in nwly flow aloft, in conjunction with lingering frontal forcing over sern NC, should result in convective redevelopment in the vicinity of the front from the srn piedmont and Sandhills to the ern piedmont and Coastal Plain on Thu, with a gradual drying/clearing trend from nw to se throughout the day. It should then be dry, and relatively cool, through early next week, though with a slight chance of a few showers late Mon-Tue in enely flow around the aforementioned offshore low. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
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As of 830 PM Tuesday... Primarily VFR conditions are expected through the TAF period. While a persistence forecast of low overcast and fog for Wed morning, like the past two mornings, seems reasonable given little change in the overall weather pattern, the models suggest this potential is relatively low. This reduced risk for sub-VFR conditions appears to be a result of a veering of the flow just above the surface from a more moist sswly one to a significant a more wly one (ie. a drier and/or drying wind direction east of the Appalachians), and also stronger winds in the low levels than previous days, which would favor mixing and an environment not conducive for the development of fog. Scattered showers or storms are likely to develop in the vicinity of a Piedmont trough near INT/GSO/RDU early Wed afternoon; and some of these will likely grow in coverage and intensity --and merge into clusters-- as they propagate ewd and affect FAY and RWI later in the afternoon through the evening. Initially sswly surface winds will become wly to nwly at Piedmont sites, owing to the ewd development/progression of the aforementioned Piedmont trough, followed by a secondary wind shift to nwly or nly with the passage of a cold front just beyond the 00Z TAF period. Outlook: The cold front will progress ssewd across central NC Wed night, with some lingering showers and storms possible at RWI and FAY until the front fully clears the area by Thu afternoon. Following Canadian high pressure will then favor generally VFR conditions through the weekend.
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&& .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Badgett NEAR TERM...Badgett SHORT TERM...BLAES LONG TERM...MWS AVIATION...MWS is the U.S. government's official web portal to all federal, state and local government web resources and services.