Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 172007

Area Forecast Discussion
National Weather Service Raleigh NC
305 PM EST Sun Dec 17 2017

High pressure will settle east across the southeastern US through
the weekend. A series of upper level disturbances will move through
the region through mid week.


As of 1040 AM Sunday...

Visible satellite and 12Z GSO upper air data suggest mid level
clouds may have a greater impact than earlier forecast on high
temperatures over the nwrn and nrn Piedmont, where highs may
struggle to hit 50 degrees. Otherwise, filtered sunshine through
considerable high cloudiness that will yield to thickening and
lowering mid-high ceilings by this afternoon, should allow
temperatures to range into the middle to upper 50s over the srn RAH
counties. KFCX radar data indicates virga has already reached as low
as 4 thousand ft AGL in the past hour over the Triad, so sprinkles
or a slight chance of barely measurable rain is on track area-wide
for the afternoon, especially across the nrn and nwrn Piedmont where
glancing mid level forcing will be relatively maximized, and into
the evening hours over the sern counties.

Previous discussion as of 315 AM Sunday follows...

Southern stream trough currently over the Oklahoma Panhandle will
undergo significant de-amplification over the next 24 hours,
shearing apart within the confluent flow as it tracks NE and east
through the Ohio Valley during the afternoon and then off the Mid-
Atlantic States late this evening, tracking along the western and
northern periphery of the mid/upper level ridge in place across the

While the better forcing/lift associated with the weakening trough
will remain well north of the area, the associated low to mid-level
moist plume, fed by a SWLY 50 kt mid-level jet, will spill east atop
the mid/upper level ridge in place. This will result in thickening
and lowering cloud ceilings during the afternoon and evening. There
could be fairly widespread virga that may reach the ground in the
form of patchy light rain or sprinkles by the late afternoon and
through the evening/overnight hours. This moisture plume will
quickly shift offshore shortly after 06z, with the potential for
skies clear out late tonight, especially across the NW Piedmont.

The thickening cloud cover could potentially temper afternoon highs,
especially across the western Piedmont where earlier onset of
thicker cloud cover would coincide with peak afternoon heating.
Highs ranging from lower 50s NW to mid/upper 50s south. Lows tonight
will be noticeably milder than previous nights, especially across
the south, where clouds may linger. Lows ranging from mid/upper 30s
NW to mid 40s SE.


As of 315 AM Sunday...

As we await for the closed low over the SW US to eject eastward, the
Carolinas will be in-between waves/systems on Monday, under the
control of the flat subtropical high ridging north into the SE
states. Deep westerly flow will result in a fair wx day with dry
conditions along with a brief return of sunshine  Afternoon
temperatures will moderate into the 60s everywhere, under light
southerly winds with the development of a surface trough in the lee
of the mountains. Mild overnight lows in the 40s.


As of 305 PM Sunday...

Frontal zone south of the area will be lifted north as a strong mid
level wave over the southwest US ejects eastward across the southern
plains on Tuesday. High cloudiness will be prevalent with an
initially dry and mild airmass in place over the area benefitting
from zonal flow to allow highs to reach the mid 60s. A moisture
plume off the western Gulf will be aimed east along the frontal
bounday, which will be in the vicinity of the NC/SC border,
producing increased cloudiness rapidly ahead of the mid level system
Tuesday night. Expect to see light rain spreading into the western
Piedmont by Wednesday morning and across the remainder of the area
Wednesday into Wednesday night. PoPs will peak on Wednesday,
graduated from 40% north to 70% across the south, and may wind up
being bumped considerably higher if we see see continued model
consensus on the timing and precip distribution. Mins Tuesday night
will be mild, mostly mid and upper 40s, with highs on Wednesday in
the mid and upper 50s.

The upper wave and associated cold front will move offshore
Wednesday night, with drier but only modestly cooler air ridging
down the Atlantic seaboard from the northeast. High temperatures
will take a short-lived tumble to near seasonable levels as the CAD
builds in, topping out mostly in the lower 50s Thursday. The surface
high associated with the CAD will be progressive, allowing the
airmass to begin scouring out Thursday night as low level veers
easterly, with highs edging back to the mid and upper 50s Friday
ahead of our next cold front, which will move into the area and
stall over the weekend. Seasonable temperatures in the 50s are
expected Saturday and Sunday, with at least chance (30%) PoPs
through the weekend.


As of 1255 PM Sunday...

A mid level trough and associated mid-high level moist axis will
lift rapidly enewd across the srn-cntl Appalachians (and NC) through
this evening, with an associated thickening and lowering of ceilings
down to between 5 and 10 thousand ft for a several hour period late
this afternoon and this evening. While most associated precipitation
will evaporate before reaching the ground (virga), some light VFR
rain or sprinkles will be possible from west to east between 20Z and
03Z Mon. Skies will also generally clear from west to east
thereafter/early tonight, although an area of lower level clouds --
in the low VFR to high VFR range-- are expected to expand/develop
enewd across SC and srn NC later tonight through early Mon. This
cloud area may result in a short period of MVFR ceilings around 2500-
3000 ft at KFAY in the several hours centered around 12Z Mon.

Outlook: The nwd retreat of a warm front, and leading edge of a more
humid air mass, will result in a medium chance of advection fog,
and/or VLIFR stratus, Mon night-Tue morning, particularly at FAY.
There will then be a high chance of rain and sub-VFR conditions,
heaviest and lowest at FAY, late Tue night through Wed night, with
the passage of a low pressure system across the sern U.S.





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