Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 230459 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 1255 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 /TONIGHT/... As of 1010 PM Monday... Convection over the south-central and southeast part of NC late afternoon and this evening has been slowly spreading out into a large showers/isolated thunderstorm area. This area of convection has been slowly building northward with time in response to the mean steering flow aloft. The main front has stalled over the region where convective outflows and ongoing convection has essentially made it nearly impossible to find. We turn to the current data and the blend of the Hi-Res models, which generally support continued development of new convection to the north of the current large "blob" of showers/isolated storms. We will place the high POP at categorical overnight for this region including up into the Triangle area. This activity may weaken in the next 3-6 hours, with additional showers/isolated thunderstorms expected across the south and west later tonight and Tuesday morning. The main threat overnight should be locally heavy rain and possible urban flooding. The severe threat is very low. Lows generally near the current readings in the mid-upper 60s. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... As of 340 PM Monday... ...Locally heavy rainfall likely Tuesday... The lead upper disturbance and attendant sfc wave will lift across central NC Tuesday morning. In its wake, there may be a temporary lull in the widespread showers and scattered storms before another disturbance and sfc wave lift northeast into our region Tuesday afternoon/evening. The available moisture coupled with decent convergence along a sfc front bisecting central NC southwest- northeast and divergence aloft supplied by a 300mb jetlet lifting ne along the spine of the Appalachians should maintain categorical PoPs through the day into the evening. These showers should dump another half to one inch of rain. This additional run-off will quickly fill creeks and streams, likely requiring the issuance of urban and small stream flood statements. If rainfall rates are greater, a flood watch may be necessary. Effective bulk shear projected to be stronger Tuesday afternoon though widespread rain showers will likely limit available buoyancy. If any locations have a threat for a strong/locally severe storm, it would be over coastal plain counties which should be on the eastern periphery of the rain showers. High temperatures will vary from near 70 NW to near 80 SE. Showers will gradually diminish in coverage and intensity Tuesday evening as the upper level support wanes. Min temps near 60 NW to the mid 60s SE. && .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/...
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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
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&& .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Badgett NEAR TERM...Badgett SHORT TERM...WSS LONG TERM...BLAES AVIATION...Hartfield

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