Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 151151 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 650 AM EST Sun Jan 15 2017 .SYNOPSIS... A cold front will drop southward across central NC today. High pressure will build in behind the front and reinforce a cold air damming airmass over the area through Monday night. && .NEAR TERM /THROUGH TONIGHT/...
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As of 350 AM Sunday... Despite low clouds over VA and fog over SC, central NC is mostly clear this morning owing to westerly low-level downslope flow. There have been a couple periods of orographic cirrus just north of the VA border, but not enough to eliminate a small chance of some patchy fog around sunrise. Today: Given a lack of precip and diabatic enhancement of the CAD airmass the past day or so, the cold front over central SC this morning will continue to weaken under good heating today. However, a secondary cold front over central VA this morning will surge south into the norther Piedmont by midday and through all of central NC by early evening, as a 1032mb surface high shifts from the Midwest to the Mid-Atlantic states. As was the case on Friday (which today`s pattern seems to strongly resemble) the cold advection isn`t terribly strong and may lag the northeast wind shift by a few hours. The front should induce some low-clouds between 2-4k ft, especially in the coastal plain per NAM/GFS forecast soundings, but the flow is pretty weak throughout the low- levels, so it`s hard to see widespread stratus until later tonight. Highs should feature a large range from north to south again, as the over all lack of greater cloud cover will allow for good heating, specially across the south through early afternoon. Most guidance suggests 50-65 for highs, though the RAP/HRRR has highs of 70 in the southern Piedmont. Tonight: The cold front will push south of the area by this evening, with weak cold advection and a light northeast wind overnight. The H9 flow will swing around to southerly after midnight, resulting weak (and shallow) isentropic lift in the western Piedmont. As a result, some patchy light rain or drizzle should break out and drift east through the Piedmont through Monday morning, with low clouds eventually engulfing the entire area. Lows 38-45.
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&& .SHORT TERM /MONDAY THROUGH MONDAY NIGHT/... As of 350 AM Sunday... CAD will persist Monday as the parent surface high shifts to the Mid-Atlantic coast and then eventually offshore. The main impacts look to be from just widespread low clouds since the H9-H85 flow is weak (< 30kt) and the flow aloft is flat. Low clouds will result in highs several degrees cooler than today, but the lack of precip should keep highs within a couple degrees but on the cooler side of guidance; 45-52. The aforementioned surface high will shift offshore Monday night, but our CAD airmass should linger without an erosion mechanism. Warm advection will start to increase over the western Piedmont, on the eastern flank of a prefrontal LLJ over the Tenn Valley. Most of the associated rain will be west of the mountains, but their could be a few areas in the Foothills and flirting with the western piedmont by Tuesday morning. Lows 42-46. && .LONG TERM /TUESDAY THROUGH SATURDAY/... As of 400 AM Sunday... The models are in relatively good agreement with the large scale pattern across NOAM through the start of this forecast period (early Tue), and this pattern will maintain continued cool and cloudy conditions --from in-situ CAD-- that will linger through midday Tue. A warming SSW low level flow will develop later Tue, and continue until a cold front accompanying a cyclone crossing the Great Lakes settles into the Srn Middle Atlantic states with an associated chance of showers centered around the day Wed. Primary forecast uncertainty for the remainder of the forecast period is how much nrn stream flow amplification of a series of nrn stream, split flow shortwave troughs occurs from Canada, and subsequently 1) to what degree they interact with a longer wavelength/less progressive srn stream, and 2) possibly slow and form a closed low across New England and the adjacent Canadian Maritimes. The handling of these features will play a role in the timing and degree of longwave riding and possible blocking over the ern U.S., and to what degree, if any, upstream shortwave energy can penetrate the ridge. It appears at this time that surface ridging, beneath the ridge aloft, will generally dominate late in the week and early next weekend, with one deamplifying shortwave trough likely to migrate into the ridge with an associated increase in cloudiness and chance of rain late Fri or Fri night. It will be a mild pattern, however, with temperatures remaining above freezing throughout the forecast period. && .AVIATION /12Z SUNDAY THROUGH THURSDAY/...
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AS OF 650 AM Sunday... VFR early this morning, with clouds at 6-8k ft drifting east across the area and some very isolated MVFR vsbys. A cold front will surge south across the area between 15z and 21z today, causing the currently light and variable winds to become northeasterly at 6- 10kt. Models have struggled with this weather pattern the last couple of days, and seem to still be in disagreement on how quickly MVFR ceilings will redevelop, but recent trends suggest some scattered MVFR ceilings this afternoon and then a better chance of cigs lowering at FAY/INT/GSO/RDU after 00Z. The lowest ceilings should be from FAY to GSO/INT after 03Z, once the low level flow swings around to southerly again, though it`s not clear how low cigs will get, Given how weak the low-level winds are, the current TAF will only reflect MVFR ceilings overnight, with light rain at INT/GSO. Outlook: Conditions are expected to at least MVFR by 12Z Monday, and possibly IFR at INT/GSO, as southerly flow reinforces a cold air damming airmass over the area. Low cigs should persist into Tuesday, when there will be a chance for low clouds to scatter ahead of a cold front approaching from the west. The cold front and associated rain and low cigs/vsby will should impact the area Wednesday. VFR Conditions will return to all terminals in the wake of the cold front Wed night and Thursday.]
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&& .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...SMITH NEAR TERM...SMITH SHORT TERM...SMITH LONG TERM...MWS AVIATION...SMITH

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