Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 032331
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
631 PM EST Sat Dec 3 2016

.SYNOPSIS...
High pressure will continue to build SE into the region tonight.
The high pressure will extend from NY state south through the
Carolinas Sunday, as a coastal front develops. A storm system
will move from the Gulf Coast states into the Southeast
states early in the week increasing the chance of rain.

&&

.NEAR TERM /THROUGH SUNDAY/...
As of 340 PM Saturday...

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.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH MONDAY/...
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.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
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
expected.

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 /00Z SUNDAY THROUGH WEDNESDAY/...
As of 630 PM SATURDAY...

24-Hour TAF period: High confidence that VFR conditions should
prevail for the first several hours of the TAF period, more likely
through 12Z Sunday, although skies will be overcast around 8-10 KFT
by Sunday morning. Aviation conditions will lower to MVFR through
the day Sunday from west (KINT/KGSO in the 12-15Z time frame) to
east (KRWI after 18Z). Rain will also move in from southwest to
northeast from 12Z Sunday to 00Z Monday, possibly holding off until
after 00Z Monday at KRWI.

Looking ahead: Confidence is high that conditions will become IFR to
LIFR Sunday evening/night and remain into Monday with periods of
rain and drizzle.

LIFR TO IFR conditions are expected Monday night and Tuesday with
the storm system arriving from the SW and the cold air damming over
the region. Rain should taper off Tuesday night but IFR/LIFR
conditions may hold through daybreak Wednesday, followed by
improvement to VFR by midday Wednesday. -Badgett

&&

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

&&

$$
SYNOPSIS...Badgett
NEAR TERM...Badgett
SHORT TERM...Badgett
LONG TERM...BSD/Badgett
AVIATION...KC/Badgett



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