Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 231922 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 320 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/... As of 320 PM Tuesday... ...Flood watch has been extended through Wednesday night ... 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(s) moving into the region. 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. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... As of 225 PM Tuesday... An initially neutral to slightly positively-tilted longwave trough from eastern Canada to the southern Plains, including an embedded mid-upper low over the mid Mississippi Valley/upper Midwest at 12Z Wednesday, will migrate eastward and assume an increasingly negative- tilt as it pivots across the Carolinas and middle Atlantic states by Thursday and Thursday night. A complex pattern will result at the surface on Wednesday and Thursday. A frontal zone will likely be draped northeast to southwest marking the southern periphery of weak ridging/CAD/low overcast extending across most of Virginia and North Carolina at the start of the day Wednesday then extending Northwestward as a warm front to a low over the western Ohio Valley. That front, acting as the edge of the CAD regime, and focus for convection, will then likely retreat northward with a narrow wedge of a warm sector airmass through southern and perhaps portions of eastern portions of the area during the day Wednesday, then through the remainder of central North Carolina early Wednesday night, all immediately ahead of a triple point low that will develop and migrate across the western Carolinas very late Wednesday afternoon and Wednesday night. Initial CAD conditions, and subsequent convection along the retreating warm front, will tend to slow the retreat of the unstable warm sector and north boundary for any severe threat through much of Wednesday, such that any strong to severe potential will likely remain from Wadesboro to Goldsboro and Rocky Mount and points southeast. That unstable warm sector should then surge Northward through the remainder of central North Carolina 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 eastward and through the Coastal Plain late Wednesday night. Brief drying and clearing will be probable through early midday Thursday, but clouds will redevelop with heating, with an additional band or bands of convection in fast, cyclonic, and Cold Air Advection bearing flow aloft, which will spread northeastward across central North Carolina through late afternoon. The western edge may result in a strongly forced line of storms along the eastern periphery of focused forcing for ascent accompanying the aforementioned upper trough/low. Cold temperatures and steep lapse rates aloft will favor small hail in even marginally strong cores; and a 50-70 kt South-southwesterly speed max on eastern 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. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... As of 225 PM Tuesday... Dry conditions will return by Thursday 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 Saturday and across the Middle Atlantic states by Sunday, with a return to warmer and more humid conditions, and a chance of convection over the weekend into early next week. && .AVIATION /18Z Tuesday through Sunday/... 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. && .RAH WATCHES/WARNINGS/ADVISORIES... Flood Watch through late Wednesday night for NCZ007>011-021>028- 038>043-073>078-083>086-088-089. && $$ SYNOPSIS...Badgett NEAR TERM...CBL SHORT TERM...MWS/Franklin LONG TERM...MWS/Franklin AVIATION...CBL is the U.S. government's official web portal to all federal, state and local government web resources and services.