Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 231943 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 343 PM EDT Sun Sep 23 2018 .SYNOPSIS... A backdoor cold front will sag slowly southward through the area today. Behind the front, cool high pressure will extend southward through central North Carolina through Tuesday. Weak low pressure will track northward over coastal North Carolina Tuesday night into Wednesday. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... As of 255 PM Sunday... The low level stable pool remains over the N Piedmont at this hour, topped with high thin clouds streaking across N/W NC. But as the last few CAM runs have foretold, the low clouds have begun to break up, and recent GOES channel 2 vis imagery reveals stable wave clouds in this region, with convective clouds along its S/E edge. We`re finally starting to see some shallow showers along the differential heating zone at the edge of the wedge front, from around Scotland Neck SW near RWI and TTA. This is along a narrow zone of marginal CAPE and around 25 kts of effective shear, and as these should both be maintained for a few more hours, we should continue to see scattered showers and perhaps an isolated storm or two through the rest of the afternoon. Loss of heating should result in a diminishing of the convection, and a backing of weak steering flow to a SSE direction should prompt a drift of any remaining isolated convection into the NW Piedmont through the rest of the evening. Weak moist upglide that has abated for much of the day is expected to resume this evening, resulting in the low clouds filling back in tonight, with the thickest/lowest clouds across the climatologically- favored Piedmont, an area that may also see patchy drizzle overnight. Expect lows mostly in the low-mid 60s, perhaps some upper 60s SE. -GIH && .SHORT TERM /MONDAY AND MONDAY NIGHT/... As of 340 PM Sunday... The offshore-centered mid level anticyclone will build briefly W over the eastern Carolinas Mon, while the surface ridge centered and drifting E over the Canadian Maritimes noses SSW through central NC. Strengthening onshore low level flow from the SE will result in increasing moist upglide atop the weakening stable wedge, and recent model runs generally indicate patchy light rain breaking out over E NC (where upglide is initially strongest) before spreading inland into central NC through the day. Will start the day mostly cloudy to cloudy areawide, with chance pops east early, spreading NW through the area through the day, before decreasing and diminishing Mon night. Cloudiness will hold firm Mon night given the weak flow and moist and somewhat stable low levels overnight. Expect highs from around 70 NW to the low 80s SE, followed by lows in the mid to upper 60s. -GIH && .LONG TERM /TUESDAY THROUGH SUNDAY/... As of 345 PM Sunday... Tuesday morning the CAD will begin to loose its grip over the Triad as a wave pulls northeast around a subtropical ridge anchored off the east coast. As this occurs, the wedge will start to fall apart and loose cohesion. High temperatures Tuesday will quickly rebound with 850 mb temperatures approaching 18 degrees C towards the Coastal Plain. Simultaneously,an axis of PV will be located over the Mountains of western North Carolina. This again will make for a bimodal distribution in QPF Tuesday afternoon into Wednesday morning. The chance of precipitation looks to move towards a minimum Wednesday morning as the wave and aforementioned PV exits the area. This should provide a brief period of subsidence. Wednesday evening, a long wave trough axis will center over the central United States with an upper level disturbance pulling northeast. The wave will quickly pull northeast though towards Lake Ontario. The main batch of PV will miss NC, but PWATs will climb towards 2.00" along with an approaching upper level jet streak. This will help to increase the chance of precipitation Wednesday evening and keep it going through Friday. The initial shortwave mentioned above will cause a cold front to stall and washout across the area late Wednesday. The ECMWF actually holds onto the front as a wedge signal late Thursday, while the GFS (and CMC) are much weaker with the front. A secondary shortwave traversing northern Ontario late Thursday into early Friday will be stronger than the initial wave, but further north. This second wave will be followed by another surface cold front. The GFS is slightly quicker and stronger with the front compared to the ECMWF. Cooler and drier air will exist with this front, but is nothing compared to what is was advertised as in previous model runs. The secondary shortwave now looks to eject far to northeast with the rex block off the west coast of the U.S finally dislodging allowing for low pressure to deepen. These dropping heights off the west coast of the United States will only aide in the demise of the long wave trough. This means that even behind the cold front temperatures will still be near to above normal. && .AVIATION /19Z SUNDAY THROUGH FRIDAY/... As of 1215 PM Sunday... At INT/GSO/RDU: IFR cigs within the cool stable air north of the backdoor front will continue to gradually lift to MVFR this afternoon, with holes breaking through the overcast skies. A short period of VFR conditions is possible, mainly 20z-00z, with a small chance of a shower near RDU. But as we approach sunset, low clouds are expected to fill back in quickly, with a return to IFR cigs and to MVFR to IFR vsbys (fog) shortly after 02z. These sub-VFR conditions should hold through at least mid morning Mon. Cigs will slowly lift to MVFR between 14z and 18z (the end of the TAF valid period. At RWI/FAY: VFR conditions will hold at FAY, while MVFR cigs at RWI should improve to VFR by 20z. These two locations will also see a chance of a shower this afternoon and early evening, until around 00z. Conditions should then begin to drop to MVFR then IFR by 05z at RWI and by 07z at FAY, where they will remain until cigs/vsbys slowly improve to VFR after 15z. Looking beyond 18z Mon: Slow improvement will continue Mon afternoon, with MVFR to low-end VFR lasting through the rest of the day. A return to IFR is expected Mon night areawide, followed by another slow improvement to VFR Tue, with a chance of showers and storms, greatest at RWI/FAY. Clouds and the shower/storm coverage will increase for Wed/Thu/Fri as a front approaches from the NW and stalls over the region. -GIH && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Hartfield NEAR TERM...Hartfield SHORT TERM...Hartfield LONG TERM...Haines AVIATION...Hartfield

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