Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 241722 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 122 PM EDT Wed May 24 2017 .SYNOPSIS... A warm front will lifting through North Carolina this afternoon and into Virginia this evening. A cold front will move sweep east across North Carolina late tonight and early Thursday morning. A second cold front will move across the region later Thursday. Fair and dry weather will return Thursday night and Friday. && .NEAR TERM /THROUGH TONIGHT/... As of 1115 AM Wednesday... ...Little change in previous thinking... ...Multiple rounds of severe storms possible this afternoon through this evening, with localized flooding possible through tonight... Widespread severe weather possible today and this evening, with several factors appearing to line up ideally to support vigorous convection, including the possibility of discrete super cells this afternoon and a more organized line favoring bowing segments crossing the area late afternoon into the evening. Additionally, with saturated soil and high water in creeks, the risk of heavy downpours producing localized flooding and high water on creeks and rivers persists through the evening. An upper low over the MS Valley this morning will continue to dig south-southeastward toward the Mid-South this afternoon and evening and then into the SE states overnight. Individual strong vort maxes rotating about the deep cyclonic flow will impact the region through Thursday. However, the upper wave of most immediate concern that will impact central NC today and tonight, is currently over the Lower MS Valley, and is forecast to lift NEWD into the Carolinas by tonight. Deep layer lifting with the approach of the wave into the area coupled with a strengthening deep layer wind field will support the development of rapidly developing and robust convection. Three noteworthy features can be seen on the latest surface analysis: a roughly west-east wedge/CAD frontal zone across the northern Piedmont, a warm front lifting into southeastern NC, and a cold front to our west through the W Ohio Valley and lower Miss Valley. The latest high-res models take the wedge front northward gradually this morning, with expectations that the warm/maritime tropical front will rapidly retreat and potentially merge with the wedge front across the central/western piedmont this afternoon. And finally, the west-to-east passage of the cold/occluded front (featuring a triple point low tracking near or just NW of the Triad) very late evening through the overnight hours. Rapid destabilization as the warm sector spreads north and westward and an improving wind field/effective bulk shear increasing to 50 to 55 kts over NC, expect development of discrete northward-moving cells, starting in the southern CWA, by early to mid afternoon. Such development is suggested by the NAM Nest, HRRR, and SPC`s SSEO. The low stratus (NW CWA) and stratocu (SE CWA) will initially limit insolation and destabilization through the morning hours, thus limiting shower coverage early today over central NC, so will start the day with low pops. But the wind field will improve through a deep layer as afternoon approaches, with a 60-80 kt 500 mb jet spreading from the FL panhandle NE through the Carolinas, and a 30- 40 kt cyclonic 850 mb SSW jet over the Southeast nosing into NC. The GFS brings MUCAPE up to 750-1500 J/kg (NW-SE) this afternoon, likely due in part to a batch of higher mid-level lapse rates now over the north-central/northeast Gulf moving NE through GA/SC/NC later today, as well as the low level heating and resident moist low levels. On top of the increasingly favorable kinematics and thermodynamics is the increase in mid level DPVA and upper divergence, the latter associated with an accelerating southerly jet streak extending from SW GA through the eastern Ohio Valley later today. All of this supports the development and maintenance of strong to severe convection this afternoon into this evening, with large hail possible, along with damaging wind gusts. And, with the retreating N Piedmont frontal zone potentially providing a rich source of low level vorticity and enhanced 0-1km SRH of 200 to 400 ms, there will be a chance for a tornado or two, with the greatest threat over the N and W Piedmont. As the cold front shifts eastward late tonight, expect a gradual tapering down of precip chances, and a trend to mostly post- convection stratiform rain with embedded thunder, from WSW to ENE overnight, before ending late. The risk of high rainfall totals possibly leading to flooding is less certain, as the highest PW values have shifted east of the CWA, and we`re unlikely to see the deep saturation through the column that we saw yesterday. But given the wet ground from recent rains, the high water levels on area creeks, and a decent chance of S-N- oriented convective bands leading to training and streaks of high rainfall totals, isolated to localized flooding threats will persist. Will continue the flood watch as is, through much of tonight, areawide. Expect additional rainfall amounts of 1-2", highest in the NW CWA where moist upglide should be strongest. Thicker stratus across the far N and NW CWA associated with the wedge/CAD airmass may be hard to dislodge and thus may hold temps down significantly for much of the day. However, across the south and along the eastern and southern fringes of the central Piedmont, any breaks will cause a quick rise in temps. Highs ranging from around 70 NW to near 80 SE. Lows tonight 59-68 W-E. && .SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/... As