Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 300121

Area Forecast Discussion
National Weather Service Raleigh NC
920 PM EDT Wed Mar 29 2017

Weak high pressure will extend across eastern NC through Thursday.
An area of low pressure will bring unsettled weather to the area
Thursday night and Friday.


As of 920 PM Wednesday...

Main forecast challenge overnight into early Thursday is whether any
rain will develop over the NW Piedmont, and if so the areal coverage.

Sfc ridge beginning to build down the coastline as evident of
pressure rises of 2 mb since 19Z. The base of the ridge expected to
settle over eastern NC, meaning the low level flow over our region
will be east-southeast. Both the KRAX VAD wind profiler and the 00Z
GSO sounding depict a easterly component in the lowest 2-3k ft of
the atmosphere with a strengthening trend noted on the KRAX
profiler. This flow will eventually advect low level moisture
residing offshore of Wilmington into the Piedmont. This will lead to
the development/deepening of low clouds along the foothills,
spreading into the western Piedmont by daybreak Thu. Best isentropic
upglide occurs closer to 12Z Thu and persist through much of Thu
morning. This may cause patches of drizzle and/or light rain to
develop, primarily in vicinity of the Triad but possibly as far
south as the southern Piedmont and as far east as Roxboro. If this
precip pattern does develop and becomes extensive enough, it may
result in even cooler temperatures than currently forecast across
the Piedmont.

Have adjusted temperatures slightly to account for observed trends,
and for the potential for cooler air to stream farther west into the
coastal plain in proximity to the dry air ridge. May adjust PoPs
once 00Z model and other near term model guidance arrives.


As of 420 PM Wednesday...

Continued southerly warm moist air advection ahead of the Plains
closed cyclone lifting northward into the Mid MS Valley, atop the
shallow cool airmass at the surface will result in the development
of CAD across the NW Piedmont, especially given the potential for
some light rain tomorrow morning. Even without any light rain
Thursday morning, a strong subsidence inversion aloft will help to
lock in the low-clouds, with fcst soundings showing ceilings likely
to remain MVFR in the Triad, which will greatly temper daytime
heating. Highs ranging from mid 50s NW to lower 70s SE.

Upper ridge aloft will shift east of the area late by mid to late
afternoon as the closed cyclone progresses across the central MS
river valley into the Ohio Valley. This will give rise to increasing
pops across the western piedmont late in the afternoon/early
evening. Rain chances will increase ten-fold between 06 to 12z, with
increasing potential for elevated showers and thunderstorms within
the strengthening low-level warm advection fueled by a 40-50kt LLJ.
As the wedge boundary retreats northward, low-level kinematics
(0 to 1km helicity ~ 300-400 m2/s2) will be maximized during
this period. However, sfc base instability of 250 J/kg or less
may prove not enough to support severe storms overnight. However,
if storms/updraft can become surface based, there will be a
threat for damaging thunderstorm winds and an isolated tornado.
Stay tune. Lows Thursday night in the 50s north to lower 60s south.


.LONG TERM /Friday through Wednesday/...
As of 300 PM Wednesday...

Fri-Fri Night: The 12Z GFS/ECMWF are in fairly close agreement
showing the upper low tracking ENE through the OH valley during the
day Friday, progressing into western PA late Friday into Friday
night. However, differences in timing persist between the GFS/ECMWF,
and the 12Z NAM (an outlier) shows the upper low tracking east
through KY into WV Fri/Fri night (much closer than the GFS or
ECMWF). Forecast confidence remains lower than normal, esp given
uncertainty in the evolution of upstream convection across the Deep
South Thu/Thu night in addition to uncertainty in the strength/
extent/evolution of the CAD wedge expected to develop over central
NC on Thu. Although confidence in precipitation chances remains
high, forecast specifics such as precip amounts, temperatures and
intensity/mode of convection remain relatively low. With the latest
guidance in mind, will indicate highs ranging from the mid 60s in
the Triad to the mid 70s in the Sandhills/SE Coastal Plain. Expect
precipitation to end from SW-NE as early as late Friday aft/eve,
with skies clearing in the wake of a cold frontal passage Fri night.
Lows Sat morning will depend primarily upon fropa timing, ranging
from the upper 40s/lower 50s (N/NW) to mid 50s (S/SE). The 12Z
GFS/ECMWF suggest a lower potential for severe weather given an
upper low track further to the north and a slightly faster
progression of the system. The 12Z NAM would suggest a more robust
potential for severe weather due to the closer proximity of the
upper low and slightly slower progression of the system which would
result in favorable diurnal timing, as well. Uncertainty remains too
high to say much more w/regard to the severe weather potential.

Sat-Sun night: Expect dry conditions and a warming trend over the
weekend as a shortwave ridge builds east across the Mid-Atlantic and
Carolinas. Expect highs in the lower/mid 70s Sat and mid 70s Sunday
as a shortwave ridge aloft tracks across the region from the west.

Mon-Wed: Expect increasing cloud cover during the day Monday and a
chance for convection Mon Night through Tue as the next upper level
low /attendant sfc cyclone/ approach from the west. -Vincent


As of 740 PM Wednesday...

24 Hour TAF Period:   High pressure will wedge south into the area
tonight and into the day on Thursday. As 925-850mb SELY moist warm
air advection overspreads the cooler near the sfc, there is medium
to high confidence that an area of MVFR stratus will develop at KINT
and KGSO.  Though confidence is not as high, ceilings at KRDU, KRWI
and KFAY are expected to remain low-end VFR, with the potential that
KRDU could be on the eastern periphery of the sub-VFR ceilings. A
strong mid-level subsidence inversion over the area on Thursday will
result in slow lift/improvement of the stratus layer, with a high
likelihood that sub-VFR ceilings at KGSO and KINT could linger
through the forecast period. Strengthening southerly WAA spreading
into the area late Thursday afternoon could support some scattered
showers at KGSO and KINT. Elsewhere, conditions are expected to
remain dry with ceilings ~5kft.

Long term: A warm front associated with a low pressure system
tracking through the Middle MS Valley will bring the chance for
showers and thunderstorms Thursday night late Thursday night through
mid to late morning hours on Friday.  High pressure with VFR
conditions should return Friday afternoon and will persist
through the weekend. The next storm system is expected to impact
the area on Monday.





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