Area Forecast Discussion
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FXUS62 KCHS 251432
Area Forecast Discussion
National Weather Service Charleston SC
1032 AM EDT Sun Sep 25 2016
High pressure will build into the area today but will weaken by
late Monday ahead of an approaching cold front. The front will
cross the forecast area and push off the coast by mid week and
high pressure will build into the region by the end of the week.
.NEAR TERM /THROUGH TONIGHT/...
Late this morning: The large cluster of offshore convection
continues on its diminishing trend and morning stratus is
scattering out in response to increasing surface heating. Overall,
not much noteworthy change to the forecast for today which is in
good shape. Water vapor imagery shows the broad upper low that
stretches from the Florida panhandle to the area just off the
South Carolina coast. Better moisture is wrapping around this
feature, but the 12z sounding still shows considerable mid/upper
level dry air and a precipitable water value well below 1.5
inches. High pressure will shift to over New England this
afternoon and will attempt to ridge down into the forecast area.
The prevailing low level flow will become more onshore through the
day and help to increase the low level moisture profile. Model
soundings are very unimpressive for today, featuring a prominent
warm layer around 800 mb which should serve to cap convective
development. We have still maintained the slight chance of showers
closer to the Altamaha which will be within the ribbon of best
moisture. It will be another warm day with temperatures in the
upper 80s and even a few places hitting 90 inland.
Tonight: The combination of high pressure moving off the mid Atlc
coast and development of a weak wedge pattern inland suggests
genesis of a weak coastal surface trough while a weak mid level
disturbance is expected to lift north off the florida coast.
Models show an increase in isentropic ascent and moisture late and
we should see an uptick in coastal showers which could start
translating more inland toward daybreak. We maintained 30-20 pop
scheme from east to west after midnight. Lows in the upper 60s
well inland to lower 70s closer to the coast.
.SHORT TERM /MONDAY THROUGH WEDNESDAY/...
Heights aloft will be falling through the period as a large cut-off
low slowly drops over the Great Lakes. Troughing overhead will spell
increasingly unstable conditions Monday and Tuesday (compared to the
weekend). At the surface, Monday will begin with onshore flow over
the area around the southern flank of high pressure wedging down the
eastern seaboard. This wedge of high pressure will weaken as the
parent high pushes further off the New England coast during the day
Monday, and our area will become entrenched in pre-frontal warm air
advection, with an inverted trough off the coast.
Thunder chances maintained over land Monday and Tuesday afternoons
thanks to ample moisture and at least modest instability.
Unimpressive lapse rates in the mid levels will keep the severe
threat low both days. Wednesday`s convective coverage is a bit more
uncertain as the axis of highest pre-frontal moisture shifts off the
coast during the daytime. Still, have maintained low-end chance POPs
Wednesday to account for possible convective enhancement immediately
along the approaching front.
Nighttime convection over the water is expected given the inverted
trough that will be in place. The potential for this convection to
bleed back over land justified keeping chance POPs in the forecast
for mainly coastal areas through the overnight hours Monday and
Despite ample cloud cover and rainfall, temps through the period
will be a couple degrees above normal owing to persistent warm air
.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
The cold front will move through the area Wednesday night, with
global models showing the front over the coastal waters by sunrise
Thursday morning. However, given the uncertainty in terms of the
weakening parent low near the Great Lakes, the front may drag its
feet getting through the area, moving over the waters and stalling
out well to our south and west by midday Thursday. Either way,
strong, dry continental high pressure will fill in behind the front
and quiet weather will ensure for the second half of the week. Temps
will be near to a few degrees below normal through the period.
.AVIATION /15Z SUNDAY THROUGH THURSDAY/...
KCHS: VFR today and this evening then late tonight, small chances
for low cigs and perhaps some developing showers in association
with building moisture convergence and a weak mid level
KSAV: VFR conditions and dry weather look to be in order today.
Tonight, there may be some deteriorating conditions late with
small chances for reduced vsbys and slightly better chances for
MVFR or even IFR cigs. Forecast soundings at this point giving
some mixed signals and confidence is low. There may also be
an increasing chance of showers late tonight as a surface coastal
Extended Aviation Outlook: Flight restrictions will be possible in
showers and thunderstorms Monday through Wednesday.
Today: Onshore flow will gradually increase as high pressure
builds north of the waters. Speeds should increase to near
15 kt later with seas 2 to 4 ft. A coastal trough is forecast
to develop tonight and this could result in a pinching surface
gradient over the waters by daybreak on monday. A bit too early
to tell if and where the gradient will tighten a bit, we
continued to indicate 15 kt and seas 3 to 4 ft for the most part.
Monday through Thursday: The gradient will weaken through the day
Monday as the high pressure wedge inland weakens. A coastal trough
over the waters will keep flow mainly 10 kt or less through midweek,
while keeping thunderstorm chances in the forecast, especially
during the early morning hours. A cold front will move over the
waters Thursday. Seas will generally range 2-4 feet through early in
the week thanks to a combination of some lingering swell from Karl
in the open Atlantic and local wind swell.
Rip Currents: Onshore flow in the 10 to 15 kt range and residual
swell from Karl will lead to a moderate risk of rip currents for all