Area Forecast Discussion
Issued by NWS Charleston, SC

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000
FXUS62 KCHS 191835
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
235 PM EDT Thu Apr 19 2018

.SYNOPSIS...
Cool and dry high pressure will build in behind a dry cold front
tonight through Saturday night. Unsettled weather is expected
late Sunday through at least Tuesday as low pressure moves up the
coast.

&&

.NEAR TERM /THROUGH TONIGHT/...
19/18z surface analysis showed the cold front beginning to move
into the far western portions of the forecast area. The front is
on target to push off the coast later this afternoon and clear
the coastal waters by mid-evening. A cooler, drier airmass will
quickly work in from the west as post-frontal cold air
advection intensifies. The front will pass through dry and rain-
free conditions will prevail. Gusty west winds will diminish
some this evening, then become gusty again overnight as a
secondary surge works north-south across the area. Lows will
range from the mid 40 inland to the mid 50s across the far south
and Georgia coast where northeast winds from off the Atlantic
will help moderate temperatures there somewhat.

Lake Winds: The shallow marine layer over the open waters of
Lake Moultrie have kept winds in check today. Mixing profiles
will become slightly more favorable for gusty winds to develop
on the lake later this evening and overnight as a cold front
pushes through and temperatures aloft begin to fall. Guidance
suggests there could be gusts to 25 kt on the open lake waters
overnight, but it is not exactly clear how frequent these gusts
will be. The situation look too marginal for a Lake Wind
Advisory at this time.

&&

.SHORT TERM /FRIDAY THROUGH SUNDAY/...
Cool and dry high pressure will build in from the northwest
Friday, then shift offshore during the day Saturday. Clear skies
will prevail Friday and Friday night, then some cumulus will
develop across southeast GA on Saturday as the flow turns
onshore. High temps will be in the upper 60s to low 70s Friday
and low to mid 70s Saturday. Saturday morning lows in the 40s
will be rather chilly for late April.

A deep cutoff low will move toward the Arklatex region on
Sunday while weak ridging persists over the local area.
Isentropic ascent ahead of the upper low will at a minimum
spread increasing clouds over the area, along with the potential
for scattered showers, mainly in southeast GA. We maintained
similar PoP schemes on Sunday with far northern areas dry
through the day with increasing PoPs as one moves to the south.
In-situ dry air to the north should limit the ability for
precipitation to spread north during the daytime. Highs will be
in the upper 60s to low 70s.

&&

.LONG TERM /SUNDAY NIGHT THROUGH WEDNESDAY/...
A blocking pattern develops aloft, with a deep cut-off spiraling
over the Tennessee valley early in the week, while an amplified
ridge is situated near or off the eastern seaboard. Strong surface
high pressure to the north, and the approach of the surface low from
along the northern Gulf coast will provide an excellent set up of
overrunning rains. The low is presently expected to track near or
just south of the region Monday, then over or just east of the
region Tuesday. Large scale lift and upper difluence, the slow
progression of the low both surface and aloft, plus PWat as high as
near the 90th percentile will bring us some much needed rainfall.
Depending upon the exact track of the low there is some potential
for t-storms, especially south and east. By mid week the cut-off low
opens up and is kicked out to the east as a strong short wave drops
through the upper Midwest. This will eventually allow for drying to
occur late in the forecast cycle. Breezy to windy weather will
prevail early in the week with a tight gradient between the low and
high pressure to the north.

&&

.AVIATION /18Z THURSDAY THROUGH TUESDAY/...
VFR. Gusty winds will continue at both KCHS and KSAV through the
overnight period in the wake of a cold front.

Extended Aviation Outlook: VFR weather will persist into early
Sunday, before flight restrictions become likely later Sunday
through Tuesday as low pressure impacts the area with rain and
breezy conditions.

&&

.MARINE...
Tonight: Winds will continue to increase overnight as high
pressure builds into the region behind a cold front. The
combination of post-frontal cold air advection and a tightening
pressure gradient will support north to northeast winds 20-25 kt
with seas building 4-6 ft nearshore waters and 6-7 ft offshore
waters. For the Charleston Harbor, winds will be north 15-20 kt
with gusts to 25 kt, but could be locally higher near the Harbor
entrance. Small Craft Advisories are in effect for all waters
tonight.

Friday through Tuesday: Ongoing Small Craft Advisories will
gradually drop off for the nearshore waters on Friday as cold
advection wanes and the northeast gradient relaxes. An SCA will
continue for the offshore GA waters until Saturday evening,
mainly due to some lingering 6 ft seas over outer portions.

The surface high moves off New England and gives way to low
pressure that impacts the area early next week with elevated
chances for rain and some stronger t-storms. The worst conditions
look to be early next week, with poor to potentially dangerous
marine weather to occur.

Rip Currents: A prolonged stretch of moderate to perhaps strong
onshore winds and larger surf, as low pressure passes nearby early
next week, will lead to an enhanced risk of rip currents at the
beaches.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory until 11 AM EDT Friday for AMZ330-350.
     Small Craft Advisory until 2 PM EDT Friday for AMZ352.
     Small Craft Advisory until midnight EDT Saturday night for
     AMZ374.
     Small Craft Advisory until 5 PM EDT Friday for AMZ354.

&&

$$


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