Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 271827

Area Forecast Discussion
National Weather Service Raleigh NC
230 PM EDT Thu Jul 27 2017

Surface ridge axis over the area will shift offshore while a weak
surface trough develops across the NC Piedmont this afternoon. A
cold front will approach from the northwest Thursday night and cross
our region Friday night.


As of 1035 AM Thursday...

Little change required to the near term forecast.

Areas of stratus across the far western Piedmont will slowly
lift/dissipate by mid day while skies will remain mostly-partly
sunny elsewhere. This strong heating will aid to push afternoon
temperatures to around 90 degrees with lower 90s probable across the
southeast sector of central NC.

Atmosphere expected to be primed by mid afternoon with slight-
moderate instability likely across the Piedmont along with effective
bulk shear values 25-30 kts (decent values for this time of
year)over the northwest. 12Z upper air analysis depicts the first in
a series of upper disturbances crossing the OH Valley and advancing
sewd toward central NC. At 500mb, morning analysis still depicts an
anti-cyclone centered over upstate SC. If this feature holds this
afternoon, it would aid to shunt the better convective activity to
our north. However, near term model guidance still suggest that this
feature will weaken after 18Z, permitting a west-nw flow to develop
and allowing the trigger mechanism for scattered convection to
approach the northwest Piedmont. While the best DCAPE values are
projected to lie east of the NW Piedmont, the available buoyancy
should lead to long lasting updrafts and the potential for
strong/locally damaging micro-bursts. Plan to highlight this threat
in the HWO.

Otherwise, timing of late afternoon-early evening still looks on
target. Still some question on how far east and in what
concentration the convection will obtain later this evening into the
overnight. If the GFS verifies, likely PoPs would be warranted for
much of central NC north of highway 64. However, last nights run of
the convective allowing model solutions have backed off on the
expected convective coverage compared to Wed 12Z run. For now chance
PoPs still appear to be necessary for the northern Piedmont and the
northern coastal plain into the overnight hours.


As of 345 AM Thursday...

Overview: An unseasonably strong upper level low over the Great
Lakes will amplify as it tracks slowly southeast into the Mid-
Atlantic late Fri/Fri night. An associated surface low is expected
to develop/ intensify along a strengthening frontal zone /cold
front/ progressing into the Mid-Atlantic Fri night, with MSLP values
~1000 mb over portions of the DELMARVA or southeast VA by 12Z Sat.
The aforementioned cold front is expected to progress SE through NC
during the day Sat as the low becomes vertically stacked along the
Mid-Atlantic coast.

Expect widespread cloud cover as cyclonic flow aloft strengthens
over the region Fri/Fri night, with some clearing possible in the
W/SW Piedmont by ~18Z Sat as the cold front progresses into the SE
Coastal Plain, however, cloud cover may persist through Sat night
(particularly across the N Piedmont/NE Coastal Plain) as shortwave
energy digs S/SSE on the western periphery of the vertically stacked
/nearly stalled/ low along the Mid-Atlantic coast. Highs on Sat
should range from the mid 80s to ~90F, coolest near the VA border
and warmest near the SC border. Lows Fri night in the upper 60s to
lower 70s, warmest S/SE. Highs on Sat should range from the lower
80s (N) to mid/upper 80s (S), and lows Sat night should be noticeably
cooler, in the lower to mid 60s.

Confidence is high w/regard to above normal cloud cover and above
normal precipitation Fri/Sat. However, convection over central NC
will be modulated by a number of factors, including small amplitudes
waves in strengthening cyclonic flow aloft and upstream convection
and/or convective remnants propagating S/SE into the area. As a
result, confidence is very low with regard to the timing/location/
intensity of convection Fri/Sat. As a result, the threat for severe
weather and/or flash flooding is unclear at this time. -Vincent


As of 230 PM Thursday...

The surface front will be in vicinity of the coastal plain Saturday
morning, with deep moisture and precip coverage on the decline as
low level flow shifts westerly. As such, will taper off morning PoPs
in the east from 60% to 30% by afternoon, with only slight chances
elsewhere. Initial cool air advection will be underway behind the
front, with highs Saturday ranging from low 80s north to mid 80s
south. The cool air will surge south Saturday night as the upper
trof rotates through, and the strong mid level forcing will spawn
scattered convection Saturday night, mainly across the northeast.
Mins Saturday night will be mostly in the mid 60s, and Sunday`s
highs will be downright pleasant, with lower humidity to accompany
temps mostly in the lower 80s, with some locations in the northeast
perhaps topping out in the upper 70s.

The cutoff low lifts northeast Sunday night and Monday, and the
remainder of the extended at least through Wednesday will be dry as
surface high pressure building into the Ohio Valley delays moisture
return into the area. Gradual extension of the Bermuda high into the
northern Gulf will eventually prevail, beginning with warm air
advection on Wednesday, to be accompanied by the return of moisture
laden air on Thursday. As such, a gradual warmup is expected through
the week, with highs Monday mostly in the mid 80s, rising into the
upper 80s Tuesday and Wednesday, and back to the upper 80s/lower 90s
on Thursday. Morning mins will follow the trend as well, from mostly
mid 60s Monday morning to around 70 by Thursday morning.


As of 105 PM Thursday...

While VFR parameters will prevail across much of central NC through
this evening, scattered convection and associated MVFR parameters
will threaten the Triad terminals primarily between 20Z-01Z. While
the bulk of the scattered convection will dissipate by mid evening,
a few showers will likely drift across the northern counties into
the overnight.

Additional scattered convection and associated MVFR parameters
appear likely across central NC Friday through Friday night as a sfc
cold front drifts southward. Areas of MVFR ceilings and a few
showers are possible Saturday as the main upper trough drifts across
the region. A drier, more stable atmosphere and attendant VFR
parameters expected Sunday through Tuesday.





SHORT TERM...Vincent
AVIATION...WSS is the U.S. government's official web portal to all federal, state and local government web resources and services.