Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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185 FXUS62 KRAH 221937 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 337 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 /THROUGH TONIGHT/... As of 1135 AM Tuesday... Current forecast is in good shape with only minor adjustments required. Late morning surface analysis shows the remains of a stationary front across SC and GA with a developing lee trough across western VA and the western Carolinas. Recent GOES-16 visible satellite imagery shows a developing cumulus field across central NC with the most widespread and thickest cumulus to the southeast across the Coastal Plain and coastal region in a region of deeper moisture and localized convergence just inland. Scattered showers and thunderstorms are developing in this region which will impact much of Sampson, and southern portions of Wayne and Cumberland Counties. Elsewhere, dry weather is expected today with partly cloudy skies. Current forecast highs in the lower 90s look on target. -Blaes Previous discussion...The strong 594 dm mid/upper level anticyclone centered over the SE US this morning will begin to break down today, with H5 heights falling slightly by the afternoon/evening with the approach of the northern stream trough and attendant cold front moving into the Ohio and Tn Valleys. At the surface, weak surface high centered off the NC coast will shift further offshore this afternoon, allowing the development of pre-frontal sfc trough in the lee of the mountains. Central NC remains in a void of appreciable synoptic scale forcing/lift through tonight. And as such, convective rain chances are expected to remain rather low and diurnally driven, confined across the far SE zones in the vicinity of the sea breeze and across the western part of the state along the developing pre-frontal trough. Nocturnal cooling/stabilization this evening should lead to dry conditions by late evening, and it should remain dry until the approach of the cold front Wednesday afternoon. Highs in the lower 90s with lows in the lower to mid 70s. && .SHORT TERM /WEDNESDAY AND WEDNESDAY NIGHT/... As of 330 AM Tuesday... On the southern periphery of the main belt of westerlies accompanying the northern stream trough and low pressure system lifting northeast through SE Canada, a cold front will advance SE into the area Wednesday afternoon and will cross Central NC Wednesday evening/night, exiting the coastal plain counties AOA daybreak Thursday. Strong insolation within the moist airmass featuring dew points in the lower to mid 70s, in advance of the cold front, will likely support moderate instability on the order of 1500-2500 J/kg across central and eastern portion of the forecast area Wednesday afternoon. A pre-frontal convection band/line is expected to develop across the western Piedmont by the early afternoon, with the potential for the line to strengthen/become better organize as it pushes eastward into the more juicier more unstable air maximized across the central Piedmont, Sandhills and Coastal Plain counties. Unidirectional SWLY shear of 20 to 25kts would support a primary threat of straight-line damaging winds with NCAPE values ~0.25 suggesting rather fat CAPE profiles along and east of US 1 which will make for prolific lightning producers. Latest Day 2 outlook from SPC has all of central NC in a marginal risk threat and will convey this threat in the HWO with the great severe threat likely to happen between 2pm to midnight. This pre-frontal band of convection will likely serve as the effective cold front, with only isolated/slight chance pops along the actual cold frontal passage late Wednesday night/early Thursday morning. Highs in the upper 80s NW to lower 90s. Lows Wednesday night will depend on the timing of the cold front, with some cooler mid to upper 60s possible across the NW Piedmont, and lower/mid 70s SE. && .LONG TERM /THURSDAY THROUGH TUESDAY/...
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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.
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&& .AVIATION /18Z TUESDAY THROUGH SATURDAY/... As of 245 PM Tuesday... 24 Hour TAF period: VFR conditions are generally expected through Wednesday afternoon with a couple of exceptions. Widely scattered showers/storms will move across the southern Coastal Plain near, but generally southeast of the KFAY terminal. An isolated shower or storm is possible across the western Piedmont including the KINT/KGSO terminal but coverage and confidence is too low to include in the TAF. Otherwise, skies will be partly to mostly clear with some patchy fog or stratus possible toward daybreak, mainly across the Coastal Plain. A strong cold front will move across the mountains Wednesday afternoon and into central NC during the evening. This feature will produce a band/broken band of widespread showers and storms that sweeps east during the afternoon and evening with local vsby and cig restrictions. Looking beyond the 24 hour TAF period: A band of convection associated with the cold front will sweep east across central NC during the afternoon and evening with adverse aviation conditions. The front will slip south on Thursday, keeping lingering convection possible across the southeast including the KFAY terminal on Thursday afternoon. Otherwise, high pressure will bring widespread VFR conditions to the area for Friday into the weekend. -Blaes && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Blaes NEAR TERM...Blaes/CBL SHORT TERM...CBL LONG TERM...MWS AVIATION...Blaes

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