Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 122028

Area Forecast Discussion
National Weather Service Raleigh NC
330 PM EST Mon Nov 12 2018

A strong low pressure system will move southwest to northeast
through the Southeast U.S. tonight through Tuesday, followed by
passage of a strong cold front Tuesday night. Cold high pressure
will return Wednesday. Another area of low pressure will approach
central NC from the southwest Thursday and Thursday night.


As of 255 PM Monday...

A steady rain will continue well into tonight, with a few storms
possible in the SE. Plenty of moisture (PW well over 1.5", with deep
moisture on WV channels) is streaming into NC, within a volatile
environment featuring strong forcing for ascent at multiple levels.
Highly amplified longwave troughing from N Ontario through the W
Great Lakes and mid Miss Valley to TX is producing a fast SW mid
level flow over the Deep South/Southeast/Mid Atlantic, while at the
surface, a damming wedge (extending from a departing high off the
Northeast coast) is surrounded by inverted troughing on the west
slopes of the Appalachians, connected to lows over the central Gulf
Coast, as a coastal front extends up through the eastern Carolinas.
A large rain shield covers NC and extends back to central TN in N
AL, while convective bands extend further south through SW GA/AL/S
MS/SE LA. The former is being forced more by strong upper divergence
and convectively-augmented DPVA, while the latter is taking
advantage of weak instability and intense kinematics, including 40-
50 kts of 0-1 km shear across AL/GA and 40-50 kts of 925 mb SE and S
nosing into AL/GA and the Carolinas. Pops aren`t the challenge
tonight -- they`ll be near 100% everywhere -- but the surface
temps/dewpoints and their impact on convection over the SE CWA are
more uncertain. Low pressure will develop over E NC along the front
and ride northward tonight, and this may allow enough warm air to
track inland to contribute to weak surface-based instability
overnight. Southport in SE NC is already 70 (while CAD-impacted RDU
sits at 43 and GSO at 39) with other coastal sites in the mid-upper
60s, so the risk for some CAPE in our SE this evening persists as
this juicy air works inland. And given the increasing and shifting
winds with height through the low levels generating high 0-1 km SRH,
a few rotating cells may occur. With the low level jetting forcing
deep moist upglide along with the upper divergence, additional
rainfall amounts of an inch or two are possible over central NC,
with the highest values over the Piedmont where the upper and lower
forcing features will be vertically juxtaposed. Peak time for the
heaviest rain in the NW CWA remains 21z-06z. Will continue a mention
of thunder through this evening over the SE CWA.

The low level jet will track off to our NE late tonight, with a
shift in 925-850 mb winds around to a more SW direction. This will
result in a downturn in deep lift (including reduced upglide),
allowing for a trend to lighter precip and lower QPF from SW to NE
late tonight. But the chances for light rain will remain high, based
on the deep moisture in place and upstream.

Temps should hold fairly steady this evening before rising slightly
overnight into the low 40s in the NW, with mid 40s to lower 50s
elsewhere, and a more dramatic rise in the far SE where some mid-
upper 60s are possible overnight. -GIH


As of 330 PM Monday...

Anomalously high PWs will hold over our area early Tue, before being
shunted to the SE and S late Tue into Tue night. The southern
portion of the mid level trough will close off over NE TX while the
northern portion remains progressive over SE Canada. While the mid
level flow will veer slightly in response, we`ll still keep a steady
stream of mid and high level moisture over the state. In the low
levels, both the low level jet and the surface low that forms along
the E NC front will both track quickly up the Mid Atlantic and
Northeast coast early Tue, allowing the front to ease eastward
during the day. The surface high attempting to build in from the W
will be initially hindered by the higher terrain, such that we`ll be
left with abundant low level moisture and weak low level flow that
will serve to keep clouds locked in for much of the day. While
forcing for ascent won`t be as strong or deep as today, the presence
of the upper jet from OH NE through the Maritimes will keep at least
weak upper divergence over our area, necessitating a continuation of
high pops Tue, trending down NW to SE Tue night as the front settles
just to our SE. Expect chilly highs from the lower 50s to lower 60s.
Lows from the mid 30s to lower 40s with partial clearing possible in
the far NW late. -GIH


As of 305 PM Monday...

