Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 230707 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 307 AM EDT Wed Aug 23 2017 .SYNOPSIS... A strong cold front approaching from the northwest will cross central NC this evening and overnight. High pressure will build into the region late Thursday and especially Friday and Saturday. && .NEAR TERM /THROUGH WEDNESDAY NIGHT/...
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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. -Badgett Wednesday and Wednesday Night: (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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&& .SHORT TERM /THURSDAY AND THURSDAY NIGHT/...
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As of 235 AM Wednesday... An upper level trough will amplify over and swing through the Eastern U.S. through Thursday night. Meanwhile, the cold front at the surface will continue to progress through Central NC, however the exact timing of the frontal passage through the region is uncertain. Some pre-frontal convection will linger, mainly across the southeast portions of the area during the day/eve Thursday, largely moving out of the area Thursday night. The location and timing of the front as it moves through the area will have an impact on both temperatures and dewpoints. For now, expect highs in the low to mid 80s and lows generally in the mid 60s.
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&& .LONG TERM /FRIDAY THROUGH WEDNESDAY/...
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As of 305 AM Wednesday... Surface high pressure will build south behind the cold front, advecting some cooler and drier air into the area. The spatial and temporal extent of the CAA is still uncertain, but best chances and longest duration will be across the north Friday and Friday night, then over northwest portions of the area Saturday through Wednesday in a CAD-type setup. Along the Carolina coast and out over the Atlantic an elongated surface trough will linger during the day Friday before the high pushes it farther out over the ocean. More interesting will be the low/potential tropical cyclone that develops off the FL coast Friday/Friday night and slowly moves northeast along the SE U.S. coast through the remainder of the extended period. Expect increasing cloud cover and precipitation chances from late this weekend through the middle of the week. Also, expect increasing northeasterly winds as the low approaches the NC coast, with high pressure continuing to ridge southwest into the NW. This could enhance the CAA over inland areas, while coastal areas may realize more of an easterly component and tropical airmass. There are still some differences between the models with regard to this system, especially into next week, thus confidence is not terribly high. Temperatures should feel much better for much of the extended period, with highs gradually decreasing from upper 70s to mid 80s on Saturday, possibly into the mid 70s NW to low 80s SE by Tuesday. Lows will generally be in the low to mid 60s from NW to SE, although areas across the northwest could see lows dip into the 50s over the weekend.
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&& .AVIATION /06Z WEDNESDAY THROUGH SUNDAY/... As of 145 AM Wednesday... A west-southwesterly LLJ of 15 to 20kts will result in good BL mixing overnight which will preclude the development of widespread fog and stratus. Only fog-prone locations like KRWI could see a brief period of sub-VFR conditions between 06 to 12z. Scattered showers or storms are likely to develop in the vicinity of a Piedmont trough near INT/GSO/RDU between 18 to 21z, with the potential for some of these to grow in coverage and intensity as they propagate ewd and affect FAY and RWI later in the afternoon through the evening. Sub-VFR conditions could accompany the stronger cores. Winds will become west-northwesterly in the wake of the convection and aforementioned Piedmont trough, followed by a secondary wind shift to nwly or nly with the passage of a cold front between 06 to 12z. Outlook: The cold front could stall briefly across southeastern NC through the first half of Thursday, with some lingering showers and storms possible at RWI and FAY until the front fully clears the area Thu afternoon. Additionally, we could see a period of MVFR ceilings Thursday morning, within the NELY low-level flow behind the front. Following Canadian high pressure will then favor generally VFR conditions through the weekend. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...CBL/Badgett NEAR TERM...Badgett SHORT TERM...KC LONG TERM...KC AVIATION...CBL

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