Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 221532

Area Forecast Discussion
National Weather Service Raleigh NC
1135 AM EDT Tue Aug 22 2017

A pre-frontal surface trough will develop across western NC today, in
advance of a strong cold front that will approach from the northwest
tonight and cross central NC late Wednesday and Wednesday night,
reaching the coast on Thursday.


As of 1135 AM Tuesday...

Current forecast is in good shape with only minor adjustments
required. Late morning surface analysis shows the remains of a
stationary front across SC and GA with a developing lee trough
across western VA and the western Carolinas. Recent GOES-16 visible
satellite imagery shows a developing cumulus field across central NC
with the most widespread and thickest cumulus to the southeast across
the Coastal Plain and coastal region in a region of deeper moisture
and localized convergence just inland. Scattered showers and
thunderstorms are developing in this region which will impact much of
Sampson, and southern portions of Wayne and Cumberland Counties.
Elsewhere, dry weather is expected today with partly cloudy skies.
Current forecast highs in the lower 90s look on target.  -Blaes

Previous discussion...The strong 594 dm mid/upper level anticyclone
centered over the SE US this morning will begin to break down today,
with H5 heights falling slightly by the afternoon/evening with the
approach of the northern stream trough and attendant cold front
moving into the Ohio and Tn Valleys. At the surface, weak surface
high centered off the NC coast will shift further offshore this
afternoon, allowing the development of pre-frontal sfc trough in the
lee of the mountains.

Central NC remains in a void of appreciable synoptic scale
forcing/lift through tonight. And as such, convective rain chances
are expected to remain rather low and diurnally driven, confined
across the far SE zones in the vicinity of the sea breeze and across
the western part of the state along the developing pre-frontal

Nocturnal cooling/stabilization this evening should lead to dry
conditions by late evening, and it should remain dry until the
approach of the cold front Wednesday afternoon.

Highs in the lower 90s with lows in the lower to mid 70s.


As of 330 AM Tuesday...

On the southern periphery of the main belt of westerlies
accompanying the northern stream trough and low pressure system
lifting northeast through SE Canada, a cold front will advance SE
into the area Wednesday afternoon and will cross Central NC
Wednesday evening/night, exiting the coastal plain counties AOA
daybreak Thursday.

Strong insolation within the moist airmass featuring dew points in
the lower to mid 70s, in advance of the cold front, will likely
support moderate instability on the order of 1500-2500 J/kg across
central and eastern portion of the forecast area Wednesday
afternoon. A pre-frontal convection band/line is expected to develop
across the western Piedmont by the early afternoon, with the
potential for the line to strengthen/become better organize as it
pushes eastward into the more juicier more unstable air maximized
across the central Piedmont, Sandhills and Coastal Plain counties.
Unidirectional SWLY shear of 20 to 25kts would support a primary
threat of straight-line damaging winds with NCAPE values ~0.25
suggesting rather fat CAPE profiles along and east of US 1 which
will make for prolific lightning producers. Latest Day 2 outlook
from SPC has all of central NC in a marginal risk threat and will
convey this threat in the HWO with the great severe threat likely to
happen between 2pm to midnight.

This pre-frontal band of convection will likely serve as the
effective cold front, with only isolated/slight chance pops along
the actual cold frontal passage late Wednesday night/early Thursday
morning. Highs in the upper 80s NW to lower 90s. Lows Wednesday
night will depend on the timing of the cold front, with some cooler
mid to upper 60s possible across the NW Piedmont, and lower/mid 70s


As of 330 AM Tuesday...

A series of s/ws rotating through a l/w trough over the eastern U.S.
will cause generally dry and not as hit conditions to central NC
this period.

The lead s/w will aid to drive a sfc cold front through central NC
early Thu morning. This feature expected to lie just south and east
of the Triangle at 12Z Thu, and should exit our southeast counties
by early afternoon. Forcing along the boundary interacting with
abundant moisture and modest instability will support scattered
showers and possibly a thunderstorm. Over the nw piedmont, partial
clearing should occur though cloudy periods will be probable at
least in the morning, and chances for rain appear to be less than 20
percent as subsidence behind the front should suppress shower

The next s/w expected to cross to our north Friday. This system will
enhance the cloud cover over our region and a stray shower or two
cannot be ruled out, primarily north and northeast of Raleigh. The
big story, will be the arrival of a notably drier and cooler air
regime as sfc dewpoints will drop into the upper 50s and lower 60s
while afternoon temperatures will be in the 80-85 degree range.
A modifying Canadian high pressure system will extend into central
NC by Saturday, enhancing the drier cooler air mass in place.
Afternoon temperatures once again will average several degrees below
normal, near 80-lower 80s with lower/comfortable humidity values.

Forecast confidence wanes Sunday into Monday as latest medium range
guidance suggesting the potential for a tropical system to develop
and cross the Atlantic waters offshore of the SE U.S. While chances
for this system to move closer to the coast appear small at this
time, this system will need to be monitored later in the week.
Circulation around this system and the sfc high to our north may
create breezy conditions over the coastal plain Sunday with a
notable uptick in cloud cover, though forcing for showers should
remain well to our east. If the parent sfc high moves east Monday,
our low level flow will become more easterly over our region. This
fetch will lead to a moistening over the lower half of the
atmosphere, resulting in more clouds than sun. If this occurs,
temperatures early next week may struggle to reach 80 over most of
central NC.


As of 732 AM Tuesday...

24 Hour TAF period:  LIFR to MVFR fog/stratus will lift and
disperse, returning to VFR AOA 15z. An isolated shower/thunderstorm
can`t be ruled out on this afternoon, but much too low to include
any mention in TAFS. Low-level flow will become more west-
southwesterly this evening and tonight, with a slight 3 to 5kt
breeze overnight, which should limit fog/stratus development.

Looking beyond the 24 hour TAF period: A cold front Wednesday will
bring adverse conditions to the area through Thursday when the front
settles along the coast. High pressure will bring a return to VFR
conditions Friday through the weekend.





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