Area Forecast Discussion
Issued by NWS Wilmington, NC

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FXUS62 KILM 180537

Area Forecast Discussion
National Weather Service Wilmington NC
1237 AM EST Sat Nov 18 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 with the possibility for Freeze warnings and/or
Frost Advisories for portions of the forecast area during
Sunday and Monday nights. Temperatures will moderate Tuesday and
Wednesday. Uncertainty increases by the mid to late week period
of next week as another cold front moves into the area and the
forecast will hinge on the track of developing low pressure in
the Gulf of Mexico.


As of 1130 PM Friday...Temperatures dropping rapidly and faster
than any available guidance suggests. Many mesonet sites now 34
or less, with widespread 36 showing up across the area. RAP
forecast soundings show temps continuing to fall for the next
several hours despite slow moistening of the upper column. For
this reason have issued a frost advisory for portions of the
area. Previous discussion below:

As of 845 PM Friday...Center of high pressure is nearly
overhead. Clear skies, subsidence and a decent sfc based
inversion after sunset, has resulted in excellent radiational
cooling conditions with winds having decoupled and temps
dropping quickly into the low 40s with upper 30s inland by 8 PM.
With sfc dewpoints in the 30s, temperatures for the remainder of
this evening and overnight will either hold steady or fall a few
more degrees as it converges toward the sfc dewpoint. Enough
moisture at ground level combined with temps dropping into the
35 to 39 degree range, patchy frost away from the immediate
coast will occur. Frost could develop as early as late this
evening but expect it during the pre-dawn Sat hours. Mid to
upper level clouds toward sunrise Sat may hamper the rad
cooling conditions enough for temps to rise a few degrees above
Frost threshold occurrence.

As of 300 PM Friday...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 06Z...High confidence in primarily VFR this valid period
although some light fog this morning may produce some short
duration restrictions.

High pressure overhead is creating nearly cloudless sky conditions
this morning. Temp/dewpoint spreads are very low, but moisture is
quite shallow so fog is not expected. Local UPS fog procedure did
produce a few areas of patchy fog, but have kept out any mention in
TAFs attm. Cannot rule out some short duration MVFR, especially at
LBT or CRE, but will handle this with an AMD if it becomes
necessary. Otherwise, VFR will be the rule through the valid period
with only FEW/SCT diurnal CU this aftn beneath increasing cirrus.
The biggest impact today will be increasing winds, especially this
evening and tonight, as speeds rise to 10-15 kts with gusts of 20-25
kts from the SW. Winds are not likely to ease after dark as the
gradient pinches ahead of a cold front. This front will bring gusty
showers to the terminals Saturday night, but most of this activity
will hold off until after 06Z Sunday, and have only a mention of
VCSH at the inland terminals before that.

Extended Outlook...Showers and possible MVFR Saturday night
into Sunday along with gusty winds. VFR Monday and Tuesday.
Showers possible Wednesday.


As of 845 PM Friday...The center of sfc high pressure will
basically be overhead this evening and overnight before
progressing off the coast and offshore during Sat. Winds will
drop to NE at 5 to 10 kt with possible calm conditions across
portions of the local waters due to a well relaxed sfc pg. The
progged sfc pressure pattern indicates once the high moves off
the coast, return flow from off the Atlantic will become common
across all waters after sunrise Sat. Significant seas initially
will run 2 to 3 ft except up to 4 ft off Cape Fear. Seas will
drop to their lowest Sat morning before picking back up during
Sat afternoon and night. A pseudo Easterly ground swell will
dominate the local seas initially given the absence of any
notable wind driven waves. This will change during Sat after the
sfc high moves offshore. With increasing wind driven waves
becoming more dominant as morning SE-S winds increase that
eventually veer to the SW and W ahead of the approaching cold
front late Sat thru Sat night.

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


SC...Frost Advisory until 8 AM EST this morning for SCZ017-023-024-
NC...Frost Advisory until 8 AM EST this morning for NCZ087-096-099-


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