Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 040548

Area Forecast Discussion
National Weather Service Raleigh NC
1245 AM EST Sun Dec 4 2016

High pressure will extend into the area from the north this morning.
Low pressure will track from the Gulf Coast across the Southeast
states late today through tonight, before moving off the coast
Monday morning. High pressure will build in briefly from the north
Monday afternoon, then a second stronger low pressure system will
cross the Gulf States and Carolinas Monday night through Tuesday.


As of 945 PM Saturday...

High clouds continue to thicken in the wly flow aloft as the first
in a series of s/ws shears out across the Deep South/southern
Appalachians. This feature responsible for a few sprinkles skirting
along the NC/SC border this evening. While upper levels have become
quite moist during the past 245 hours, a decent layer of dry air
still resides in the lower half of the atmosphere per 00Z GSO
sounding. The passing of the 850/700mb ridge axis overnight will
allow for better moisture advection by early Sunday, leading to
increased chances for rain later Sunday morning across the
western/southern Piedmont, and areawide Sunday afternoon-Sunday
evening.  Have adjusted PoPs overnight in the far west, and
across the Piedmont Sunday morning based on observed radar/surface
obs and trends of WRF/HRRR model runs.

As cloud cover continues to thicken/lower, temperatures will slowly
fall or hold steady overnight. Min temps lower 30s far north-
northeast to the upper 30s south-southwest. ~WSS

340 PM discussion...

First CAD event in quite a while expected to develop Sunday
afternoon, with some rain and chilly temperatures.

The 1028 MB surface high pressure extending from the Great Lakes
SE through the Ohio Valley and into the central Appalachians will
continue to move east to a position along the eastern seaboard
Sunday. Dry and cool weather is expected overnight as high cloudiness
is forecast to continue to overspread the region from the southwest.
This increase in clouds is in response to the SW to NE mid/upper
level jet extending across the southern tier of the U.S (out ahead
of the deep west-central U.S. trough).

This mid/upper level trough is forecast to lift NE during the next
few days. In response, the surface high will align into a position
that will extend down the eastern seaboard Sunday. This will set the
stage for a Cold Air Damming (CAD) episode over the Piedmont
Damming Region later Sunday into Monday. The strength of the CAD will
be partly dependent on enough rain (QPF) to allow for evaporative
cooling of the initially dry low level dry air in place. Models are
in general agreement in a tenth to a quarter inch of rain (enough to
allow for evaporative cooling and diabatic affects over the Piedmont
Sunday and Sunday evening, which will lead to hybrid CAD conditions.

This first shot of rain will occur with increasing moisture/lift
with the approaching disturbance aloft Sunday, then aided by the
WAA/ moist air being lifted up and over the coastal front and
developing CAD. Models also suggest an initial weak surface wave to
track SW to NE along the coastal front as well later Sunday and
Sunday evening enhancing the rain. The higher QPF of near a half
inch should occur near or just northwest of the coastal front where
the convergence is maximized.

Summary... tonight, increasing cloudiness with a light north breeze
less than 10 mph. Lows generally 35-40. A few sprinkles possible
late in the west. Sunday the WAA moist flow from the surface to H85
and developing mid level lift will lead to light rain
developing/spreading NE across the region. Timing should bring
highest POP to the west during the early to mid afternoon spreading
throughout the entire region during the late day and Sunday night.
Highs Sunday will be dependent on onset of rain and the amount of
rain in the damming region. Highs should hold in the 40s Piedmont,
with 50s SE. -Badgett


As of 340 PM Saturday...

Rain Sunday night will taper to light rain or drizzle late as the
initial surge of lift shifts to our NE and the initial wave of low
pressure is forecast to move NE off the coast by 12Z/Monday. Temps
should hold steady in the 40s/lower 50s NW to SE.

The passage of the wave will bring a break in the rain chances for
most of Monday. This occurs as another (this time weaker) surface
high builds in from the north Monday. Some residual affects of the
CAD should linger Monday in the heart of the Piedmont with plenty of
clouds and some light mist or drizzle. Low clouds early may give way
to breaks in the clouds around the edges of the damming Monday
afternoon. However, we will undercut highs Monday going below
statistical guidance by 5 degrees in the W-N Piedmont (near 50), but
go close to guidance where breaks should occur (Sandhills, Coastal
Plain) may see lower 60s.


As of 400 PM Saturday...

The parent storm is still expected to take on a "Miller B" scenario
Monday night and Tuesday. This means the parent storm tracks NE from
the Gulf of Mexico through the Ohio Valley Mon night. As this storm
weakens, another develops along the SC coast (coastal front) and
tracks along the NC coast Tuesday and Tuesday night.

Another hybrid CAD event is expected as surface high pressure is
forecast to become established from NY to western SC in the Miller B
scenario. Lows Monday night generally in the 40s. Highs Tuesday will
hold in the 40s in the Piedmont, with near 60 SE. Some lingering
light rain or drizzle expected Tuesday night with temperatures
holding steady or slowly rising.

Ample moisture from both the Atlantic and Gulf will be pulled into
the system and the result will be rain developing again Monday night
and continuing Tuesday. The back edge or end of the significant rain
should arrive very late Tuesday with models in general agreement on
this timing. QPF this time should be heavier, with 1 to 1.5 inches

The first very cold air mass originating in Alaska and the Yukon
will arrive in the mountains as early as Thursday. Models are
trending faster and are more in line with the GFS (preferred
solution) in pushing this air mass across the mountains to the coast
late Thursday and Friday. Any showers would be rain and prefrontal
in the warm air.

Wednesday will be mild with decreasing clouds and temperatures in
the 60s. Much colder temperatures are expected Thursday through
Saturday with the heart of the cold for our region Friday into next
weekend. Our coldest lows should be 20-25 and highs mid 30s to mid
40s Friday and 40-45 Saturday.


.AVIATION /06Z Sunday through Thursday/...
As of 1245 AM Sunday...

VFR conditions will hold through this morning across Central NC,
although mid and high clouds will continue to overspread the area
from the west. Cigs will stay above 5,000 ft AGL areawide through at
least 17z. But conditions will gradually deteriorate this afternoon
from SW to NE as surface low pressure and an upper level disturbance
track across the Gulf States toward the Southeast. Cigs will drop to
MVFR at INT/GSO starting around 18z with light rain spreading in,
then at RDU/FAY starting around 21z, and at RWI starting around 02z
this evening. Cigs are then expected to drop to IFR at INT/GSO after
21z and at RDU/FAY after 02z this evening. The rain will initially
be light as it first arrives today with VFR vsbys holding, however
as the rain intensity picks up a bit later today, vsbys will drop to
MVFR in the late afternoon (INT/GSO) through evening (RDU/FAY/RWI).
Winds will be light mainly from the NE initially before shifting to
blow from the E and SE late today into tonight, remaining light.

Looking beyond 06z late tonight, high confidence in poor aviation
conditions, primarily IFR/LIFR cigs and MVFR/IFR vsbys, dominating
later tonight through daybreak Mon. Rain will gradually taper off
from NW to SE Monday morning with conditions rebounding to VFR
areawide. VFR conditions will dominate from around 18z Mon until
around 06z Tue. Then another storm system will cause IFR conditions
to return after 06z early Tue morning, lasting through much of Tue
with steady widespread rain returning. Rain will end late Tue,
although IFR clouds will hold into Tue night. Improvement to VFR is
expected Wed, then will drop back to MVFR/IFR Wed night. Cold front
passage may bring VFR conditions once again Thu. -GIH




SHORT TERM...Badgett
AVIATION...Hartfield is the U.S. government's official web portal to all federal, state and local government web resources and services.