Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 230058

Area Forecast Discussion
National Weather Service Raleigh NC
858 PM EDT Tue Aug 22 2017

A pre-frontal surface trough extending from the mid-Atlantic south
into the Carolinas will persist into Wednesday. A strong cold front
will move across central NC Wednesday afternoon and Wednesday night.
High pressure will build into the region late Thursday and
especially Friday and Saturday.


As of 750 PM Tuesday...

Isolated showers have had a tough time developing late this
afternoon, even along the surface trough over the Piedmont.
Convection over SC has been weakening and decreasing in coverage as
well along the Piedmont trough there. It appears that the chance of
any additional convection is 10 percent or less. We will update the
forecast now that we have sunset and the CINH will only be
increasing as the evening rolls on. Upstream convection moving
toward the Appalachians is expected to diminish before crossing the
mountain barrier later tonight.

Another warm and humid night is expected with lows in the 70s, upper
60s in the normally cool spots in the northern and central Piedmont.


As of 330 PM Tuesday...

A broad, 590m west to east ridge at 500 hPa extending from the
northern Gulf of Mexico into the southwestern Atlantic on Wednesday
morning, relaxes as troughing develops across the eastern Great
lakes and the Northeast. Modest height falls are noted across
central NC on Wednesday afternoon and evening as an initial
shearing short wave trough shifts east during the day with a
second stronger short wave approaching the region from the
northwest on Wednesday night. A prefrontal surface trough
located across the Piedmont will shift east during the day as
the main cold front moves across the mountains during the
afternoon and into central NC Wednesday night.

These features when combined with a weak to moderately unstable
air mass will result in the development of scattered to
possibly numerous showers and thunderstorms. The various
features are disjointed and will likely result in the
development of a couple of bands/broken lines of storms across
central NC during the afternoon, one associated with the pre-
frontal trough and lead short wave and the other with the cold
front and more pronounced trough at 850/700 hPa with these
clusters likely merging into a singular line toward the Coastal
Plain during the evening.

SPC currently has most of central NC outlooked in a marginal
severe weather risk. Destablization is perhaps not as impressive
as previously progged with afternoon MLCAPE values generally
ranging from 800 to 1800 J/Kg. The instability is muted by warm
mid levels and weak mid-level lapse rates. Bulk shear values
range from 15 to 25kts with a fairly unidirectional southwest to
westerly flow which would support line clusters. The end result
would be a primary severe weather threat from straight-line
damaging wind gusts. Will retain current wording in the HWO with
the great severe threat likely to happen between 2pm and

Highs on Wednesday should range between 88 and 94. Expect the
cold advection to lag a bit so probably not a lot of cooling
on Wednesday night when lows will range between 67 and 74. -Blaes


As of 335 PM Tuesday...

At the base of a vortex over Quebec, a shortwave trough in NW flow
aloft will amplify across the OH valley and nrn Middle Atlantic
states on Thu, then lift up the Northeast coast through Fri,
followed by a lingering trough aloft over the Northeast and Middle
Atlantic through the weekend. A positive height anomaly/ridge aloft
initially over central Canada will meanwhile migrate ewd, to ern
Canada and the Northeast US through early-mid next week. Relatively
strong (1025-1030 mb) Canadian high pressure will precede the upper
ridge and extend south across most of the Ern US throughout the
period. Lastly, the models continue to indicate an area of disturbed
weather initially over the Bahamas will strengthen as it drifts
slowly newd, roughly parallel to and offshore the Sern US coast.
While a significant wly component to the flow aloft should keep
whatever becomes of this system too far east to directly impact
central NC, the MSL pressure gradient between this system and the
aforementioned Canadian surface high would result in breezy
conditions for central NC, particularly the ern half, both Mon and

The amplification of the aforementioned shortwave trough in nwly
flow aloft, in conjunction with lingering frontal forcing over sern
NC, should result in convective redevelopment in the vicinity of the
front from the srn piedmont and Sandhills to the ern piedmont and
Coastal Plain on Thu, with a gradual drying/clearing trend from nw
to se throughout the day. It should then be dry, and relatively
cool, through early next week, though with a slight chance of a few
showers late Mon-Tue in enely flow around the aforementioned
offshore low.


As of 830 PM Tuesday...

Primarily VFR conditions are expected through the TAF period.

While a persistence forecast of low overcast and fog for Wed
morning, like the past two mornings, seems reasonable given little
change in the overall weather pattern, the models suggest this
potential is relatively low. This reduced risk for sub-VFR
conditions appears to be a result of a veering of the flow just
above the surface from a more moist sswly one to a significant a
more wly one (ie. a drier and/or drying wind direction east of the
Appalachians), and also stronger winds in the low levels than
previous days, which would favor mixing and an environment not
conducive for the development of fog.

Scattered showers or storms are likely to develop in the vicinity of
a Piedmont trough near INT/GSO/RDU early Wed afternoon; and some of
these will likely grow in coverage and intensity --and merge into
clusters-- as they propagate ewd and affect FAY and RWI later in the
afternoon through the evening. Initially sswly surface winds will
become wly to nwly at Piedmont sites, owing to the ewd
development/progression of the aforementioned Piedmont trough,
followed by a secondary wind shift to nwly or nly with the passage
of a cold front just beyond the 00Z TAF period.

Outlook: The cold front will progress ssewd across central NC Wed
night, with some lingering showers and storms possible at RWI and
FAY until the front fully clears the area by Thu afternoon.
Following Canadian high pressure will then favor generally VFR
conditions through the weekend.




NEAR TERM...Badgett
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