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672
FXUS62 KILM 240613
AFDILM

Area Forecast Discussion
National Weather Service Wilmington NC
213 AM EDT Thu Aug 24 2017

.SYNOPSIS...
An approaching cold front will bring thunderstorms into
Thursday, some of which may be strong. Canadian high pressure
will bring an early fall-like feel Friday and through the
weekend. We will be watching the tropics for possible
development of low pressure off the Southeast coast early next
week.

&&

.NEAR TERM /THROUGH THURSDAY/...
As of 915 PM Wednesday...The big convective line is weakening
slightly as it approaches the coast of SE North Carolina.
Ingesting the very humid but slightly cooler (and therefore
initially capped) boundary layer is likely responsible for the
storms` recent behavior. Although weaker, they should continue
to sag southeastward and to the beaches in the next hour. Heavy
rainfall, gusty winds, and frequent lightning are all expected.

Redevelopment of storms off to the west from Marion and
Johnsonville, SC all the way back through Timmonsville and
Camden is a bit healthier than I would have thought two hours
ago. Surface parcels are heavily capped back in this region, but
recent SPC mesoanalysis graphics indicate central and eastern
South Carolina is coming under the influence of a 250 mb jet
streak located over the Mid-Atlantic region. Upper level
divergence located within the right-entrance region of this jet
is probably helping lift elevated parcels starting in the
5000-1000 foot AGL range. There`s no reason to expect this
activity to dissipate given its disconnect from the boundary
layer and the likely persistence of the upper level pattern.
Likely (60-70 percent) PoPs have been expanded across a good
portion of eastern South Carolina through early Thursday
morning.

Discussion from 730 PM follows...

A synoptic cold front across central North Carolina extends
from Greenville, NC to Fayetteville to near Charlotte and is
dropping southward. A large cluster of thunderstorms has
developed just ahead of the boundary, taking advantage of
uncapped surface-based CAPE of 2500 J/kg. Precipitable water
values near 2.0 inches normally doesn`t coexist with large DCAPE
values, but in this case just large enough temp-dewpoint
spreads exist above the boundary layer to support DCAPE as large
as 1300 J/kg -- leading to the gusty winds we have seen in
today`s storms. A gust to 55 mph was measured at the ASOS in
Lumberton, NC, and trees were blown down in Dillon, SC.

Evolution of this event through the evening is shown very well
on the last few HRRR runs as well as the 18Z NAM. A massed-line
of storms should work down into Wilmington and the Cape Fear
region by late evening, joined by scattered showers and storms
across the Myrtle Beach area. One weak upper level vort max is
supporting our current storms, but a second vort digging across
eastern Tennessee and western North Carolina later tonight may
begin to back the mean flow just enough late tonight to cause
activity to cease its eastward progress and hang up across the
coastal waters and perhaps the coastal counties as well. On this
latest update PoPs were increased to 70-100 percent for SE North
Carolina, with QPF raised to 1+ inches in spots.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
As of 215 AM Thursday...A cooling trend back towards and then
ultimately below seasonal readings will commence on Friday as
high pressure builds in from the north and a series of waves
move NE along a stalled frontal boundry offshore. These waves
plus the strengthening high will gradually increase the NEly
gradient, allowing daytime highs to drop back into the mid 80s
throughout the forecast area on Saturday after a day of mid to
upper 80s on Friday. Lows Saturday night will drop into the mid
and upper 60s. The real difference will come from the drop in
dew points, which will drop back into the lower and mid 60s by
Saturday, allowing for a more autumnal-like feel for the first
day of the upcoming weekend. A consensus of guidance keeps in
slight chance pops for showers nearest the coast for both days
in closest vicinity to the offshore front.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
As of 300 PM Wednesday...A stationary front will be located
well offshore and S of the area Sat with an area of low pressure
positioned on the front, but well offshore. High pressure will
be ridging across the Carolinas from New England. The pressure
gradient between these two features will keep a fairly brisk NE
wind in place across the Forecast Area through the weekend.
This will ensure the cool air is continually reinforced and
highs during the weekend should only be in the lower to mid 80s
and this will be coupled with comfortable humidity levels. Model
profiles continue to be quite dry with precipitable water
values below climatological normals and thus will not include
any mention of a shower or thunderstorm during the weekend. This
pattern looks to hold through Sun. The flow becomes more
onshore and the column begins to moisten Mon and Tue. There may
also be some mid- level shortwave energy impinging on the area
from the SW. Will paint more in the way of cloudiness as a wedge
likely sets up and introduce the potential for some showers and
perhaps a thunderstorm. The probabilities for precipitation
will be highest at the coast.

