Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 211905 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 305 PM EDT Sat Oct 21 2017 .SYNOPSIS... High pressure will extend across the Carolinas through Sunday. A strong cold front will approach from the west Monday, and cross central NC on Tuesday. && .NEAR TERM /THROUGH TONIGHT/...
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As of 305 PM Saturday... The sfc ridge associated with the mid/upper level high will drift offshore tonight through early Sunday. The resultant ely low level flow should advect enough warm moist air to produce patchy fog over most of the region by daybreak Sunday, though model signals/depiction of fog are rather weak per current guidance . Not expecting dense fog, visibilities on the order of 2-4 miles will be common, primarily along and east of highway 1. It will be a bit milder compared to previous nights with low temps in the upper 40s to around 50, and the low-mid 50s southeast.
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&& .SHORT TERM /SUNDAY AND SUNDAY NIGHT/...
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As of 305 PM Saturday... High pressure at the surface and aloft will drift offshore Sunday as a s/w digs into the lower MS Valley. The backing mid-upper level flow will usher moisture into our region, leading to an increase in mid-upper level cloudiness, primarily in the late afternoon through Sunday night. Still expecting enough sunshine to boost temperatures well into the 70s with locations across the south half hitting 80 degrees. Clouds will continue to increase and thicken Sunday night as the s/w deepens and becomes a closed low over the western TN Valley by early Monday. Strengthening isentropic upglide ahead of this system may generate a shower or two late Sunday night, primarily across the sw third of the forecast area. Southerly flow and thickening cloud cover will yield overnight temperatures mainly in the 55-60 degree range.
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&& .LONG TERM /MONDAY THROUGH SATURDAY/...
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As of 155 PM Saturday... Model consensus has improved greatly concerning the cutting off of a vigorous short wave moving into the Tennessee Valley Sunday night. This short wave subsequently lifts northeast into the Ohio Valley as it is absorbed into a stronger long wave trof which will be digging deeply into the eastern CONUS through midweek. A strong cold front associated with the lead short wave will move into the mountains early Monday, and race east across central NC during the afternoon and evening. Low level flow increases dramatically and very quickly ahead of this stacked system, with southerly low level jetting exceeding 35Kt by midday providing a deep surge of moisture (PW`s rapidly approaching 2.0 inches) ahead of the surface front. This initial surge of low level moisture and warm air advection will likely produce a pre- frontal QLCS, which would develop and move into the southwest Piedmont by early afternoon. The low level jetting continues to strengthen, with H85 winds exceeding 50 knots as the initial line of convection moves across central NC, and will likely be followed by another line of convection accompanying the surface front a couple of hours later. Instability will be unimpressive and somewhat of limiting factor, but the low level forcing/convergence and upper diffluence associated with the short wave lifting north of the area will be very strong. Damaging wind gusts could accompany stronger storms, and the strongly sheared low level profile would support rotating storms, so a tornado or two look to be possible. Will transition categorical PoPs across the area from 18-06Z, with a 6- hour window of 90% PoPs area-wide centered around 00Z tomorrow evening. Warm air advection will partially offset increasing cloudiness and highs will reach the low 70s west to near 80 in the southeast. The convective zone will move rapidly east of the area after midnight, with clearing and weak initial cool air advection over at least the western half of the area by sunrise. Mins will be mostly in the lower 60s, with some upper 50s in the west. Drier and cooler air will be filtering into the area on Tuesday, with potentially some instability showers across the northern tier as the main upper trof rotates across the Ohio Valley. Highs Tuesday will be mostly in the upper 60s to lower 70s, with further cooling into the low and mid 60s for Wednesday through Thursday. Mins will bottom out Wednesday night as the upper trof axis shifts east of the area and the drier airmass cools under better radiational conditions. Some climatologically cooler locations will fall to the upper 30s, with widespread lower 40s elsewhere. Shortwave ridging and return flow as surface high pressure moves offshore will give us a warmup late in the week, with highs Friday and Saturday mostly in the mid 60s to lower 70s.
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&& .AVIATION /18Z SATURDAY THROUGH THURSDAY/... As of 140 PM Saturday... There is a high probability that VFR conditions will persist across central NC through 00Z Monday as high pressure at the surface and aloft controls the weather across central NC. The exception will be after 04Z tonight as patchy fog will develop, likely reducing the visibility into the MVFR range, particularly in vicinity of KRWI and KFAY, and possibly at KRDU. Pockets of IFR/LIFR visibility may occur close to daybreak in proximity of KRWI. Pockets of fog will disperse within an hour or sunrise. The good aviation conditions are expected to persists through early Sunday evening. Later Sunday night and on through Tuesday, an approaching frontal system will draw a moist low level air mass into central NC leading to an increased threat for sub VFR ceilings as well as increasing coverage of showers and a few thunderstorms, especially late Monday through early Tuesday. A return to VFR parameters anticipated for Tuesday night through Thursday. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...WSS NEAR TERM...WSS SHORT TERM...WSS LONG TERM...mlm AVIATION...WSS

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