Area Forecast Discussion
Issued by NWS Wilmington, NC

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FXUS62 KILM 192248

Area Forecast Discussion
National Weather Service Wilmington NC
648 PM EDT Sat Aug 19 2017

A cold front will stall and weaken near the coast this weekend.
High pressure will expand across the western Atlantic in the
upcoming week, bringing waves of tropical moisture through mid
week. A cold front will approach from the northwest Wednesday
through Thursday moving off the coast Friday.


As of 7 PM Saturday...The last of this afternoons showers are
moving offshore and other than the odd sprinkle along the coast
expect that will be it for the night as diurnal cooling takes
hold. Have allowed the Heat Advisory to expire on time at 6 PM
as heat index values are now well below criteria throughout the
forecast area. Have updated the forecast and gridded database
accordingly. Relative portion of previous discussion follows:

Partial clearing and low- level saturation with light wind will
support pockets of dense fog from 4 am through sunrise Sunday.

Drier mid-level air and lower surface dewpoints Sunday will reduce
convective coverage compared to today, while lowering heat indices
across the region, and no  `heat advisories` are needed for Sunday.
The better rain chance will reside near the coast in vicinity of the
sea breeze front and its interaction with a decaying frontal
boundary. Winds in the lowest several thousand feet remain light
Sunday and the severe threat remains minimal, although downdraft
CAPES will be higher, so a localized downburst cannot be written
off entirely.


As of 300 PM Saturday...Eclipse-viewing prospects continue to worsen
unfortunately as several ingredients coming into play to increase
cloud cover and rain chances. The first `culprit` may be the upper
low/surface wave retrograding across the Gulf of Mexico. Locally
this leads to an enhanced mid and upper SSE flow of moisture as
early as Sunday night. The GFS is much more aggressive than the WRF
in its depiction of this moisture. Monday morning even as this upper
mositure may wane, models are then showing low and mid level
moisture moving ashore as the remnant tropical wave in the Bahamas
starts moving around the SW periphery of the Atlantic ridge,
possibly even accompanied by weak mid level vorticity centers.
Lastly assuming Monday`s airmass is not dissimilar from today`s
(barring Sunday`s slight drying trend) thunderstorm initiation will
coincide very roughly with the eclipse start time (partiality begins
at 119 PM, max eclipse is at 248 PM). Even the seabreeze will likely
not represent a clearing line (like today) as it will simply draw in
moisture along the coast from the Bahamas system. Assuming the
cirrus does not hang tough like the more agressive solutions I`m
afraid eclipse viewing will be quite luck-based and be relegated to
areas that have holes in the storms and deeper towering cumulus.
Locally these lucky spots look to be rather few and far between.


As of 300 PM Saturday...High pressure will reside just off
shore extending down from off the coast of the Mid Atlantic
region on Tues. At the same time a broad ridge aloft on Tues
will be displaced by a trough digging down from the northwest
through mid week. This trough will push a cold front into the
Carolinas on Wed reaching the Eastern Carolinas late Wed into
Thurs and slowly moving off the coast into Fri. The models show
it pushing far south and east to bring drier weather on Friday
into Fri night across the forecast area.

Initially expect moisture convergence along the coast early
Tues along periphery of high with some convection along or off
shore becoming enhanced by sea breeze front Tues aftn. By Wed,
cold front reaches into the Carolinas with gradient tightening
ahead of it with moisture pooling over the Eastern Carolinas and
dynamics aloft on the increase. Pcp water values will be above 2
inches with convection on the increase ahead of the front as it
drops southeast into the area Wed night into Thurs. Drier air with
lower temps expected by weeks end as front moves off the coast
by Fri. Looks like we may even see a decent northeast surge
behind front by late Fri. As it looks now, inland should begin
to dry out Thurs night with moisture slow to leave the coast on

Temps will start out warm into mid week ahead of the front, near
90, but will drop off to the mid 80s Thurs into Fri as front
pushes south and east.


As of 00Z...Showers and thunderstorms have moved offshore so
that should be it for the night as far as cig/vsby restrictions
go due to precipitation. Good confidence IFR vsby conditions
will develop in the early morning hours for at least some of the
terminals, possibly all, due to fog. Clearing skies this
evening will enhance fog development, especially where heavy
rain fell today.

Extended Outlook...Expect flight restrictions from isolated to
widely scattered TSTMs Sun aftn, increasing in coverage by Tue
through Wed. Flight restrictions will also be possible each
early morning due to stratus/fog Wed.


As of 7 PM Saturday...Latest obs show quite variable winds due
to convection moving across the waters, with seas in the 3 to 4
ft range. Thunderstorms will move east of the waters, after
which winds will settle back into a SW direction of around 10
kts. Forecast in good shape. Have adjusted timing of showers and
thunderstorms activity with the latest update. Previous
discussion follows:

South waves will govern the sea spectrum, running in 5-6 second
intervals, around 3 feet, but up to 4 ft outer waters tonight.
Isolated to scattered TSTMS will move to the NNE-NE through the
weekend. Winds will be of a light character this period as the
far western edge of the Bermuda high extends to shore, with a
weakening front wavering near the coast. As a result chop will
be on the light side through Sunday. Isolated TSTMS will impact
the waters through the weekend.

As of 300 PM Saturday...Light southwesterly flow Sunday night may get
disrupted for a time and go somewhat variable on Monday as poorly
defined front/trough moves close to the area.  Not much changes
Monday night as the boundary remains very weak and possibly
retrogrades slightly. Over the marine environment this may allow a
southerly wind direction to slightly dominate even as speeds remain
capped at 10kt and seas 2ft.

As of 300 PM Saturday...Southerly winds around high pressure
extending down from off the Mid Atlantic coast will become more
SE Tues aftn in sea breeze but will remain basically 10 kts or
less. Winds will begin to veer and increase out of the
southwest Tues night into Wed as a cold front makes its way into
the Carolinas tightening the overall pressure gradient. This
front will make it into the Eastern Carolinas Wed night into
Thurs with stiff SW winds continuing ahead of it. The winds will
begin to veer late Thurs into Fri as the front makes its way
into and through the waters. Shower and thunderstorm activity
will be on the rise ahead of the front through mid week.





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