Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 231841 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 242 PM EDT Tue May 23 2017 .SYNOPSIS... A series of low pressure systems will cross central NC through mid- week, causing periods of showers and thunderstorms. Improving weather conditions are anticipated by late in the week. && .NEAR TERM /today through tonight/...
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As of 240 PM Tuesday... ...Flood watch continues through this evening... Mid-level perturbations, embedded within the deep cyclonic flow around the digging upper low centered over MN/IA and associated long wave trough encompassing much of the central/eastern CONUS, will continue to eject northeastward, atop a quasi-stationary frontal zone bisecting the SE states. With high PWAT of 1.5-1.8"(150 to 200 percent of normal and near daily record values) continuing to pool along this complex frontal zone through tonight, the threat of flooding will persist, with the next batch of moderate to heavy rain expected to move into the area this afternoon as the next prominent upper level disturbance currently over the Deep South/western Florida Pan Handle moves towards the Carolinas. With the approach of this disturbance, a wave of low pressure will develop along the front, which will serve to strengthen the low-level moisture convergence, particularly along and just on the cool side of the frontal zone where low-level f-gen will be maximized. Additional storm total amounts of 1-2" are likely with locally higher totals probable. Review of 24 hour rainfall totals over the past 24 hours indicate two areas of heavy rainfall: 1)Across the Sandhills and southern coastal plain where yesterdays convection produced an expansive area of 1 to 2 inches with some localized higher amounts up to 3 to 3.5 inches; 2) More localized/isolated 1 to 2 inches across portions of Forsyth and Guilford counties. In between these two areas, 24 hour rainfall amounts have generally been around 0.50". While the two highlighted areas above will likely be most susceptible to flooding, heavy rainfall of 2 to 3 inches in a short period of time could cause flooding anywhere, thus flood watch will continue for all of central NC. The threat of the heavy rain and flooding looks to be over aoa 06z, once the upper disturbance pushes NE of the area. Hampered by widespread cloud cover and rain, severe threat will be isolated and confined to the southern coastal plain(Sampson/Wayne counties), weak to moderate buoyancy developing east of the sfc low combined with strengthening effective shear of 40 to 50 kts could support a non-zero threat for an isolated tornado and straight line damaging winds. Highs ranging from the mid/upper 60s NW to mid/upper 70s SE. Lows tonight 58-65.
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&& .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
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As of 410 AM Tuesday... Little change in forecast weather pattern and forecaster rationale since 24 hours ago. Potential severe weather event on Wed looks very similar to the surface evolution/pattern, storm mode, and timing experienced a couple of weeks ago/on May 5th. An initially (slightly) positively-tilted longwave trough from ern Canada to the srn Plains, including an embedded mid-upper low over the mid MS Valley/upper Midwest at 12Z Wed, will migrate ewd and assume an increasingly negative-tilt as it pivots across the Carolinas and middle Atlantic states by Thu and Thu night. Fast, perturbed, and moist SSWly to Sly flow aloft will precede the trough. A complex pattern will result at the surface through the middle of the week. A frontal zone will likely be draped SWwd from a series of waves passing across and offshore the Middle Atlantic coast, marking the srn periphery of weak ridging/CAD/low ovc extending across most of VA/NC at the start of the day Wed, then extending NWwd as a warm front to a low over the wrn OH Valley. That front, edge of the CAD regime, and focus for convection, will then likely retreat Nwd with a narrow wedge of warm sector through srn and perhaps portions of ern NC during the day Wed, then through the remainder of central NC early Wed night, all immediately ahead of, and associated with, a triple point low that will develop and migrate across the wrn Carolinas very late Wed afternoon and Wed night. Initial CAD conditions, and subsequent convection along the retreating warm front, will tend to slow the retreat of the unstable warm sector and nrn bound for any severe threat through much of Wed, such that any strong to severe potential will likely remain from Wadesboro to Goldsboro and Rocky Mount and points SEwd. That unstable warm sector should then surge Nwd through the remainder of central NC early Wed night, immediately ahead of the triple point low, yielding brief --but likely sufficient-- (weak) destabilization to support surface-based QLCS storms capable of producing both swaths of damaging straight line winds and isolated tornadoes along embedded mesovortices/bows/lewps. The cold/occluded front will cause the warm sector and associated severe threat to collapse Ewd and through the Coastal Plain late Wed night-early Thu. Brief drying and clearing will be probable through early midday Thu, but clouds will redevelop with heating, with an additional band or bands of convection --in fast,cyclonic, and CAA-bearing flow aloft-- which will spread NEwd across central NC through late afternoon, the wrn edge of which may result in a strongly forced line of storms along the ern periphery of focused forcing for ascent accompanying the aforementioned upper trough/low. Cold temperatures/steep lapse rates aloft will favor small hail in even marginally strong cores; and a 50-70 kt SSwly speed max on ern fringe of the upper trough will support organized storm modes, including low-topped (splitting) supercells, given long and relatively straight forecast hodographs, which would be capable of producing severe hail and damaging straight line winds. Any tornado threat would be mitigated by both a veering of the low level flow to SWly (Ie. no backing of the sfc flow) and sfc dewpoints aob 60 degrees that would favor evaporatively-cooled downdrafts and cool RFD`s. Temperatures will be slow to climb into/through the 60s on Wed, probably reaching calendar day highs at Piedmont sites between 00Z and midnight, with afternoon highs in the 70s probable over the sern tier or two of counties. Lows in the 60s Wed night, and highs Thu in the upper 60s to 70s, inhibited by both variably to mostly cloudy skies and convection during the afternoon.
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&& .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
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As of 415 AM Tuesday... 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 --some probably severe owing to 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 by the weekend-- focused along and north of the front late Sat afternoon onward.
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&& .AVIATION /18Z Tuesday through Sunday/...
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As of 240 PM Tuesday... Upper level disturbances overunning a quasi-stationary frontal zone, with anomalously moist air in place, will result in a continuation of adverse aviation conditions through Wednesday night as a series of low pressure waves track along the front. Ceilings are expected to remain LIFR to IFR through the period, with visibilities decreasing to IFR to MVFR in the heavier rain showers. Additionally, with the wavy frontal zone bisecting the area, expect erratic/variable winds at central NC terminals. The heaviest rain is expected to impact the terminals between 21Z to 06z. Although there will be a lull in precip/convection until the arrival of the next surface wave late Wednesday afternoon/evening ceilings are expected to remain IFR to MVFR. A closed upper low digging into the southern Appalachians Wednesday night will bring another round of heavy showers and a few potentially strong thunderstorms to the area late Wednesday afternoon/evening, lingering into the predawn morning hours on Thursday. Improvement to mostly VFR is expected by midday Thu, although scattered afternoon showers generating MVFR conditions are possible. VFR conditions should return for Fri/Sat as a weak high pressure ridge extends into the area.
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&& .RAH WATCHES/WARNINGS/ADVISORIES... Flood Watch through late tonight for NCZ007>011-021>028- 038>043-073>078-083>086-088-089. && $$ SYNOPSIS...Badgett NEAR TERM...CBL SHORT TERM...MWS LONG TERM...MWS AVIATION...CBL

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