The heat is not to be found, even early to the middle of next
week with highs only in the lower to mid 80s. Lows will be in
the mid 60s to near 70 through the period.

As a caveat, we will be watching the tropics early next week to
see if a weak disturbance is able to develop somewhere off the
SE coast. Confidence in any development is very low as is its
eventual track.

&&

.AVIATION /06Z THURSDAY THROUGH MONDAY/...
As of 06Z...Messy night continues with an area of heavy rainfall and
thunderstorms progressing across the SC terminals. This area of rain
is expected to slowly migrate E/SE through the next few hours,
eventually moving offshore and eroding. However, the Myrtles will
likely see at least TEMPO IFR in TSRA while FLO/ILM experience MVFR
in showers. A cold front west of the terminals will begin to migrate
E/SE as well, and IFR stratus is forecast to develop behind the
boundary as it shifts through LBT/FLO overnight. Due to the presence
of this front, winds will be variable at all the terminals
overnight, with gusts likely confined to convection at CRE/MYR.

After daybreak, stratus will slowly erode and the front will very
gradually shift SE, exiting the area only late on Thursday. This
will persist light and variable winds, possibly becoming SE at the
coast with the sea breeze for a time this aftn, while taking on a NE
direction inland. Late morning CU will lift only to MVFR much of the
day leading to the development of scattered showers and tstms once
again. These will be most likely along and east of the front, but
coverage and intensity is expected to be lighter than what occurred
tonight. As the front pushes away finally by the end of the period,
VFR will gradually develop as NE winds usher in drier air to the
region.

Extended Outlook...MVFR ceilings could linger, especially near
the coast Friday. VFR through the weekend. Showers may increase
beginning Monday as a coastal low passes well offshore.

&&

.MARINE...
NEAR TERM /THROUGH THURSDAY/...
As of 915 PM Wednesday...Storms are moving rapidly toward the
beaches of SE North Carolina. They are weakening but are still
capable of producing some hefty wind gusts as they move
offshore. Please reference the latest Marine Weather Statement
for details. Additional storms are developing across central and
eastern South Carolina and will likely affect the areas south
of Cape Fear closer to midnight and into the early hours of
Thursday morning. No changes have been made to synoptic forecast
reasoning through the night. Discussion from 730 PM follows...

A cold front currently across central North Carolina is moving
southeastward and should reach Cape Fear by daybreak Thursday.
Steady southwesterly winds around 15 knots ahead of the front
should become disturbed by late evening as a large line of
thunderstorms across the interior Coastal Plain moves out across
the waters. Variable wind gusts in and near these storms should
settle down to a weaker, more westerly average wind after
midnight. Mariners planning to be out on the waters, especially
north of Myrtle Beach, should closely watch these storms now
through midnight.

SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
As of 215 AM Thursday...A gradually tightening gradient between
high pressure building in from the north and a deepening trough
of low pressure east of the waters will lead to steadily
increasing NEly winds through the period, with Small Craft
Advisory conditions likely developing on Saturday for 20 to 25
kt winds and 4 to 6 ft seas. The period will start off a bit
more benign, with 10 to 15 kt winds from the NE and 2 to 4 ft
seas on Friday morning.

LONG TERM /SUNDAY THROUGH MONDAY/...
As of 300 PM Wednesday...Persistent and brisk NE winds will
likely require a Small Craft Advisory for much of the period.
The tightening pressure gradient will be the result of high
pressure ridging down into the Carolinas from the N and an
offshore front and area of low pressure. NE winds will be around
25 kt for much of the period. Seas will be 5 to 7 ft and
perhaps up to 8 ft by Mon.

&&

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

&&

$$

SYNOPSIS...REK
NEAR TERM...JDW
SHORT TERM...REK
LONG TERM...RJD
AVIATION...JDW
MARINE...REK/RJD/JDW