Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 201950

Area Forecast Discussion
National Weather Service Charleston SC
350 PM EDT Fri Oct 20 2017

Strong high pressure will continue over the area tonight, then
will shift offshore this weekend. A cold front will likely move
through Monday night into Tuesday before a secondary cold front
moves through Tuesday night ushering in much cooler and drier
weather through late next week.


This Evening and Tonight: At this sfc, dry high pressure centered to
the north will slowly shift east toward the coast this evening and
overnight. Aloft, a mid/upper lvl ridge of high pressure will remain
centered over the Southeast United States. The pattern will favor
a strong radiational night under clear skies and decoupling winds.
There is even a hint of patchy ground fog developing away from
the coast late tonight as temps cool, but moisture remains too shallow
for anything significant. Patchy fog has been included from most
locations away from the coast, but greatest chances should occur
well inland near bodies of water. Overnight lows will likely dip
to around 50 degrees inland and between the upper 50s to lower
60s closer to the coast.


Saturday: A highly amplified mid and upper pattern will exist,
featuring strong ridging in the east and a deep trough moving
through the Rockies. We`re situated underneath a 5910-5920 meter H5
anticyclone that is shifting east into the nearby Atlantic, and
takes with it a 1027-1028 mb surface high that moves east off the
Delmarva. Given the large scale subsidence, the lack of moisture
with PWat less than 50% of normal, and no forcing, we`ll experience
another rainfree across the entire CWFA. We stayed close to the low
level thickness consensus which supports another day with temps
above 80F west of US-17.

Saturday night: While there does remain a prominent subsidence cap
around 800-850 mb, a subtle inverted trough will form across the
nearby coastal waters. The associated low level moisture
convergence/transport and isentropic ascent will allow for a slight
chance of showers advecting onshore, mainly from Hilton Head south
to McIntosh County, GA. temps won`t be anywhere near as cool as the
past several nights, and will actually revert from below average of
late back to above normal due to the onshore trajectories and
increase in cloud cover. Not sold on any significant fog at this
time, especially since there is close to zero probabilities from the
SREF and condensation pressure deficits are generally no less than
50 mb.

Sunday: The mid and upper anticyclone off the SE coast of the U.S.
is stuck between a blocking low spinning about midway between Puerto
Rico and Bermuda, and a potent short wave driving slowly but
steadily east through the central and southern Great Plains. Despite
this, a proximity to the weak inverted trough that lifts onshore, a
deep SE`erly flow surface to around 500 mb will support a chance of
showers in the 30-50% range, mainly south of the Charleston tri-
county district where there is more available moisture and better
isentropic upglide. The onshore trajectories and the higher rain
chances will be offset some by the warm advection, meaning that most
places inland from the beaches will achieve near 80F for highs.

Monday: The short wave upstream looks to close off across AL and MS,
which nudges the SW Atlantic ridge a little east as it weakens. A
solidly S-SW flow aloft, and a SE flow at the surface and low level
between a cold front moving through the TN valley and the northern
Gulf states will transport considerable moisture into the area.
Combined with better dynamics we`re looking at scattered to numerous
showers. While there is an increase in shear and helicity,
instability and CAPE are lacking, so we have nothing more than
isolated non-severe t-storms. Continued WAA will again produce above
normal max temps, although the diurnal curve could be significantly
altered pending timing of convection.


A potent upper shortwave and/or closed low will affect the area
Monday night into Tuesday, pushing an initial cold front through the
area Monday night into Tuesday. Expect a good chance for showers and
some thunderstorms along and ahead of the cold front, followed by
drier and cooler high pressure. Tuesday should be mainly dry,
although some showers could linger, especially near the SC coast
early in the day. A secondary cold front will then move through
Tuesday night as the main upper trough envelopes the eastern U.S..
Dry, but cooler conditions will then prevail through Thursday before
some moderation occurs late in the week as high pressure quickly
shifts offshore allowing a return flow to set up. Temperatures
should stay above normal through Tuesday before falling at or below
normal through Thursday night. Many inland areas should get into the
40s Thursday/Friday mornings.


VFR conditions will prevail at both CHS and SAV terminals through
18Z Saturday. However, patchy ground fog can not be ruled out late
tonight to around daybreak, mainly at the SAV terminal. Confidence
remains too low to include lower vsbys in response to fog at this

Extended Aviation Outlook: Brief flight restrictions are possible
Sunday due to low clouds/showers. Higher chances of flight restrictions
are expected Sunday night through Monday night, possibly into Tuesday,
as a cold front affects the area with heavier showers and potentially
some thunderstorms.


This Evening and Tonight: Sfc high pressure centered north of the
area will shift east off the Mid-Atlantic Coast, helping weaken the
pressure gradient over local waters. As a result, east/northeast
winds and seas will steadily decrease this evening and tonight,
remaining below Small Craft Advisory levels. In general, winds
will peak around 10 kt in South Carolina waters. However, winds
should peak near 15-20 kt in Georgia waters where the pressure
gradient is stronger. Seas will range between 2-3 ft in nearshore
waters and 4-5 ft in offshore Georgia waters.

Saturday: The local waters will be situated on the southern side of
strong high pressure moving east off the Delmarva and to the south
of Long island. This allows for a fairly steady E/NE flow at 10-15
kt, with seas to average 3 or 4 ft.

Saturday night and Sunday: The surface high becomes absorbed by a
stronger high over eastern Canada, allowing for a weak coastal
trough that forms over or near the local waters to eventually move
onshore. Easterly winds will slowly climb as the gradient starts to
tighten, and this allows for seas to rise about 1 or 2 ft, enough so
we could be close to SCA conditions on the outer GA waters,
especially Sunday.

Monday through Wednesday: Strong high pressure covering much of the
west and SW Atlantic will give way to a cold front that sweeps
through late Monday night and/or early Tuesday. This is followed by
high pressure that begins to build from the W-SW during the middle
of the week. The gradient tightens further in advance of the cold
front early next week, but this is tempered somewhat by WAA. Still,
we anticipate SCA`s across much of the marine area as seas build
further. CAA behind the front is overly strong given an air mass of
Pacific origin, so there should be some improvement in winds and
seas by late in the forecast period.

Mariners should stay alert for at least isolated t-storms that
impacts the area Sunday into early Tuesday, producing locally heavy
rains, lightning strikes and perhaps gusty winds.

Rip Currents...Onshore winds, small swells and lingering astronomical
influences will produce a moderate risk of rip currents at the
GA beaches through Saturday. Conditions will be close to a moderate
risk on the SC beaches, but the flow is a little weaker than further
south, so a low risk is maintained for now.


Tides/Coastal Flooding...Since there is a little more onshore wind
than there was today, we could come close to the requirement for a
Coastal Flood Advisory for parts of the SC coast around 10 am Saturday.




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