of 115 PM Wednesday... Another convectively active day expected on Thursday although with a different character and a more limited coverage and threat. A vigorous closed mid and upper-level low and upper-level trough axis over the eastern OH and TN valleys on Thursday morning will become negatively tilted and cross central NC between 18 and 00 UTC. At the surface, an initial cold front will extend north to south across the eastern Piedmont of NC at 12 UTC and then push east reaching the coast by 18 UTC as a second cold front reaches the Foothills of NC by 18 UTC and then the coastal region toward 00 UTC. The radar should be relatively quiet during the morning rush hour although some scattered showers are possible along the initial cold front that will be moving from the Piedmont into the Coastal Plain. Ahead of the front, dew points will range in the mid and even upper 60s supportive of convection. As the front exits, dew points will fall into the mid and upper 50s. Ahead of the approaching upper trough, large scale forcing for ascent will increase during the day Thursday. This will be supported by increasingly cold air aloft with 500 MB temperatures of -18 to -20 deg C which combined with steep low level lapse rates will lead to widespread cloudiness and the development of scattered showers and thunderstorms very late in the morning and into the afternoon. Cold air aloft resulting in suppressed freezing levels and an evolving but still robust wind field supporting shallow supercells, will support numerous cells with small hail and a few storms producing larger hail. As the previous shift noted, long and relatively straight forecast hodographs would lead to some splitting supercells and the potential for severe hail and damaging straight line winds. Limiting factor for severe weather will be the amount of instability while high cloud bases will reduce the tornado threat, they will support enhanced downburst winds. The greatest severe weather threat will be from 16 to 22Z, especially across the northern and western Piedmont. Convection will decrease from west to east during the late afternoon as the secondary cold front moves through and the air mass stabilizes. High temperatures will range in the lower 70s northwest to upper 70s southeast. Fair and dry conditions are expected Thursday night as heights rise and a surface high pressure extends into the region from the south. Low temperatures will range in the mid 50s to around 60. -Blaes && .LONG TERM /FRIDAY THROUGH TUESDAY/... As of 350 AM Wednesday... Dry conditions will return by Thu night, as the upper trough lifts away and weak high pressure expands across the sern states through early Sat. A warm front will develop newd across the central Appalachians later Sat and across the Middle Atlantic states by Sun; with a return to warmer and more humid conditions, and a chance of convection focused along and north of the front this weekend. Some storms may be severe, given relatively strong wind fields, and steep lapse rates related to a large plume of EML that will likely have overspread much of the sern U.S. and srn Middle Atlantic states, around the top of a progressive sub-tropical ridge along the Gulf coast. The next trough aloft will amplify and progress across the central U.S. early next week; and the associated surface low will lift across, and occlude over, the Great Lakes. The related cold front will likely settle east of the Appalachians and serve as a focus for primarily diurnal convection Mon-Tue. Continued warm, in the 80s. && .AVIATION /17Z WEDNESDAY THROUGH SUNDAY/... As of 630 AM Wednesday... IFR to LIFR conditions will dominate across central NC through mid morning, as a warm moist air mass combined with light surface winds supports low stratus. FAY, however, will see a few periods of MVFR to briefly VFR clouds through this morning. Cigs are expected to rise to MVFR, with mostly VFR vsbys, at all sites by late morning (around 15z), perhaps reaching low-end VFR by early to mid afternoon (around 18z) as a warm front surges northward through the area with increasing mixing. Patchy light showers are expected through early afternoon, followed by more widespread showers/storms moving in from the south and west during the mid to late afternoon. A period of IFR conditions in heavy showers and storms with gusty erratic winds are possible after 18z at INT/GSO and after 19z at RDU/FAY/RWI. Some storms this afternoon into early evening may be strong to severe, with large hail and intense wind gusts. Then, conditions will trend to MVFR then VFR from west to east in the evening with decreasing rain coverage, after 02z at INT/GSO, after 04z at RDU, and after 06z at RWI/FAY, although patchy clouds based below 900 ft AGL may form late tonight. Some storms this afternoon into early evening may be strong to severe, with large hail and intense wind gusts. Looking beyond 12z early Thu morning, cigs should rise to VFR after 13z or 14z Thu, but there will be a good chance for showers with small graupel/hail during the day Thu as the axis of a powerful mid level trough swings through the area. Skies will clear out late Thu night, then VFR conditions should dominate Fri through Sun, although there will be a chance for a few storms late Sat into Sun as a cold front approaches. -GIH && .RAH WATCHES/WARNINGS/ADVISORIES... Flood Watch through late tonight for NCZ007>011-021>028- 038>043-073>078-083>086-088-089. && $$ SYNOPSIS...Blaes NEAR TERM...CBL/HARTFIELD SHORT TERM...BLAES LONG TERM...MWS AVIATION...HARTFIELD

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