Area Forecast Discussion
Issued by NWS Wilmington, NC

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FXUS62 KILM 212347
AFDILM

Area Forecast Discussion
National Weather Service Wilmington NC
647 PM EST Tue Nov 21 2017

.SYNOPSIS...
A coastal wave will move northward overnight, bringing a chance
of showers. A cold front will press southward across the
Carolinas on Wednesday. A few coastal showers are possible
Thanksgiving Day, as a low pressure wave passes offshore.
Seasonably cool and dry conditions will prevail into the weekend
and early next week, as a series cold fronts sweep off the
coast, and low pressure areas remain offshore. Strong Canadian
high pressure will pass across the area early next week.

&&

.NEAR TERM /THROUGH WEDNESDAY/...
As of 630 PM Tuesday...Isolated light showers moving ashore from
the south this evening. HRRR handling situation quite well so
far and has rain coverage expanding through the evening hours
from S to N, peaking around midnight or so with a focus on the
coast, then out of the area by around daybreak. This scenario is
close to what we already have in the forecast so if any changes
are made they will be minor tweaks to coverage and timing. Still
looks like a low QPF/stratiform event. Previous discussion from
this afternoon follows:

Coastal trough sitting offshore will attempt to move onshore
this evening and confidence is low on whether or not this will
happen. Should it move onshore the most noticeable difference
would be increased rainfall rates within showers east of the
trough as opposed to lighter rain on the west side. In the grand
scheme of things the increase in rainfall would not be
significant, a couple tenths of an inch at most as opposed to
around a tenth of an inch. Temperatures would also briefly spike
up as the trough moves onshore.

Regardless of where the trough ends up this evening it will be
lifted north and then pushed east later tonight. A shortwave
rounding the base of the high amplitude 5h trough over the
eastern CONUS will move northeast just off the Southeast coast.
Surface wave/low associated with the wave will lift northeast
along the coastal trough, helping move the feature out of the
region. Light rain will spread over the area this evening as
isentropic lift on the 295k surface increases and deeper
moisture arrives. Precipitable water increases from half an inch
around midday to around 1.3 inches overnight which should allow
for measurable rainfall across much of the area. Rainfall
amounts and coverage will be highest closer to the coast.
Isentropic lift starts to wind down around midnight with any
rainfall expected to come to an end shortly thereafter. Westerly
flow below 7k ft as the surface and mid-level wave exit help
dry the region out during the second half of the overnight
period. Lack of any significant cold advection, combined with
lingering cloud cover and mixing helps to keep lows above climo.

Cold front trailing northern stream low moving into eastern
Canada late tonight moves across the area Wed. The front is
starved of moisture by the time it reaches the area and it will
pass dry while helping to clear out any lingering clouds. Some
modest cold advection behind the front but the air arrives too
late to have any real impact on highs, which will end up a
couple degrees above climo. Winds will become breezy following
the passage of the front in the afternoon, especially as
sunshine increases and stronger winds aloft start mixing to the
surface.

&&

.SHORT TERM /WEDNESDAY NIGHT THROUGH THURSDAY NIGHT/...
As of 230 PM Tuesday...H5 ridge across the Desert SW and a
trough will be positioned across the Eastern U.S. through the
short term period. At the surface, high pressure will prevail as
it builds across the Carolinas, possibly supplanting the areas
of low pressure far enough offshore to limit rainfall chances
during Thanksgiving. Isentropic lift is shown to be lacking as
well. Currently, we will maintain the highest POPs across the
coastal zones during Thanksgiving and keep QPF low unless the
low can evolve farther west. Maintained a blend of MET/MAV
temperatures through the period.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
As of 230 PM Tuesday...A progressive yet overall dry extended
forecast prevails with seasonably cool temperatures, and dry
largely in part, because the atmosphere remains dry above 8000
feet. A moderately strong cold front Saturday night may bring a
few clouds but column moisture appears too scarce for rain. The
dynamics aloft will be running strong, so a brief shower over SE
NC cannot be ruled out but no QPF expectations at this time.
Sunday should bring brisk north wind, as high pressure builds
from the west. The core of the high, and clear skies will team
up overhead daybreak on Monday, and may greet some inland areas
with sub-freezing air.

&&

.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
As of 23Z...Latest radar and satellite pics showing area of light to
some patches of moderate rain off the GA coast moving to the NNE.
Developing low pressure system near Florida will track up the coast
this evening, bringing the chance for rain. The rain will mainly be
at the coastal terminals at least in the prevailing group and in the
tempo group across the inland terminals. After the low passes to the
northeast, winds will swing around to northwest, there will be a
brief window of IFR ceilings/vis expected. Conditions are expected
to improve within a few hours after daybreak as a cold front pushes
across the region and helps to clear out the fog and low ceilings.

Extended Outlook...VFR conditions expected by Wednesday afternoon.
VFR Thursday. Low confidence for MVFR to IFR conditions near the
coast late Thursday through Friday night in low clouds and rain.
&&

.MARINE...
NEAR TERM /THROUGH WEDNESDAY/...
As of 630 PM Tuesday...Winds are in the 10 to 15 kt range this
evening with a variable direction due to the proximity of the
coastal trough. Seas range right around 3 ft. May update the
forecast again for wind direction once we get a better sense as
to the progression, or lack thereof, of the coastal trough.
Given low wind speeds though, any changes will prove to be
minor. We do expect increasing rain coverage through the evening
hours, with the rain being mainly on the light, drizzly side.
Previous discussion from this afternoon follows:

Coastal trough continues to linger in the region with light
southeast winds over the eastern waters and northeast along the
coast. The trough will try to move onshore, and may briefly do
so this evening. This would result in east to southeast winds
across all of the waters but the weak gradient will keep speeds
10 kt or less. Surface wave moves northeast along the trough
tonight, with winds backing to southwest and then west as night
turns into day. Offshore flow following the wave/trough is not
particularly strong, 10 to 15 kt, but a dry cold front will
cross the waters late in the day. This feature is likely to be
followed by a stronger surge of cold air, albeit after the end
of the period. Seas will steadily build through the period with
2 to 3 ft this afternoon increasing to 2 to 4 ft this evening.
Seas continue to build as the wave approaches and then, with the
switch to southwest and then west, keep increasing, peaking at
3 to 5 ft during Wed.

SHORT TERM /WEDNESDAY NIGHT THROUGH THURSDAY NIGHT/...
As of 230 PM Tuesday...High pressure building across the waters
from the north Wednesday night will maintain a northeasterly
fetch and choppy seas. During Thursday, the area of high
pressure will likely suppress areas of low pressure farther off
the Southeast U.S. coast. This pattern will shift the stronger
pressure gradient offshore by Thursday night allowing winds to
gradually diminish across the coastal waters. At this time think
seas will remain just below Small Craft thresholds across the
outer waters but will need to monitor.

LONG TERM /FRIDAY THROUGH SUNDAY/...
As of 230 PM Tuesday...An easing trend in NE winds Friday will
begin to back to the NW and increase on Saturday as deepening
low pressure passes well offshore. The low will pull well away
to the NE Sunday, with moderate N-NNW winds as high pressure
builds from the west. An exercise caution period is possible
Sunday for N wind, and elevated seas offshore. No 0-20 NM TSTMS
this period but a few showers possible late SAT/early SUN as a
cold front moves through.

&&

.ILM WATCHES/WARNINGS/ADVISORIES...
SC...None.
NC...None.
MARINE...None.

&&

$$
SYNOPSIS...99
NEAR TERM...REK/III
SHORT TERM...SRP
LONG TERM...MJC
AVIATION...MAC
MARINE...MJC/REK/III/SRP



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