Area Forecast Discussion
Issued by NWS Wilmington, NC

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FXUS62 KILM 172033

Area Forecast Discussion
National Weather Service Wilmington NC
333 PM EST Fri Nov 17 2017

A cool night tonight will be followed by a breezy warmup on
Saturday ahead of a strong cold front. The front may be
accompanied by some gusty showers as it sweeps across the area
and offshore late Saturday night and early Sunday morning. Much
colder and drier air will be ushered in for Sunday and into
early next week and we have a reasonable chance of a widespread
freeze. Temperatures will moderate Tuesday and Wednesday.
Uncertainty increases mid and late next week as another cold
front moves into the area and forecast will hinge on the track
of developing low pressure in the Gulf of Mexico.


As of 300 PM Friday...Cool and dry airmass will remain in place
through tonight as Canadian high pressure migrates south and
east. The center of the high will settle almost directly
overhead tonight before shifting off the coast by daybreak
Saturday. This will produce ideal radiational cooling with near
very light to calm winds and clear skies. Once the atmosphere
decouples after sunset temps should drop off quite rapidly. The
very dry air mass in place will produce a large diurnal swing
with temps dropping into the mid to upper 30s most places just
inland. Inland Soundings show temps dropping just enough to
produce a very shallow saturated layer. There may be just enough
very shallow moisture to produce some patchy frost which I have
included in the grids inland. Places along the coast may see
some marine moisture produce some fog or a few lower clouds
advecting on shore under subsidence inversion toward morning.

Heading through Saturday, the winds will shift to the south in a
deeper increasing low level return flow as a cold front begins to
make its way toward the Carolinas from the northwest later in the
day. Pcp water values down near a quarter of inch through this
evening race back up as high shifts off the coast into Sat. Moisture
profiles show potential for some mainly lower and high clouds
through the day on Saturday, but plenty of mid level dry air and
subsidence will keep any pcp out of the forecast until after the
near term period when cold front moves in. May see decent cloud
layer under subsidence inversion just under 6k ft by 21z.

Overall expect a breezy and warm day as temps recover nicely in
WAA. The 850 temps jump back up to near 12c through Sat aftn.
Temps will reach up into the 70s most places, but an increase
in clouds may hold back temps a bit.


As of 300 PM Friday...Impressive cold front due Saturday night.
And though the baroclinicity is considerable and wind fields
quite strong guidance continues to insist on a low QPF event.
Given the quick movement of the boundary and its associated
forcing this seems very plausible. Forecast soundings show
enough lack of instability to keep thunder out of the
forecast...but a deep enough convective shower may be able to
mix down 40-50kt winds to the surface as such wind speeds may be
lurking a mere 2000-2500ft up. Cold advection Sunday drops
850mb temps from 8C to 2C from 12Z to 00Z. NW flow continues
Sunday night but warm advection kicks in as low level thermal
ridge builds in from the west. This advection will occur above
the surface-based boundary layer however and Sunday night lows
will be in the low to mid 30s and a freeze warning may be needed
for northwestern zones.


As of 300 PM Friday...A split upper pattern will exist next week
across North America. A series of shortwaves moving across
southeastern Canada in the northern stream will have relatively
little impact on the Carolinas, however a pair of shortwaves in the
southern stream will have impacts on our sensible weather Tuesday
and Thursday Night into Friday.

Canadian high pressure at the surface Monday will move eastward and
off the coast late Monday night. Model support for this idea is
quite high. Veering low-level winds and a warm advection pattern
developing Tuesday and Tuesday night should support the development
of a coastal trough as a weak shortwave approaches from the
southwest. There are significant model differences in the track the
shortwave takes (ECMWF from south GA to Cape Fear; GFS from central
GA to near Norfolk) which then determine how well-developed the
surface system can become. A compromise solution should advect
enough shallow Atlantic moisture back onshore for a 20-30 percent
chance of showers Tuesday night, clearing out by Wednesday morning
as the shortwave moves offshore. Cold air will sneak south as
another Canadian high moves across the Ohio Valley. This should keep
highs on Thanksgiving Day only in the upper 50s to around 60, a good
6-8 degrees below normal.

Attention then turns to a much stronger shortwave diving into the
western and central Gulf of Mexico Wednesday night and Thursday.
Just like yesterday there are still model differences here, with the
ECMWF slower than the GFS with the eastward movement of the trough
across the Gulf of Mexico Thursday and Friday. A compromise solution
of the operational GFS, ECMWF, and their ensembles brings rain
chances into the area Thursday night through Friday Night, but given
all the uncertainty (and to better blend with surrounding NWS
offices) I`m capping PoPs no higher than 30 percent for now.


As of 18Z...High pressure will remain in control for one more
day. Winds are a bit more gusty than anticipated, but they will
diminish diurnally this evening. Some light fog is possible at
all terminals after 08Z, with moderately favorable moisture
profiles. Saturday, increasing cloudiness in advance of the next
system. Winds will shift to the south as high pressure moves

Extended Outlook...VFR with a chance of showers and possible
MVFR/SHRA Saturday night into Sunday.


As of 300 PM Friday...Winds will diminish further as high
pressure shifts closer overhead through tonight. This will also
allow for a veering of winds from NE to E by midnight, shifting
further around to the SE and S through Sat morning as the
center of the high shifts off the coast of Hatteras. Overall,
winds will drop off into tonight to under 10 kts before picking
up again through Saturday.

Gradient will tighten through Saturday aftn between high
pressure to the east and a cold front approaching the Carolinas
from the NW. Expect winds increasing up to 15 kts by end of
period with gusts up to 20 kts. Seas will remain less than 3 ft
through Sat morning, but increasing southerly winds Sat aftn,
will push seas up to 3 to 4 ft by Sat eve.

As of 300 PM Friday...Powerful cold front approaching Saturday
night. A small craft advisory will be issued next forecast cycle
or thereabouts. Both the pre-frontal and post-frontal flow
regimes will support advisory-worthy wind and seas. The actual
FROPA and wind shift will come right around daybreak Sunday. At
this time it seems that the SSTs being much cooler than the
airmass will preclude pre-frontal Gale conditions and later in
the period as the airmass cools the wind fields will weaken
sufficiently to do the same.

As of 300 PM Friday...High pressure will move across the Carolinas
Monday, then offshore late Monday night. By Tuesday return flow
behind the departing high will buckle as a coastal trough develops
along the Southeast coast. Winds should turn easterly on Tuesday,
then perhaps southeasterly for a time Tuesday night as the trough
makes its closest approach to shore, but by Wednesday morning the
feature should retreat back offshore as high pressure advances
across the Ohio Valley and shifts our winds to a more northerly





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