Area Forecast Discussion
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FXUS62 KRAH 100121
Area Forecast Discussion
National Weather Service Raleigh NC
815 PM EST Fri Dec 9 2016
Cold high pressure will build across the area through Saturday
night, then shift offshore Sunday. A cold front will approach from
the west late Sunday and move through the area on Monday.
.NEAR TERM /THROUGH TONIGHT/...
As of 815 PM Friday...
1040+ mb surface high pressure was centered over the Northern
Plains states this evening. Bitterly cold air was associated with
this high. The high pressure continued to build and extend SE to
encompass much of the central and eastern United States. Even though
the arctic air is greatly modified as it reaches our latitude, the
temperatures were still around 15 degree below normal for mid-
December with the air mass. These numbers seem almost mild if
compared to the temperatures directly under the high pressure over
MT/ND/MN, where readings were 25 to 35 degrees below normal.
Cold air advection continued over our region with the NW flow. The
pressure gradient will continue to weaken overnight as the core of
the high shifts SE toward the mid-Mississippi Valley region. Clear
skies are expected with good to excellent radiational cooling as
winds die off to less than 5 mph. Dew points were already in the
teens and lower 20s. Expect lows to fall to 20-25 except around 18
in the rural Piedmont locations.
.SHORT TERM /Saturday through Sunday night/...
As of 335 PM Friday...
Sat/Sat night: The surface high builds overhead through Sat night,
as the mid level flow becomes more zonal. We should see an increase
in high thin clouds, especially late Sat into Sat night, as weak
perturbations and high level moisture move out of the Front Range
and across the Mid-Atlantic region. Thicknesses will start out a bit
cooler Sat morning than this morning, although this will be
partially offset by some afternoon recovery, which should lead to
highs similar to today, in the 40-45 range. Lows Sat night in the
low-mid 20s, with light to calm surface winds and a veil of high
thin clouds and patchy mid clouds overnight.
Sun/Sun night: The 850 mb anticyclone will shift off the Southeast
coast late Sat night into early Sun morning, inducing low level warm
advection just above the slowly-departing surface high. This warm
advection will strengthen over time Sun/Sun night, and models agree
on increasing and deepening moist isentropic upglide, starting at
285K-290K, focused on the eastern CWA. Expect increasing clouds
roughly from S to N Sun afternoon, trending to mostly cloudy
areawide by sunset, and remaining cloudy Sun night. The column
remains dry above the freezing level through Sun night, so any
precip should be fairly light with pops no better than chance. With
rising thicknesses balancing increasing clouds, expect still-cool
highs in the 43-50 range. After a slight dip in temps Sun evening,
readings should hold steady or rise a bit overnight. Lows 39-46. -GIH
.LONG TERM /MONDAY THROUGH FRIDAY/...
As of 210 PM Friday...
Initial low amplitude long wave troffing will produce fast and near-
zonal flow across the central and southern tiers of the Conus
through mid week. Return flow ahead of an initial front on Monday
will produce a strong surge of warm air advection with highs ranging
from upper 50s across the north to mid 60s in the south...5 to 10
degrees above normal. Moisture advection will cut off pretty quickly
as low level flow turns westerly across the Gulf states...with
chance PoPs during the day shifting to the east and diminishing
post-fropa on Monday night.
The frontal zone aligns parallel to the flow and forecast details
are murkier heading towards midweek with considerable variability in
model solutions. The frontal zone looks to remain south of the
area...with small chance PoPs mainly across the southern tier...
both Tue and Wed as surface high pressure races across the Ohio
Valley. Highs will be on a gradual fall as the upper trof amplifies
a bit and we should be in a predominantly cloudy regime. Highs
Tuesday in the mid to upper 50s will fall to the low and mid 50s on
Confidence in model solutions for the late week is even lower
although colder air will continue edging southward. The upper flow
is not highly amplified, however, which will block off the really
frigid airmass north of the area. Regardless, highs will probably
top out in the mid 40s after morning lows in the mid to upper
20s both Thu and Fri.
.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
As of 745 PM Friday...
Sprawling arctic high pressure will maintain continued VFR
conditions and calm to light Nly winds through Sat. Cirrus will fill
an otherwise clear sky beginning late tonight or early Sat.
Outlook: VFR conditions should hold through the first half of Sunday
as high pressure builds overhead. As a warm front approaches from
the south, moisture will increase, and cigs will trend to MVFR, then
IFR Sunday afternoon from south to north. There is a good chance for
low level wind shear conditions Sun night into Mon morning with 35-
40 kt winds from the SW at 1500-1800 ft AGL. Adverse aviation
conditions, including sub-VFR cigs and vsbys, are expected to
dominate from late Sunday through at least Monday night -- and
perhaps into Wednesday -- as waves of low pressure track along the
frontal zone as it holds over or just south of the area.