Area Forecast Discussion
Issued by NWS Wilmington, NC

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FXUS62 KILM 240020

Area Forecast Discussion
National Weather Service Wilmington NC
820 PM EDT Fri Sep 23 2016

A cold frontal passage followed by modest and dry high pressure
will build southward across the Great Lakes and into the Mid-
Atlantic region this weekend. Moisture and rain chances will
return by Tuesday as the high moves offshore. The next cold front
will arrive from the west Wednesday, followed by autumn high
pressure accompanied by a dry and cool airmass.


As of 700 PM Friday...The Flood Watch has been cancelled do to
lack of additional pcpn expected.

The evening will feature a diminishing and dissipating trend to
the showers and isolated thunderstorms as witnessed by the latest
mosaic 88D trends and the latest near term models, HRRR RAP and
HiResWrf. Will indicate a lowering of POPs over land areas this
evening, leaving widely scattered activity over the adjacent Atl
local waters. Trajectories in the lower levels tonight indicate
the pcpn that develops over the Atl Waters will not have that
onshore and inland movement like the past 5 days. May see a few
shra develop nearshore and clip possibly Cape Fear or Cape Romain.
For the pre-dawn Sat hrs, low level moisture will remain avbl as
drier air aloft works its way southward. Will either see low
level stratus or ground based fog. Not bought on which wx
phenomena will dominate. For now, will include variable amounts of
clouds and patchy to areas of fog during the pre-dawn Sat hrs up
to a few hrs after daybreak. No tweaks to the min temps needed at
this time.

As of 300 PM Friday...Surface low pressure is centered about 20
miles south of Georgetown, SC, and should settle farther south
just off the South Carolina coast tonight. This feature has been
on our weather maps for a good portion of our intern`s career now!
Dense stratus took many hours to burn off earlier today, but once
surface temperatures reached 80-81 degrees and uncapped the
atmosphere, numerous showers have developed.

Weak steering flow within the tropical airmass surrounding the low
has led to another day of slow moving heavy rain. The combination of
the best instability and low-level convergence has set up across SE
North Carolina just inland from the coast. Rainfall rates over 1
inch per hour have so far remained on the west side of US Highway
17, fortunately not aggravating the areas which picked up 5-8 inches
of rain and significant flash flooding today. The Flash Flood Watch
for Pender and New Hanover counties will remain posted through this
evening, at which point the inland showers and storms should be
weakening and drifting southward (offshore) behind the departing
surface low.

With drying mid-levels but very rich low level moisture in place
with light winds, fog and low stratus is again expected to develop
after midnight. A check of GFS and NAM MOS performance over the past
few days shows a persistent -2 degree bias in these products when
verified against actual low temperatures, so our forecast lows are
about 2 degrees above MOS consensus across the board.


As of 300 PM Friday...As our low drifts farther south Saturday and
high pressure to the north begins to build in, the airmass should
dry appreciably from the top down. Precipitable water values in the
1.8 to 2.0 inch range today should fall to 1.5 inches Saturday
afternoon and to 1.25 inches Saturday night. Using 850 mb relative
humidity as a guide, I am limiting showers Saturday afternoon to
just the seabreeze convergence zone of Horry and Georgetown counties
in South Carolina, anticipating the airmass will become too dry
elsewhere to overcome the convective cap. With skies becoming mostly
sunny by late morning, highs should reach the upper 80s inland, 82-
86 closer to the coast.

High pressure north of the Great Lakes Saturday will build southward
down the Mid-Atlantic coast on Sunday. This should push a cold front
through the coastal Carolinas early Sunday. With a building ridge
aloft and meager moisture, this front will probably come through dry
with only an increase in low-level wind speeds expected. Winds
behind the front Sunday should become east-northeasterly, perhaps
picking up enough Atlantic moisture for scattered showers to develop
and advect onshore. Since this is heavily modified Canadian air that
will have spent a thousand miles or so over the warm western
Atlantic, low-level lapse rates should be steep which may help
support these shallow convective showers.

Given the significant post-frontal influence off the Atlantic, I am
not forecasting Saturday night and Sunday night low temperatures to
fall quite as low as recent MOS guidance would indicate,
particularly along the coast.


