Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 171210
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
710 AM EST Sun Dec 17 2017

.SYNOPSIS...
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.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 315 AM Sunday...

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
SE US.

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.

&&

.SHORT TERM /MONDAY AND MONDAY NIGHT/...
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.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
As of 212 AM Sunday...

Below average confidence continues in this part of the forecast, but
generally models are starting to trend toward one another,
increasing confidence from previous days. The temperature forecast
will be somewhat of a roller coaster, but mainly above to well above
normal for mid-December. Best chances for rain will be Tuesday night
through Wednesday and again Saturday and Sunday (next weekend).

Through Wednesday: The upper level wave (the deamplifying
low/trough) exiting the Southwest US on Tuesday will move eastward
through the Midwest toward the Mid-Atlantic. Meanwhile, the flow
over Central NC will be nearly zonal Tuesday and Tuesday night ahead
of the approaching wave. The model differences with this wave as it
moves into the region continue with the latest runs as the EC still
holds onto a slightly stronger low/wave while the GFS continues to
be more open/weaker. Regardless, both models indicate likely
rainfall moving into the area from the west-southwest on Wednesday,
lingering into Wednesday night. At the surface, a Piedmont trough
sets up on Tuesday, keeping the better warm moist advection south of
the area. South-southwesterly flow increases over the region Tuesday
night into Wednesday as the surface low over the ARKLATEX moves
eastward, impinging on the Carolinas and increasing the warm moist
advection into Central NC. The placement of the surface low varies
between the NAM, GFS and EC, thus so does the southward progression
of the approaching cold front toward/into the area. The track of the
low will also have an impact on the precipitation forecast as well,
but chances increase farther south. Temps Tuesday/Tuesday night will
be well above normal, highs in the mid 60s and lows in the low to
mid 40s. Wednesday highs will depend on the location of the low and
advancement of the front, generally in the mid 50s to low 60s.

Wednesday Night through Saturday: In the wake of the upper wave and
surface low, the upper level ridge will again extend northward into
the Carolinas. A surface cold front will sink through the region,
setting up a brief wedge Wednesday, which could stick around
(primarily in the far NW) into Friday. The more interesting weather
feature will be the upper level trough the digs southward through
the Rockies and into Texas, swinging through the Midwest through
Friday. A reflection of the upper level system will be extremely
evident at the surface as a strong low develops over the
Midwest/Plains on Thursday. The surface cold front associated with
this low will strengthen as it progresses eastward with a strong
Arctic high digging south out of Canada in it wake and increasing
south-southwest flow ahead of it. As of the latest model runs, the
front appears to get hung up over the Appalachians Friday night into
Saturday, however the strong southwesterly flow into Central NC
ahead of it will result in increasing chances for rain on Saturday.
The coolest day of the period will be Thursday with the wedge/high
lingering over the region. However, expect temperatures to moderate
ahead of the next system to well above normal again by Friday and
Saturday.

&&

.AVIATION /12Z SUNDAY THROUGH THURSDAY/...
As of 710 AM Sunday...

Low to mid-level moisture plume associated with the de-amplifying
southern stream shortwave that will track on the western and
northern periphery of the mid/upper level ridge in place across the
SE US will result in ceilings gradually lowering to the 4-5kft
thousand foot range this afternoon and evening.  There could be
fairly widespread virga that may reach the ground in the form of
patchy light rain or sprinkles between 18Z Sun through 03Z Mon. It`s
possible that ceilings could briefly lower to MVFR at KFAY between
06 to 12z, however confidence is too low at this time to include any
MVFR ceilings.

Outlook: Drier air will build into the area on Monday and Tuesday.
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.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

SYNOPSIS...CBL
NEAR TERM...CBL
SHORT TERM...CBL
LONG TERM...KC
AVIATION...CBL/MWS



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