Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 300527
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
127 AM EDT Thu Mar 30 2017

.SYNOPSIS...
A weak stationary front will linger near the Altamaha River
tonight, then move north as a warm front Thursday and Thursday
night. A cold front will move through Friday night followed by
dry high pressure into Sunday. Unsettled weather should return
again next Monday, possibly lasting into Tuesday.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
At midnight: Low clouds were mainly north of the Savannah River
but slowly building inland and south. We tweaked some very near
term trends but the flavor of the forecast remains unchanged for
the overnight.

The main concern overnight is for the potential for fog and low
stratus as low-level moisture advects inland from off the
Atlantic. The best chances for fog will occur across mainly the
central and southern portions of the forecast areas overnight
closer to the decaying front. There does look to be enough wind
in the boundary layer across Southeast Georgia and far southern
South Carolina to keep any fog from becoming too dense, but
locally dense fog will be possible, especially from Beaufort and
Hilton Head south through the Savannah Metro Are to other areas
along the Georgia coast where the corridor of highest surface
dewpoints and potential for stratus build down will be found.
Any fog that develops at the coast will gradually advect inland.
Across the Charleston Tri- County area, there could be a stint
of patchy or even areas of fog by late evening, but this should
translate to low stratus with the onset of stronger boundary
layer winds. Dense fog is not expected in this area. Overnight
lows from the upper 50s/lower 60s inland to the lower-mid 60s at
the beaches look right on track.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH SATURDAY/...
A stalled front across southeast GA Thursday will move northward as
a warm front Thursday night. A cold front is expected to cross
through the region Friday night. Models have remained relatively
consistent, showing rain chances increasing Thursday afternoon
when deeper moisture and isentropic ascent increase across the area.
Thus, showers and possibly a few thunderstorms (mainly across GA)
are expected Thursday afternoon/evening. A brief lull in the
precipitation is Thursday night, especially across our
southernmost counties, before increasing again toward daybreak
Friday as stronger synoptic forcing arrives. Precipitation should
taper off later Friday as the deepest moisture shifts northeast
of the area. Although wind fields will be strong Friday, instability
is minimal. Models keep the bulk of convection far to our north,
closer to an area of low pressure, and far to our south, where
a jet streak could provide some support. Since we should remain
between these two areas, the threat for severe weather is low.
More so given the best moisture and instability appear to be
early in the morning, well before peak heating. The trend has
been earlier and weaker, and the forecast reflects this, the
main threat being in the morning hours, diminishing in the
afternoon. But even that may be a bit overdone at this point.
Either way it should be a breezy day with gusts up to around
25-30 mph. Any lingering showers near the SC coast should end
Friday evening as the cold front moves. Dry conditions will
prevail into Saturday.

With the first front to our south on Thursday, temperatures will be
a bit cooler. It should stay below 80 degrees across SC, but rise
into the 80s across GA. Temperatures Friday will be moderated by the
clouds/rain, likely only reaching close to 80 degrees inland with a
slight warmup Saturday as slight cool advection likely gets overcome
by some downslope warming.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
Dry high pressure to prevail through Sunday night although moisture
will be increasing toward Monday morning as low-level jetting ramps
up ahead of approaching low pressure from the west. Unsettled
weather is expected Monday into Tuesday although timing is a bit
uncertain at this point. Mid-level ridging should keep things dry
and quite warm Wednesday, possibly near record levels in GA.
Otherwise, temperatures should remain above normal through the
period.

&&

.AVIATION /06Z THURSDAY THROUGH MONDAY/...
KCHS: Low level winds were increasing and the stratus had lifted
to stratocumulus and MVFR should be persistent overnight. Fog
potential is unlikely given the latest trends and winds in the
forecast soundings. There is some uncertainty how quickly cigs
will rise above MVFR levels Thursday morning, but data generally
support a 14-15z timeframe. Convective rains are certainly later
in the day but until then, only shallow showers are possible.

KSAV: The primary concern is low-stratus and fog. Low-level
moisture will steadily increase from off the Atlantic tonight as
a stationary front lingers near the Altamaha River. The terminal
looks to become embedded within the corridor of highest surface
dewpoints so expect extensive low-stratus and possibly some fog
(stratus build down) to occur. Will go ahead and show prevailing
IFR cigs right at alternate minimums and MVFR vsbys 07-14z with
a tempo group 08-12z with LIFR cigs and IFR vsbys. There is a
potential for lower conditions, including conditions below
airfield minimums, but it still appears the boundary layer
winds will be too strong to support this. This will be monitored
carefully and amendments issued as needed. Similar to KCHS,
there is some uncertainty how quickly cigs will rise above MVFR
levels Thursday morning, but data generally support a 14-15z
timeframe.

EXTENDED AVIATION OUTLOOK: Flight restrictions are likely due to
showers/thunderstorms/low clouds, late Thursday into Friday morning.
Breezy conditions are expected on Friday.

&&

.MARINE...
Tonight: NE to E winds expected tonight as the surface high
remains north of the waters and a modest gradient persists.
Stratus is expected to develop across the nearshore waters south
of Charleston where moist onshore flow persists tonight.
Visibility could be limited at the surface at times where the
stratus builds down to the surface. Guidance is hitting the fog
prospects harder with time, so included "areas of fog" for the
nearshore waters from roughly Hilton Head south along the
Georgia coast. Will have to monitor for a possible Marine Dense
Fog Advisory.

Thursday through Monday: A warm front will shift north through the
area Thursday night/Friday. A cold front will then move through
Friday night/early Saturday. At some point prior to the cold frontal
passage sea fog could develop across the cooler near shore waters
and lead to low visibilities. Confidence in this occurring is very
low. High pressure is expected this weekend followed by deteriorating
conditions on Monday as a storm system approaches from the
west.

Expect a high chance for Advisories Thursday night across the outer
waters, expanding northward into the SC waters into Friday night as
strengthening winds build seas to 6-7 feet, highest toward the Gulf
Stream.

Rip Currents: Moderate risk for rip currents along the SC coast
through Thursday. Onshore winds along with a 2-3 foot long period
swell may produce rip currents. The enhanced risk for rip currents
could persist into Friday.

&&

.TIDES/COASTAL FLOODING...
Tides are poised to peaking 7.0-7.3 ft MLLW in Charleston Harbor
this evening and a Coastal Flood Advisory has been posted for
coastal portions of Charleston, Beaufort and Colleton counties
through midnight.

Onshore winds combined with a long period swell impacting the coast
will cause saltwater inundation, especially in vulnerable coastal
areas. The high tides Thursday morning could reach advisory
levels, especially in SC. The Thursday evening high tide could
approach warning levels. Coastal Flood headlines are expected
with each of these tide cycles.

&&

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

&&

$$
NEAR TERM...
SHORT TERM...
LONG TERM...
AVIATION...
MARINE...
TIDES/COASTAL FLOODING...


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