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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS62 KRAH 100153
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
850 PM EST Sat Dec 9 2023

.SYNOPSIS...
High pressure will extend from near Bermuda to the South Atlantic
states ahead of a powerful low pressure and frontal system that will
move across the region Sunday and Sunday night.

&&

.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --
As of 850 PM Saturday...

WAA and isentropic lift atop the lingering CAD airmass and
associated stratus cloud layer in place across western NC will
result in occasional patchy light rain rain and sprinkles across the
western Piedmont through the remainder of the evening and early
overnight hours. Then during the predawn hours, strong moisture flux
convergence coincident with the arrival of a 35 to 45 knot low-level
jet will steer the lead band of heavier shower activity and possible
embedded thunder east into the western Piedmont and Triad. Do not
anticipate any severe threat during this time frame as instability
appears to weak and elevated. Rainfall amounts  of one-tenth to one-
quarter of an inch will be possible through daybreak across the
western counties.

The far eastern/coastal plain counties could also see some isolated
showers by the predawn morning hours as the nearby offshore shower
activity moves onshore.

Elsewhere it should remain mostly dry. However, there is the
potential for fog, potentially dense, to spread north and inland
from off the SE coast. Will need to monitor this hazard.

Temperatures may cool another degree or two over the next few hours,
followed by rising temperatures between 06 to 12z. Lows in
the lower/mid 50 north to upper 50s/near 60 SE.
-- End Changed Discussion --

&&

.SHORT TERM /SUNDAY AND SUNDAY NIGHT/...
As of 350 PM Saturday...

A high amplitude upper trough, extending south from a low over
Hudson Bay to the Gulf Coast, will sharpen as it shifts eastward on
Sun. A few embedded s/w disturbances moving through the parent
trough will further impact the progression and orientation of the
trough. A pair of disturbances, one over the Deep South and the
other over the TN/OH Valley, will lift ENE in tandem through the
Southeast and mid-Atlantic late Sun/Sun night. The disturbances
could help the trough become quasi-negatively tilted over the
region. At the surface, a strengthening cold front will approach the
Appalachians on Sun, then move into and through central NC Sun
eve/night. An area of low pressure may develop along the front as it
moves into the Carolinas. Expect both the front and low to move out
of the area early Mon morning. Confluent SSW flow off the Gulf ahead
of the front and SSE return flow around the Bermuda high will result
in anomalously high moisture advection into the area. PWATs should
increase into the 1.4 to 1.6 inch range through Sun aft/eve. There
will likely be multiple rounds of convection, one moving in from the
west early Sun and the other late Sun aft/eve into Sun night. Rain
should be exiting the area to the northeast by daybreak Mon.

Severe Potential: One of the big questions will be how how far NW
the warm front will retreat through the area, and whether or not the
rain falling over the cooler air in the NW Piedmont will keep them
locked into the stable side of the boundary. MUCAPE of generally 500-
800 J/Kg is expected across the area, highest across the south and
east and peaking during the evening, although the forecast MLCAPE is
slightly lower. The effective shear should be around 40 kts. There
is at least a chance of thunder in the NW Piedmont on Sun, but the
better chance for thunder and potentially strong to severe storms
late Sun morn/early aft will be across the southern and eastern
Piedmont. The second round of convection is a bit less certain wrt
severe potential and overall character. While the convective
parameters appear to maximize during the late aft/eve, it depends
how much the earlier showers and storms work the environment over.
If storms are able to tap into some higher instability late, then
additional strong to severe storms may occur.

In addition to the wind potential associated with the storms, wind
gusts of 20 to 30 mph are possible. Also, hi-res model guidance
suggest there may be a brief period of stronger wind gusts, 35 to 45
mph, mainly over the Sandhills and Coastal Plain as the surface low
and rain lift NE out of the area Sun night.

Precipitation: Confidence is increasing that rainfall totals could
be in the 2-2.5 inch range by the end of the event, with anomalously
high PWATs of 1.4 to 1.6 inches. While there is still a Marginal
risk for isolated flash flooding, the system still appears fairly
progressive and training of storms are not anticipated. Best chances
for isolated flash flooding will generally be over the urban areas
and areas of poor drainage. Lastly, have introduced a slight chance
for a brief period of snow mixing in with the rain as the system
lifts out Sun night. Confidence is relatively low, but the chance is
there nonetheless. No impacts or accumulation expected.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
As of 350 PM Saturday...

Dry weather and mostly clear skies will prevail from Monday through
Friday under the influence of surface high pressure and W/NW flow
aloft. A strong pressure gradient between the deepening surface low
over the New England coast and high pressure building in from the
Deep South will result in strong NW winds continuing from Monday
morning into early afternoon behind the cold frontal passage,
gusting up to 20-25 mph. Thus much cooler temperatures are in store,
with highs only in the mid-40s to lower-50s (5-10 degrees below
normal). Winds will gradually relax in the afternoon and evening,
becoming largely calm on Monday night as the surface high
strengthens and becomes centered over the NC. This will allow for
decent radiational cooling conditions. Lows will be mostly mid-to-
upper-20s, but some isolated lower-20s will be possible in the
coldest outlying areas.

High temperatures will slowly moderate from slightly below normal on
Tuesday to near or slightly above normal on Wednesday. As the
surface high over the Carolinas on Tuesday gets absorbed by another
cool ~1040 mb high building SE into the OH Valley and Mid-Atlantic,
temperatures may drop a bit again on Thursday. The high will then
weaken and lift north on Friday, helping temperatures climb back
into the mid-to-upper-50s. Lows will be mid-20s to lower-30s each
morning with continued good radiational cooling.

Confidence in the sensible weather forecast decreases for Friday
night and Saturday. Both the 12z GFS and 12z ECMWF depict a closed
mid/upper low over the Southern Plains phasing with a northern
stream shortwave energy diving down from the northwestern US.
However, details including to what extent this influences the
surface pattern are very uncertain. The GFS and a minority of GEFS
develop a surface low that moves along the Southeast US coast on
Friday night and Saturday, bringing some rain into central NC.
However, the ECMWF has the phasing occurring much farther west, with
surface cyclogenesis occurring over the Midwest instead. Given the
wide range in deterministic and ensemble solutions, confidence is
too low for anything beyond slight to low chance POPs (and
increasing clouds) during this period.

&&

.AVIATION /00Z SUNDAY THROUGH THURSDAY/...
As of 715 PM Saturday...

Strengthening southerly flow through the lower atmosphere will
rapidly bring in IFR/LIFR conditions as well as LLWS ahead of cold
front, currently moving through OH, TN, and lower MS Valley. The
cold front and a band of pre-frontal showers and storms are forecast
to approach the Triad terminals around 10-12z Sun morning, RDU/FAY
15-18z and RWI around 18z. Periods of light to moderate rain will
continue behind the first wave, with a second moderate to heavy area
of rain/storms along the cold front expected to cross from the Triad
terminals during the late afternoon through RWI around midnight. -
Swiggett

Beyond 00Z Monday: Strong and gusty nwly winds 20-25 kts will result
behind the front Sunday night-Monday, including some locally
enhanced ones between 30-40 kts at FAY immediately behind the front
early Sunday night. Otherwise, VFR conditions are expected Monday
through Thursday.

&&

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

&&

$$

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