Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 250744
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
344 AM EDT Wed Apr 25 2018

.SYNOPSIS...
Low pressure will develop across the lower Gulf Coast tonight
then move north of the region by early Friday, followed by a
cold front Friday night. High pressure will then build into the
region through Sunday and will prevail through next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Pre-Dawn: Temps have been slow to fall across the forecast area
overnight as southwest winds have been able to keep the lowest
levels stirred up to this point. Skies have cleared and winds
may tend to weakly decouple inland toward daybreak, thus some
upper 50s are still expected west of the coastal corridor.

Today: Low pressure along the Mid Atlantic coast this morning
is forecast to lift toward southeast New England by early this
evening. Upper level troughing will lose influence over the
Carolinas, while deep layered flow over the region becomes more
west-northwest. Mid level subsidence and low level offshore
flow will result in dry weather today with plenty of morning
sunshine warming temps into the mid 70s by midday most areas.

This afternoon, scattered to broken cumulus and stratocu are
expected to develop across South Carolina and then flatten out
below a strong subsidence inversion aloft later in the day.
Some of these clouds could result in slight cooler late day
temps across northern zones but the sinking west flow is
expected to allow much of the coastal corridor to reach 80
degrees this afternoon with at least upper 70s well inland. West
winds will average 10-15 mph this afternoon but it should be
locally breezy along the Charleston County resultant sea breeze
during the afternoon, especially if skies remain sunnier rather
than cloudier between 2 PM and 6 PM.

Tonight: Quiet weather is on tap with most areas under mainly
clear skies. A weak backdoor front may sneak into the SC zones
overnight but have little impact to forecast elements. Lows will
be in the 56-60 degree range most areas.

&&

.SHORT TERM /THURSDAY THROUGH SATURDAY/...
Thursday and Thursday night: A vigorous upper trough and associated
surface low pressure will approach the region from the west. Forcing
for ascent should support an area of showers/thunderstorms west of
the region Thursday, but guidance suggests that any precipitation
will not arrive in our area before late afternoon, followed by
highest POPs during the first half of Thursday night.

While skies will become mostly cloudy Thursday, daytime POPs are
limited to slight chance/chance inland during the late afternoon.
Near normal high temps in the upper 70s/lower 80s are expected away
from the beaches.

The upper trough will open, become negatively tilted and will begin
to lift northeast Thursday night. However, guidance differs
regarding the details/timing of the transition of the upper trough
from a closed low to a weaker open wave, and this in turn will
determine whether decent coverage of showers arrives to produce
measurable rainfall at many locations or if upstream convection
tends to fall apart as it approaches the Southeast coast. The latest
forecast indicates Thursday evening POPs ranging from 40-50 percent
north/west to 15-25 percent far south, but adjustments will no doubt
be required within subsequent forecasts. Also, while instability
remains unimpressive, dynamic forcing for ascent within the
warm/moist conveyor belt should support eastward translation of
upstream convection, justifying a slight chance of thunderstorms
mainly north/west Thursday evening.

Friday: Cannot rule out a few showers within a persistent surface-
850 mb trough, especially along the sea breeze during the afternoon.
However, residual moisture appears meager, so rather than blanket
the entire area with slight chance POPs opted to hold POPs below 15
percent with no explicit mention of precipitation. Even if a couple
of showers manage to develop, most locations will remain rain-free.
Otherwise, expect high temps in the mid/upper 70s away from the
beaches.

Friday night and Saturday: Another upper trough will quickly advance
east and will push a cold front through the region. Latest guidance
generally agrees that moisture return ahead of these features should
remain off the coast, so slight chance POPs are limited to coastal
waters Friday night into Saturday. Otherwise, the cold front will
bring drier air but little change in temps as highs average close to
80F Saturday afternoon.

&&

.LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/...
No rain is expected through at least the middle of next week.
Surface high pressure will build from the north this weekend. Then,
the surface high will shift south over the region as an upper ridge
builds over the region by early next week, and this regime will then
prevail through the week. As the surface high migrates south,
persistent onshore flow will commence later this weekend and will
continue next week. Thus, temperatures will remain close to normal
for late April/early May, featuring highs in the upper 70s/lower 80s
inland and upper 60s/lower 70s on the beaches and lows in the 50s
inland/60s at the coast.

&&

.AVIATION /07Z WEDNESDAY THROUGH SUNDAY/...
VFR conditions will prevail at KCHS and KSAV through 06z Thursday.

Extended Aviation Outlook: Brief flight restrictions possible in
showers Thursday night. Otherwise, VFR through this period.

&&

.MARINE...
A modest surface pressure gradient keeping winds closer to 15 kt
over the coastal waters early this morning. The flow is gradually
veering to a more west component and seas were slowly subsiding
pre-dawn. We lowered the SCA for Charleston County waters given
latest trends. Winds later today will be mainly SW with speeds
near 15 kt. Weaker SW to W flow tonight as the gradient slackens.
Seas will range from 4-5 ft near shore Charleston waters to 3-4
ft GA waters today. We maintained the SCA for outer GA waters
until mid afternoon but it could be lowered a bit sooner.

Thursday through Monday: Winds/seas will remain below SCA levels
through this weekend. Low pressure passing inland Thursday night
into Friday and a cold front pushing through the waters Friday
night/early Saturday will contribute to changing wind directions.
Seas of 2 to 4 ft Thursday-Friday will subside to 1 to 3 ft for a
time this weekend.

Seas will build in response to increasing east/northeast winds as
high pressure builds from the north then shifts south later this
weekend into early next week, and 6 ft seas could push into GA
waters beyond 20 nm by early next week.


&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory until 2 PM EDT this afternoon for AMZ374.

&&

$$
NEAR TERM...
SHORT TERM...SPR
LONG TERM...SPR
AVIATION...SPR
MARINE...SPR



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