As of 300 PM Friday...High pressure will be to our north on
Monday but rather than a wedge-like NE flow wind will remain out
of the E. This will keep shower chances in the forecast as
Atlantic moisture is handily advected ashore. They may tend to
favor coastal areas though as mid level ridging keeps the upper
levels of the column quite dry. This onshore flow will turn more
southerly with the approach of a cold front on Tuesday, its
passage slated for Tuesday night. Wednesday and Thursday could
feature the strongest cool advection we`ve seen in a while and
bring some unseasonably cool weather with Thursday night possibly
slipping below 60 away from the coast. A wedge then sets up on
Friday continuing the cool weather.


As of 00Z...Expect a mixture of VFR/MVFR/IFR tonight through
Saturday morning due to passing low cigs, isolated showers and
developing fog towards morning. Otherwise expect VFR with isolated
showers on Saturday.

Latest guidance and weather pattern trend continue to suggest
potential for widespread fog late overnight into the early morning
hours. Thus have noted in going forecast with light winds and
passing low cigs. After daybreak, expect conditions to gradually
improve to VFR and continue through the remainder of the TAF period.
Northeasterly winds will increase to 5 to 8 kts, becoming east-
southeasterly in the afternoon hours.

EXTENDED OUTLOOK...Chance for SHRA/TEMPO MVFR Sunday. Chance for
SHRA/TSRA Tue through Wed. Otherwise expect VFR.


As of 730 PM Friday...Rather weak sfc pg and somewhat diffuse
pressure pattern across the local waters due to the weak sfc low
now well south of the ILM CWA. Will continue with the NE wind
directions and speeds at 5 to 10 kt. The pcpn coverage over the
local waters will slowly shrink this evening but could temporarily
peak back up mainly over the offshore waters south of Cape Fear
during the pre-dawn Sat hours. With pcpn movements slated to have
mainly a southward trajectory, the majority of the pcpn will
likely stay or remain over the offshore waters. Significant seas
will be totally a function of the 2.0 to 3.5 foot e to ese ground
swell from Tropical Cyclone Karl. The higher swell will occur
north of Cape Fear, with 3 to 4 foot significant seas being
fcst...with 2 to 3 foot south of the Cape.

As of 300 PM Friday...Low pressure is centered about 20 miles south
of Georgetown, SC, and should slip farther down the South Carolina
coast overnight. Cyclonic flow will maintain mainly northeasterly
winds overnight, with wind speeds generally 10 knots or less. The
bulk of the showers and thunderstorms are located inland this
afternoon with only isolated activity over the coastal waters east
of Cape Fear. Models suggest that this evening the showers in the
Wilmington area could slide south and off the coast, migrating
farther offshore overnight.

Seas are still 2-3 feet, mainly in easterly 12-second swell being
produced by distance Tropical Storm Karl. Combined seas could build
to as high as 4 feet in the Cape Fear area coastal waters tonight.

As of 300 PM Friday...Surface high pressure north of the Great Lakes
on Saturday will build southward down the Mid-Atlantic coast Sunday.
This should push a cold front through the coastal Carolinas early
Sunday. Winds both ahead and behind the front will be northeasterly,
and since the front will come through dry the only change heralding
the front`s arrival will be an increase in wind speed. Winds
Saturday and Saturday night around 10 knots should increase to
around 15 knots for Sunday. These winds, in combination with swells
from Tropical Storm (or Hurricane) Karl out near Bermuda, will build
seas to 3-4 feet with some 5-footers probably lurking not too far
from 20 miles from shore near Cape Fear.

As of 300 PM Friday...Monday starts off with easterly winds as
high pressure is centered well to our north. This flow will then
veer as the day wears on as the high progresses eastward and a
cold front approaches from the northwest. This boundary comes
through Tuesday night and it will usher in an increasingly strong
NE surge of cooler air. Ahead of the front on Monday seas will be
quite manageable and any prefrontal increase in seas Tuesday
should be quite gradual. This may not hold true with and following
FROPA Tuesday night into Wednesday where headlines may become




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