Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 230048 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 848 PM EDT Mon May 22 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 /UNTIL 6 AM TUESDAY MORNING/... As of 320 PM Monday... Atmosphere has become slightly to moderately unstable this afternoon across central NC excluding the far northeast piedmont and northern coastal plain. This air mass will support/maintain scattered convection across the region. The effective bulk shear is marginal at best (~25kts) and mostly confined to the southern half of the forecast area. This shear may be sufficient to organize the storms into segments, mainly south of highway 64, with locally strong winds associated with bowing segments. Convective parameters weaker to the north and northeast of Raleigh so expect any convection in this region to be rather tame. Convection will slowly diminish late this evening/early overnight before showers will increase in coverage from the southwest as an upper disturbance and attendant sfc low approaches. Prior to daybreak, expect moderate/locally heavy showers to cover most of the Sandhills and Piedmont. With precipitable water values hovering around 1.8 inches, the showers will likely be efficient rain producers. Thus, most locations will likely see a solid half inch of rain by daybreak Tuesday, with some some places in the western Piedmont approaching an inch. Min temps generally 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 /00Z TUESDAY THROUGH SATURDAY/... As of 840 PM Monday... 24 Hour TAF Period: Scattered to widespread showers and storms will be possible across southern portions of the area this evening, mainly at KFAY. Sub-VFR conditions in association with the convection is expected at KFAY as well. We may see some of this activity spreading northward to the northern TAF sites late this evening into early Tuesday morning. However, another area of low pressure is expected to lift northeastward tonight from the Deep South and cross central NC Tuesday morning. This will spread widespread showers and isolated storms across the area Tuesday morning. Another disturbance will follow quickly on the heels of the first helping to continue to spread scattered showers and storms across the area on Tuesday afternoon. Sub-VFR conditions are expected to develop early Tuesday morning, generally IFR/Low end MVFR. IFR cigs are expected to persist at KGSO/KINT and maybe even KRDU (where a stalled frontal zone will be very near). Locations to the east and south of the stalled frontal zone may see some slow improvement in cigs Tuesday afternoon, but will keep cigs in the MVFR range at KRWI and KFAY. Outlook: Adverse aviation conditions are expected to continue through Wednesday night, with a continued threat of precip and sub- VFR conditions. Lingering precip chances and sub-VFR conditions will be possible through Thursday, before VFR conditions return for good on Friday. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...WSS NEAR TERM...WSS SHORT TERM...WSS LONG TERM...BLAES AVIATION...BSD

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