Wednesday we will get a little break from the heavy precipitation as
a surface high pressure moves across the Ohio Valley and into the
mid-Atlantic states. Most of the area should remain dry, especially
north of US-64 but the GFS never does clear out precipitation across
the southern areas. That being said both the NAM and ECMWF do in
fact clear the state so will only carry slight chance across the
south. Cold air advection around the front of the high will bring
northerly flow and high temperatures only topping out in the mid

By Thursday morning both the GFS and the ECMWF bring the surface
high into northern New England and introduce precipitation across
central during the early morning hours. With the high not able to
filter enough cold air in from the north in time, temperatures in
the Triad and the VA border counties should stay slightly above
freezing and keep precipitation all liquid with the exception of a
few inconsequential sleet pellets. One thing to keep an eye on
however is that the NAM becomes a bit of an outlier by Thursday
morning in a key way as it is further south with the surface high
across the mid-Atlantic which keeps precipitation out of the area
till after 12z Thursday but as a consequence, filter colder air into
the Triad and NW Piedmont. Looking at forecast soundings, this would
present an opportunity for a couple hours of freezing rain in the
Triad. With antecedent temperatures well above freezing, it is
likely that even in this scenario, any ice accrual would be very
minimal but it is worth keeping an eye on over the next couple of
model cycles to see if it falls more in line with the GFS and the
ECMWF. For the forecast, have kept temperatures slightly above
freezing and all precipitation liquid at this time. Rain will
continue through the day on Thursday and into Thursday night before
clearing out before 12z on Friday. Rainfall amounts looks to be
slightly lower than the early week system and so have a storm total
QPF of between 1.5 and 2 inches at this time which should put us
somewhere between 4-5 inches for the week. Will need to keep an eye
on how much rain we get in our river basins to identify any
potential for river flooding later in the weekend as water works its
way downstream. Low temps should be in the mid 30s but if the
precipitation clears out quicker, the Triad could drop into the
lower 30s.

Clearing will commence on Friday with high pressure moving into the
area which will remain in place through the weekend. Highs in the
50s with lows in the mid 30s.


As of 1150 AM Monday...

Poor aviation conditions will dominate over central NC over the next
24 hrs. A strong storm system will move SW to NE through the region
today and tonight, bringing a large shield of rain and embedded
storms. Current MVFR cigs over the area will drop further to IFR
during the 18z to 21z time window, and vsbys will also become MVFR
to IFR, lowest within moderate to heavy rain. A period of LIFR
conditions is possible, especially during the early-mid part of this
evening and again Tue morning. These poor aviation conditions will
hold through at least 18z Tue. The greatest chance for isolated
thunder will be at FAY and RWI 05z-09z tonight and again 15z-18z

Low level wind shear is also possible 00z-08z tonight, particularly
at FAY/RWI where light winds at the surface will strengthen to 35-45
kts from the SE at 1500-2000 ft AGL. LLWS cannot be ruled out as far
W as RDU and will closely monitor for this potential.

Looking beyond 18z Tue, as a front slowly eases eastward over
coastal NC Tue afternoon with high pressure gradually building in
from the W, sites should see a slow trend to VFR conditions with the
following likely timing: late Tue afternoon at INT/GSO, late evening
at RDU, and very late overnight into early Wed morning at RWI/FAY.
VFR conditions should then dominate on Wed, but it will be short-
lived as another low pressure system moves in from the SSW and
brings a return to IFR/LIFR conditions and high rain chances Wed
night, lasting through Thu night. VFR conditions will follow on
Fri/Sat as high pressure builds in from the SW. -GIH




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SHORT TERM...Hartfield
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