Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 210234
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1034 PM EDT Fri Oct 20 2017

.SYNOPSIS...
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.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
Late this evening: A very quiet night remains on tap. Surface
high pressure and deep dry air will remain across the forecast
area which sits directly under the ridge aloft. Other than a few
thin cirrus that could move through late tonight, skies will be
clear. Lows will range from the upper 40s inland to the low/mid
60s at the coast. We could see some patchy ground fog, but the
available moisture is quite shallow so it will be quite limited
in depth and extent, if it occurs at all.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH MONDAY/...
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.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
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.

&&

.AVIATION /03Z SATURDAY THROUGH WEDNESDAY/...
VFR conditions will prevail at both KCHS and KSAV through 00Z
Sunday. Could see some patchy ground fog late tonight but no
restrictions are expected.

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.

&&

.MARINE...
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...
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.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.

&&

$$

NEAR TERM...BSH
SHORT TERM...33
LONG TERM...33
AVIATION...BSH/33
MARINE...DPB/33
TIDES/COASTAL FLOODING...



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