Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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636 FXUS62 KRAH 230713 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 310 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/...
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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
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&& .SHORT TERM / /...
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As of 340 PM Monday... To be updated.
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&& .LONG TERM /WEDNESDAY THROUGH MONDAY/... As of 300 PM Monday... The upper-air pattern at the beginning of the long term period features a high amplitude trough extending southwesterly from the Great Lakes into the MS Valley with an embedded close low centered near MO/IL. The upper trough axis will slide east on Thursday and take on an increasingly negative tilt with both the GFS and EC moving the long wave trough axis off the mid- Atlantic coast by 12 UTC Friday. The southwesterly flow ahead of the trough strengthens with 75 to 85kts progged at 500 mb over the eastern Carolinas early Thursday. While NWP guidance is similar in the large scale pattern there are differences in some smaller scale features that will play an important role in the forecast details. At the surface, the pattern is rather complex with an existing weak CAD wedge in place across central NC on Wednesday morning with dew points in the upper 50s near the VA border and the upper 60s in the southern Coastal Plain. Guidance is probably too aggressive in driving the warm sector northward to the VA border by mid-afternoon, a more reasonable approach would lift the wedge front more slowly, eventually reaching the VA Wednesday evening as a surface low develops along the front in upstate SC Wednesday evening and moves to southwestern VA Thursday morning. A trailing cold front will then sweep east across the Carolinas and Virginia Thursday morning. Given the strong forcing for ascent ahead of the upper trough, low- level warm advection, localized ascent with the wedge front and then the cold front along with precipitable water values climbing in excess of 1.75 inches, another robust precipitation event is expected Wednesday morning through midday Thursday with general rainfall amounts of 1 to 1.5 inches with localized amounts in excess of 2 inches. Currently SPC has our region outlooked in general thunder and a marginal severe weather risk for the Wednesday and Wednesday night period. Initially, the severe weather risk should be confined to the southeast portion of the area but as the warm sector expands north and with the strong low to mid level flow producing 0-6km bulk shear values in excess of 40-50kts with a weak to perhaps borderline moderately unstable air mass. An expanding severe weather threat is expected, initially in proximity of the wedge front and then ahead of the advancing cold front. The significant precipitation should be over by Thursday but scattered showers and possibly some storms are expected ahead of the upper trough that will be located near the Appalachians. The combination of a relatively moist environment combined with steep low- level lapse rates should support scattered convection and fairly widespread cumulus field. All of the precipitation should end late Thursday night as the air mass stabilizes and larger scale subsidence develops. Dry weather is expected on Friday and Friday night. The pattern becomes less clear over the weekend as troughiness begins to develop over the mid and upper MS valley and a warm front lifts north across the region on Saturday and then stalls near or just north of NC for late Saturday and Sunday. A limited threat of convection persists into Monday but confidence in the forecast over the end of the holiday weekend is limited. Highs on Wednesday and Thursday will be muted by cloud cover and precipitation and modulated by the placement of the warm front. Highs should range around 70 in the Triad to around 80 across the southeast. Highs should moderate into the upper 70s to lower 80s on Friday and the mid to possibly upper 80s on Saturday through Monday. -Blaes && .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...
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Flood Watch through late tonight for NCZ007>011-021>028- 038>043-073>078-083>086-088-089.
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&& $$ SYNOPSIS...Badgett NEAR TERM...Hartfield SHORT TERM... LONG TERM...BLAES AVIATION...Hartfield

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