Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 250829 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 429 AM EDT Thu May 25 2017 .SYNOPSIS... A cold front will cross the area this morning, followed by the passage of a strong upper level trough this afternoon. Quiet and dry weather will return Friday. Warmer temperatures and chance for showers and storms will return over the weekend. A cold front will approach from the west on Memorial Day. && .NEAR TERM /THROUGH TONIGHT/...
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As of 425 AM Wednesday... 00Z/25th upper air/RAOB data and more recent WV satellite imagery depict a double-barreled mid-upper low featuring centers over wrn OH and nrn AL. A pronounced lead perturbation in SSWly flow on the ern periphery of the parent trough aloft is also apparent over west- central NC, with associated forcing for ascent and leading edge of colder temperatures aloft/lowering freezing levels likely having contributed to the earlier uptick in coverage and intensity of a band of (briefly severe) storms and related cold/occluded surface front now nearing the I-95 corridor. There will likely be a lull in precipitation behind this band this morning. Thereafter, the srn-most mid-upper low now over nrn AL will lift in an increasingly negatively-tilted fashion across the Carolinas -- directly over central NC by ~18Z-- to just off the srn Middle Atlantic coast by 00Z Fri. Although surface dewpoints will have dried into the 50s to around 60 degrees behind the aforementioned cold/occluded front, diabatic warming into the 70s, beneath a pocket of very cold temperatures/steep lapse rates aloft (characterized by near minus 20C temps at 500 mb), and strong quasi-geostrophic forcing for ascent accompanying the upper low/trough, will favor the development of around 500-750 J/kg of mixed layer instability --and the re-development of scattered to numerous showers and storms over central NC-- after 15-16Z. The aforementioned cold temperatures/steep lapse rates aloft, and associated low freezing levels generally between 8-10 thousand ft, will favor small hail in even marginally deep/reflective cores. In addition, a 60-70 kt SSwly mid level jet on the ern fringe of the upper trough will be supportive of organized storm modes over thje ern half of NC, including low-topped (splitting) supercells, given long and relatively straight forecast hodographs, particularly between 3-6 km. There will be a related conditional threat of severe hail, particularly east of U.S. Hwy 1, where both instability and SSWly mid level flow/bulk shear will be relatively maximized in a brief window between 17-21Z. Strong to damaging straight line winds, driven by both the background mid level flow and sub-cloud evaporational/diabatic cooling, may also result. After about 21Z, the passage of the trough axis will cause the mid level flow to weaken and veer and consequently mitigate an earlier severe threat, while small hail will remain possible through early evening. Lastly, mixed character precipitation (Ie. a stratiform shield of light rain with some convective elements), similar to that now near Cincinnati, will likely pivot across the nrn piedmont as the parent mid level moist axis swings around the base and wrn side of the upper trough; and this may result in a few sprinkles there this evening, until the moist axis lifts away after 03Z. High temperatures mostly in the 70s. Lows in the 50s, with a light but steady Swly breeze in the MSL pressure gradient between sub-1000 mb low pressure that will be lifting away from the Middle Atlantic coast, and incoming 1015 mb high pressure from the nrn GOM and Gulf coast.
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&& .SHORT TERM /Friday through Friday night/... As of 115 AM Thursday... The mid level low over the Northeast early Fri will continue moving to the NE along the coast of the Canadian Maritimes through Fri night, with its trailing trough axis lifting NE away from NC Friday morning. Rising heights aloft and a drying/stabilizing air mass -- as agreed upon by the latest model runs -- supports plenty of sunshine with no pops. Low level thicknesses start the day well below normal but do start to recover in the afternoon, indicating highs just a tad under normal, 79-84. Fast mid level winds from the WNW Fri night will likely draw some high level moisture across the region, and we could see a little orographic enhancement of these high clouds, based on the forecast thermal and moisture profiles. Expect lows of 60-64. -GIH && .LONG TERM /Saturday through Wednesday/... As of 410 AM Thursday... Sat/Sat night: A very broad and flat anticyclonic flow will stretch from NW Mexico across the southern Plains and mid Miss Valley to the Mid Atlantic region, north of a weak ridge centered over the NW Gulf, to begin the holiday weekend. At the surface, a weak and elongated Bermuda high will extend across the Southeast states, with a weak frontal zone to our north across VA/KY. Models show at least one subtle wave (which may be an old MCV) tracking within this fairly fast mid level flow across the Mid Atlantic Sat evening/night, prompting convection chances across northern and eastern NC late Sat but particularly Sat evening and into the night, accompanying a weak surface low tracking along the surface front boundary. With good model agreement on this general picture, will ramp up pops across the north late Sat afternoon, showing a max in the NE CWA Sat evening before decreasing slowly post-wave overnight. Thicknesses will have rebounded back above normal, so expect highs from the mid 80s far NW and near the VA border to the lower 90s SE. Lows Sat night 65-70. Sun-Mon: Starting Sun, a mid level low over W Ontario will deepen and drop SE across the N Great Lakes, as broad troughing begins to dig over the central and eastern CONUS. This trend from gently anticyclonic or zonal to cyclonic flow with the trough axis to our west indicates a moist return flow and increasing chances for mainly afternoon/evening convection, triggered in part by weak perturbations tracking from the W Gulf Coast northeastward across the Carolinas. At the surface, we`re likely to see a cold front holding well to our NW, but with a weak prefrontal surface trough extending through the Piedmont. This will likely serve as a focus for the expected storms, with models suggesting the best coverage during the afternoon and evening of Memorial Day (unfortunately for many). Will go with a good chance of showers/storms each afternoon and evening. The aforementioned cold front is expected to drop SE into NC by late Mon, but appears likely to wash out before having any major impact on temps, as the mid level trough axis will still be to our west. Highs both days in the 80s to around 90 (with Sun perhaps a degree or two hotter than Mon) with muggy lows in the 60s to near 70. Tue-Wed: As the mid level trough over central and eastern NOAM broadens and shifts eastward, we should see a reinforcing front dropping through the area, helping to bring in drier and slightly cooler temps by mid week. Expect low shower/storm chances to be confined to SE sections Tue, with dry weather everywhere Wed as weak high pressure starts to build in from the W. Highs from around 80 to the mid 80s, and lows around 60 to the mid 60s. -GIH && .AVIATION /06Z THURSDAY THROUGH TUESDAY/... As of 220 AM Thursday... A band of strong storms now extending from near HBI to RCZ to FLO will sweep northeastward and affect RDU, FAY, and RWI during the next few hours. Small hail and wind gusts up to 40 kts will accompany the storms. Otherwise, IFR-MVFR ceilings will gradually lift and scatter from west to east through 12Z, as a lead cold/occluded front and following drier air in the low levels spreads east across central NC this morning. Scattered to numerous showers and storms, again with hail and strong wind gusts, will redevelop by late morning to midday, as a potent upper level trough and associated very cold temperatures aloft pivot northeastward across the forecast area. While mostly small hail will be common, some may exceed one inch in diameter between 17-23Z. Outlook: At least a couple of clusters of showers and storms, some strong to severe, and with gusty outflow winds surging well away from them, will return to portions of central and/or northern NC for Sat afternoon through Sunday. The approach and stall of a couple of cold fronts east of the Appalachians may result in a band of MVFR ceilings over central NC Mon night, and also a small chance of diurnal convection both Mon and Tue. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Hartfield NEAR TERM...MWS SHORT TERM...Hartfield LONG TERM...Hartfield AVIATION...MWS

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