Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 230820 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 420 AM 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 310 AM Tuesday... ...Flood watch in effect today through this evening... Coverage of showers and embedded storms has fallen to a relative minimum early this morning, however slow-moving convective cells with 1-2"/hr rates have produced pockets of high totals over parts of the Piedmont in the last several hours. Areas of showers and storms will continue to spread over central NC from the SW through the day, as mid level perturbations track northeastward in the cyclonic flow around an expansive low centered over MN/IA and covering much of the central/eastern CONUS. The surface pattern is quite messy this morning, muddied by subtle synoptic-scale features and convectively-induced boundaries and mesoscale highs within a light low-level flow. There appears to be two main dewpoint discontinuities, one just NW of the forecast area and another to the south across central GA and central SC. PW values along and south of the latter frontal zone are close to daily record values, and these (as well as the front itself) are expected to surge northward into central/eastern NC through the day. Forecast soundings continue to depict a deep saturated warm layer (LCL-0C of nearly 4 km) over central NC later today, favoring warm rain processes, and the initial weak flow will lead to slow-moving and poorly organized cells through this morning, until the steering flow picks up a bit toward midday. Most models with both parameterized and explicit convection generate areas of heavy rainfall totals today through this evening, although they differ on location, likely a function of the murky and diffuse surface pattern and resultant variations in low level mass convergence that are tipping the scales to heavy rain production. With the high water content and strengthening deep layer forcing for ascent as the most prominent mid level wave tracks NE through the Carolinas today, expect likely to categorical pops areawide today, with a risk of high rain totals and pockets of flooding possible anywhere. Based on the pace of movement of the rain area now extending back across GA/AL and southern MS and the model projections of the movement of the upper divergence maxima across the Carolinas today, expect the peak threat of flooding rains to occur late morning through mid afternoon, gradually tapering down and pushing to our ENE this evening. Will issue a flood watch in effect from 5 am this morning until 2 am tonight, with storm total amounts of 1-3" likely and locally higher totals probable. Will taper down pops west to east tonight as the mid level wave and upper jet core shift to our NE, although a moist and stable pool overnight will lead to lingering areas of drizzle and thick stratus. Temps have a high error potential today, as any amount of sunshine in the SE CWA could send readings soaring. Will go with highs from the upper 60s NW to the upper 70s SE. Lows tonight 58-65. -GIH && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... 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. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... 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. && .AVIATION /06Z Tuesday through Saturday/... As of 1255 AM Tuesday... Lower than usual confidence this morning. Aviation conditions currently vary from VFR at INT/RDU/RWI to MVFR at GSO and IFR at FAY, and this variation will continue through 09z this morning, after which time we should see a trend to prevailing MVFR/IFR at INT/GSO/RDU and perhaps RWI, with MVFR to VFR conditions at FAY this morning. A large area of showers and isolated storms is expected to spread over central NC throughout the day, lasting into the evening, and a trend back down to prevailing IFR conditions are anticipated at all sites by late morning, with low cigs and occasional MVFR to IFR vsbys in heavy rain. Unfortunately, details regarding the specific timing of this heavy rain and storms is uncertain, but we do have confidence in poor aviation conditions dominating through tonight, as low (mainly IFR) cigs are expected to persist overnight even as the heavier rain departs from west to east between 00z and 06z. A surface front oriented SW to NE across central NC will hold in place today as low pressure tracks northeastward along it, leading to erratic winds at central NC terminals, mainly from the SE/S shifting to SW at FAY, and mainly from the NE elsewhere through tonight. Looking beyond 06z late tonight/early Wed morning, adverse aviation conditions are likely to hold Wed through Wed night as a deep mid level trough and surface cold front approach from the west, bringing another round of heavy showers and a few potentially strong thunderstorms. Improvement to mostly VFR is expected 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. -GIH && .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...Hartfield SHORT TERM...MWS LONG TERM...MWS AVIATION...Hartfield

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