FXUS62 KCHS 241516

Area Forecast Discussion
National Weather Service Charleston SC
1116 AM EDT Mon Sep 24 2018

A stationary front will linger over the area through Tuesday.
Weak low pressure is expected to advance from the Atlantic
tonight into Tuesday, then weaken as it moves northward. A cold
front will approach from the west Wednesday, then possibly
stall over the area Thursday into Friday. A second cold front
is expected over the weekend.


Today: A cold front will slowly stall across the area late this
morning into the afternoon with a light northeast wind and some
stratus clouds in its wake. Aloft, high pressure will extend from
the western Atlantic and over the Southeast United States, favoring
a somewhat later start to most convection across/near the area.
Despite clouds and the northeast flow, PWATs approaching 2.0 inches,
SBCAPE around 2500 J/kg and sfc temps already in the lower 80s
should support a few showers and/or isolated thunderstorms by early-
mid afternoon, initially across Southeast Georgia where instability
is greatest, followed by locations near the Southeast South Carolina
coast by mid afternoon when instability increases, then inland
along the stalled front/boundary mid-late afternoon. Expect
greatest precip coverage during peak diurnal heating when temps
reach the mid/upper 80s. A few locations well inland across
Southeast Georgia could approach 90 degrees. Convective activity
should then slowly wane during evening hours while sfc temps
begin to cool.

Tonight: Lingering inland convection will quickly dissipate as
sunset approaches. The far outer bands of convection rotating
around the large area of low pressure located between the
Southeast U.S. coast and Bermuda are forecast to move into the
South Carolina coastal waters early Tuesday as the system moves
north/northwest. Some of this activity should brush the
Charleston County coast closer to daybreak. 20% pops will be
highlighted for this area with rain-free conditions elsewhere.
Low stratus and areas of fog are expected to develop across
central Georgia into the South Carolina Midlands after midnight.
Some of this could ooze into the far interior counties after
4 AM, but latest SREF data suggest the bulk of this activity
will remain west of the local forecast area. Lows will range
from the upper 60s/near 70 well inland to the upper 70s at the


Tuesday: The mid-levels will consist of high pressure off the
Southeast coast, with its western periphery stretching into our
area. The high is forecasted to weaken a bit overnight, allowing
our heights to drop slightly. At the surface, a stationary
front will be spread across our area during the day, then
dissipate overnight. Additionally, weak low pressure over the
Atlantic will approach our area in the morning, then make a
northward turn in the afternoon. As it does this it's expected
to weaken and then get absorbed in the overall flow during the
nighttime hours. PWATs are lower than the past few days.
However, models indicate a dry slot should move from east to
west across our are during the day. This could impact the POP
forecast. Models have the best rainfall potential over the
Charleston Tri-County area during the day and this is where we
have the highest POPs. Instability is highest near the coast,
then quickly lowering inland. GFS BLCAPES may reach 2,000 J/kg
over the Charleston Tri-County area while Showalter values are
around minus 2. This is the area where thunderstorms have the
best chance of developing. Given the weak steering winds higher
up, it's possible there could be locally heavy rainfall if a
storm stalls or is very slow to move. Regardless, the
precipitation will quickly dissipate in the evening. However,
more showers are expected to develop over the coastal waters
overnight and move towards the coast. They could make it to the
Charleston County coast just before daybreak, so this is the
only spot we have precipitation over land at that time.
Temperatures will be several degrees above normal due to heights
and warm 850 mb temperatures.

Wednesday: The mid-levels will consist of high pressure off the
Southeast coast and long wave troughing over the Central U.S.
The axis of the trough will slowly move eastward, which will push
the high further offshore and cause our heights to lower. At the
surface, a cold front will approach from the northwest, almost
making it into our area late at night. PWATs rise ahead of the
front. Models hint that they could exceed 2" overnight. This is
above normal for this time of year. Instability also increases
across the area, with values possible the highest we've seen in a
few days or longer. We'll certainly have to monitor this for
stronger storms. POPs increase from west to east as the day
progresses. Unlike previous days, the overnight hours will see more
precipitation coverage as the front approaches. Heights and 850 mb
temperatures will again lead to another day of well above normal

Thursday: The mid-levels will consist of high pressure off the
Southeast coast in the morning and long wave troughing over the
Central U.S. The trough will slowly deamplify as time progresses.
Though, the high is expected to maintain it's intensity. At the
surface, a front may be stalled over or just west of our area.
Plenty of moisture in the form of above normal PWATs will be in
place. There will be some instability too. Hence, showers and
thunderstorms are in the forecast, especially far inland. We have
POPs capped at chance for now, but they may need to be raised based
on the location of the front. Since we remain on the warm side of
the front, highs still appear to be several degrees above normal.


Thursday night into Friday a front is expected to be stalled over or
near our area, bringing above normal POPs. A cold front is then
forecasted to approach from the northwest this weekend, continuing
the rainfall potential. Models differ on the exact location of this
second front and that will dictate the forecast. For now it appears
to equate to above normal POPs. However, more adjustments to the
forecast are expected.


KCHS: Two concerns for the 12z TAF period. Showers activity is
expected to increase around the terminal, some of which may
briefly impact the terminal through 15z. Will include VCSH and
possibly a TEMPO group for marginal MVFR conditions pending
radar trends a issuance time. Scattered showers/tstms are
expected to develop near the terminal by early-mid afternoon as
instability peaks with a southward moving cold front. There are
some uncertainties on timing/placement this far out, but there
is enough agreement in the various high resolution data to
support a TEMPO MVFR group for TSRA from 18-21z. Adjustments
will be needed later today as convective trends become more

KSAV: VFR through the period. Isolated showers/tstms are
expected to develop later this afternoon ahead of a southward
moving cold front. Latest high resolution data suggest the bulk
of this activity will remain north and northwest of the
terminal. Will not include a mention of TSRA or SHRA at this
time for this reason.

Extended Aviation Outlook: Multiple atmospheric features could
bring brief periods of convection, especially during the
daylight hours. Brief flight restrictions are possible,
especially Thursday and Friday.


Today and Tonight: A broad easterly flow regime will hold in
place through tonight as a front stalls over/near the coastal
waters while Atlantic high pressure prevails aloft. Expect
winds/seas to remain well below Small Craft Advisory levels
through tonight. In general, east winds will average between
10-15 kt with seas no higher than 2-4 ft.

Tuesday through Friday: A stationary front will hover over the
area Tuesday, then dissipate Tuesday night. At the same time,
weak low pressure is expected to advance from the Atlantic, then
weaken as it moves northward. A cold front is forecasted to
approach from the west Wednesday, then possibly stall over the
area Thursday into Friday. Despite all of these features, no
Small Craft Advisories are anticipated.

Marine guidance indicates weak long period swell originating from TD
Kirk could arrive towards the end of the week. However, given the
latest status of Kirk from the NHC. It appears the swell forecast is

Rip Currents: WW3 and NWPS output show easterly swell across the
coastal waters today. Guidance is similar in dampening the 2 ft
swell at the buoys to 1 ft or less as the swell waves approach
the beaches. Despite influences from this evening's full moon
and moderate easterly flow, there is just not enough swell wave
to generate frequent rip currents. With the lack of reports
yesterday and even slightly lower swell numbers noted in the
buoy spectral plots, will go with a low risk for rip currents
for all beaches today. Conditions in the surf zone will
certainly be chaotic at times, especially heading from high to
low tide.

Distance features in the tropics could send some weak long
period swell into our area this week. However, given the latest
forecasts from the NHC on these systems, it's fair to say the
wave models may be overdoing the swell. The risk for our area
may be Marginal at worst.




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