Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 191508 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 1108 AM EDT Fri May 19 2017 .SYNOPSIS... High pressure anchored offshore will slowly weaken its grip on central NC Friday. This will allow a cold front to drop southward into our region late Saturday and Saturday night, then retreat north as a warm front on Sunday. && .NEAR TERM /THROUGH TONIGHT/...
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As of 1105 AM Friday... Latest surface analysis shows a cold front extending from the NYC area southwest to the Ohio River. A surface ridge of high pressure is present well off the southeast coast. The pattern aloft features a 590dm ridge centered just off the Florida east coast and extending into the Carolinas. A southwesterly flow across Texas extending into the mid Mississippi Valley and then bending east into the mid- Atlantic extends along the periphery of the ridge. Water vapor imagery and NWP analysis shows a couple of disturbances rotating around the ridge. These features initiated and sustained convection across the Tennessee Valley and southern Appalachians overnight and this morning in a region of warm advection and favorable mid-level lapse rates of 6.5 to 7 deg C/km. Observational trends and guidance suggest these features will shift east and will promote convective activity across the northwest Piedmont of NC that will shift east across the northern Piedmont and northern Coastal Plain, especially near the Virginia border. Mid- level lapse rates should remain unimpressive to the south, closer to the ridge, so while this afternoon and evening should feature better convective coverage than prior days, most of it should be confined near and north of U.S. 64. Weak deep layer shear of around 20 to 25 kts with some support for enhanced downdrafts and cold pools, could support some limited storm organization in clusters or small line segments. This would support a low- end/marginal severe threat, mainly across the north with isolated straight line winds being the most prominent threat. As the first area of isolated to scattered convection pushes east during the late afternoon and evening, it should weaken with the loss of heating. Convection allowing models suggest that another area of weak convection associated with another disturbance in the westerly flow aloft may move from the mountains into the western Piedmont during the mid evening hours before weakening overnight. Will retain a slight chance of shower or thunderstorm across the north to handle this possibility. Highs today should range in the upper 80s to near 90 with overnight lows generally in in the mid to upper 60s. -Blaes .SHORT TERM /SATURDAY AND SATURDAY NIGHT/... As of 400 AM Friday...
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Model differences with the timing of the back-door cold front into the area on Saturday is resulting in lower than normal forecast confidence for the day 2 period. Downstream of the closed low lifting northeast across the central and northern Plains, mid/upper level ridge will actually strengthen across the region on Saturday. So it`s really not a surprise to see models trending slower with the timing of the back-door cold front into the area, potentially delayed until the afternoon/early evening. The pre-frontal warm sector which was once thought to lie across the southern tier counties, now could potentially encompass all but the far northeast Piedmont/coastal plain counties. Highs Saturday ranging from near 80 NE to mid/upper 80s central areas, to lower 90s south. Due to uncertainty in the location of the front, convective details/coverage are difficult to ascertain as this time, but as the warm sector destabilizes with moderate instability likely to develop across the area, scattered to possibly numerous showers and storms are possible Saturday afternoon/early evening. For now will go with a good/high chance across the central/northern Piedmont and coastal plain, which is hedging towards the slower model solutions. Deep layer shear remains on the order of 20 to 25 kts and with the presence of the upper ridge overhead deflecting and appreciable support/forcing aloft, severe threat should be rather localized/isolated. Front should eventually sag south to along the SC/NC border Saturday evening/night with post-frontal low clouds/stratus on the cool side of the boundary Saturday night. Will keep a slight chance of showers/rain Saturday night owing to proximity to the front. Lows in the 60s.
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&& .SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/...
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Model differences with the timing of the back-door cold front into the area on Saturday is resulting in lower than normal forecast confidence for the day 2 period. Downstream of the closed low lifting northeast across the central and northern Plains, mid/upper level ridge will actually strengthen across the region on Saturday. So it`s really not a surprise to see models trending slower with the timing of the back-door cold front into the area, potentially delayed until the afternoon/early evening. The pre-frontal warm sector which was once thought to lie across the southern tier counties, now could potentially encompass all but the far northeast Piedmont/coastal plain counties. Highs Saturday ranging from near 80 NE to mid/upper 80s central areas, to lower 90s south. Due to uncertainty in the location of the front, convective details/coverage are difficult to ascertain as this time, but as the warm sector destabilizes with moderate instability likely to develop across the area, scattered to possibly numerous showers and storms are possible Saturday afternoon/early evening. For now will go with a good/high chance across the central/northern Piedmont and coastal plain, which is hedging towards the slower model solutions. Deep layer shear remains on the order of 20 to 25 kts and with the presence of the upper ridge overhead deflecting and appreciable support/forcing aloft, severe threat should be rather localized/isolated. Front should eventually sag south to along the SC/NC border Saturday evening/night with post-frontal low clouds/stratus on the cool side of the boundary Saturday night. Will keep a slight chance of showers/rain Saturday night owing to proximity to the front. Lows in the 60s.
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&& .LONG TERM /SUNDAY THROUGH THURSDAY/...
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As of 350 AM EDT Friday... Mid-level ridge axis positioned over the SE states Sunday morning will gradually shift east during the day. Meanwhile at the sfc, an old boundary along the NC/SC coast will gradually dissipate as flow veers to a more southerly direction with the aforementioned ridge shifting east. Models are suggesting that a few showers will be possible, particularly across our southern zones and later in the day, invof of the old boundary. Highs will range from the upper 70s north in the cooler airmass north of the boundary, to mid 80s south near the SC border along and south of the boundary. On Monday, a cold front assoc with a developing longwave trough over the Great Lakes, Ohio Valley, and Midwest regions will approach from the west and move across central NC during the afternoon. A band of showers and thunderstorms assoc with this front will move across the area during the daytime, thus likely POPs in the forecast for daytime Monday. The risk for rain will gradually end front west to east Monday night as the front exits. The period Tuesday through Wednesday looks potentially unsettled as the aforementioned longwave trough further deepens to our west and slowly moves east across the eastern third of the country, but the GFS and ECMWF differ in the finer details. For example, the GFS features dry/fair weather for Tuesday in the wake of Monday`s front, then rain on Wednesday as the next front moves across. Meanwhile, the ECMWF transports copious moisture northeastward across our area late Tuesday ahead of Wednesday`s front, resulting in good chance POPs both days. For now, will keep pops near climo on Tuesday, with increasing pops Tuesday night and Wednesday. Under the influence of ample cloudiness, daytime temps will be held to near normal values with above normal temps at night.
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&& .AVIATION /15Z FRIDAY THROUGH TUESDAY/... As of 115 AM Friday... With the exception of some isolated pockets of fog/stratus, expect generally VFR conditions overnight. Shortwave disturbances traversing the region along with pre-frontal surface trough over western NC will support scattered convection this afternoon/early evening, subsequently affecting the northern terminals. Will include prob30 for convection at KINT and KGSO with the 06z taf package, with future updates possibly needing to add KRDU and KRWI. Surface winds will remain light from the SSW to SW through the period. Outlook: A cold front will settle into and at least partially through central NC Sat and early Sat night, then retreat back NEwd through central NC as a warm front on Sun. This feature will be accompanied by both scattered convection along it, and post-frontal IFR-MVFR ceilings on the cool side of it. Another cold front, numerous showers and storms, and sub-VFR conditions, will move east across central NC Mon through early Mon night. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...WSS/CBL NEAR TERM...Blaes SHORT TERM...CBL LONG TERM...NP AVIATION...CBL/MWS

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