Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
636 PM EDT sun Aug 2 2015
a weak low pressure will lift northeastward along a surface trough
through Monday night as a stationary front remains well inland.
High pressure will build in Tuesday and Wednesday before an inland
trough of low pressure develops Thursday. A cold front will drop
into the area next weekend.
Near term /until 6 am Monday morning/...
for the early evening update...a nose of strong low-level
convergence is helping to maintain convection over the far
southern zones early this evening. Have adjusted near term probability of precipitation to
increase values to the 70 percent range from Ludowici to Savannah
metropolitan to account for this. Already seeing impressive surface
moisture flux convergence in excess of 45 g/kg/12hr over the
coastal waters. This will continue to intensify this evening and
02/22z surface...radar and satellite analysis placed a stationary
front from eastern Alabama through central Georgia...the central
midlands into coastal plain of North Carolina. Weak low pressure
/1009 mb/ was located right along the extreme eastern coast of
Apalachee Bay south of Adams Beach flordia. The low was attached
to southwest-northeast oriented surface trough which extended
from the eastern Gulf of Mexico into far southeast Georgia. The
low is forecast to meander north/northeast along the trough
tonight while remaining well detached from the inland stationary
The broad cyclonic flow induced by the surface low will support
increasing nocturnal speed convergence along the Georgia and lower
South Carolina coast overnight as nocturnal jetting takes hold
along the western wall of the stream. This coupled with high
atmospheric moisture characterized by a ribbon of precipitable waters 2.10-2.30
inches will support a gradual uptick in showers/thunderstorms over the
coastal waters after midnight. Low-level trajectories favor this
activity moving onshore along the southern South Carolina coast
and possibly parts of the Georgia coast early Monday. Will show
probability of precipitation ramping up east of the I-95 corridor midnight-sunrise with
probability of precipitation ranging from 20 percent to 40-50 percent at the coast...
highest in the Hilton Head-Summerville-Moncks Corner corridor.
Lows will range from the Lower-Middle 70s inland with upper 70s/near
80 at the beaches/downtown Charleston.
There will be a continued risk for localized flooding of low lying
and poor drainage areas.
Short term /6 am Monday morning through Wednesday/...
Monday and Monday night...significant changes were made to the
forecast as models have trended much wetter through Monday night.
The main cause of this trend is the weak area of low pressure
currently near The Big Bend of Florida that is forecast to lift
northeastward along a stalled front. As this occurs...a deep south
to southeast moisture feed will become enhanced ahead of the surface
low and into coastal southeast South Carolina and southeast Georgia.
The percent of normal precipitable water satellite product currently
shows values in the 120-130 percent range near the low...and this
airmass will become solidly entrenched across the forecast area
beginning Monday morning. Such percent of normal values point to
precipitable waters on the order of 2.25 inches for much of the day on Monday and
into Monday night. The models...especially the 02/12z European model (ecmwf)...put
the quantitative precipitation forecast bullseye right on the forecast area and mainly along the
South Carolina coast. This seems quite reasonable given the degree
of moisture...presence of a low level feature...and even some
upper level divergence in the entrance of a 250-300 mb jet. Probability of precipitation
have been increased into the 60 percent range for mainly the tri-
County region...and this may need to be expanded depending on how
precipitation evolves overnight into Monday morning.
Overnight...the low will begin to lift northeastward and the axis
of deeper moisture and best coverage will shift up toward the
NC/SC border. Have maintained at least chance probability of precipitation across the east
until a more definitive trend can be identified. Temperatures for
Monday will be tricky as areas along and east of I-95 will likely
see little to no direct sunlight. This could keep highs down
significantly...with still low 90s possible across interior
Tuesday through Wednesday...much drier deep layer air will work into
the area and probability of precipitation are no more than 20 percent as high pressure
builds in behind the departing surface wave. Warm temperatures will
return...with highs reaching the middle to upper 90s away from the
Long term /Wednesday night through Sunday/...
a relatively wet pattern is anticipated Wednesday night through
Sunday as a longwave trough takes shape in the upper levels. A
pronounced Lee trough will develop late week...then a cold front
will drop into the area over the weekend. Precipitable waters surge back above 2"
late week with values approaching 2.3" Saturday and Saturday night.
We show increasing probability of precipitation during much of the period...especially
during daytime hours. Temperatures will be moderated by lower
heights and greater cloud cover...thus highs will mainly reach no
higher than the lower 90s.
Aviation /22z Sunday through Friday/...
kchs...VFR this evening with increasing chances for MVFR
ceilings/visibilities after 08z as scattered showers and possibly thunderstorms
redevelop over the area. Timing for impacts is a bit in
question...but the trend for scattered coverage is there. Will
introduce vcsh by 08z with tempo group for MVFR visibilities/ceilings in
moderate rain 09-12z to trend. The window for impacts may need to
be moved or expanded based on later guidance and radar trends.
Scattered showers will likely remain around the terminal for much
of the morning hours with activity diminishing some by afternoon.
Additional mention of showers/thunderstorms may be needed after 15z with
the 06z taf cycle.
Ksav...light shower activity could impact the terminal 00-02z as a
band of mainly light rain skirts by to the south. Current
indications are that much of the night will remain VFR. Shower
activity is expected to develop and remain east of the
terminal...although it could come close at times after 09z.
Rap/NAM soundings are similar in showing MVFR ceilings 09-14z so will
go ahead an include this. Scattered showers/thunderstorms will likely be
around the terminal for much of the afternoon Monday. The
introduction of showers/thunderstorms may be considered with the 06z taf
cycle as timing for possible impacts become a bit more certain.
Extended aviation outlook...elevated chances of direct impacts
from showers and thunderstorms Monday afternoon/evening.
tonight...a weak area of low pressure near the Florida Big Bend
area will move northeast into southeast Georgia. This will place
the tightest pressure gradient/low-level jet mainly beyond 20 nm
and this is where we have raised a Small Craft Advisory. Across
the near shore waters southerly winds will surge but remain
mainly 20 knots or less with seas building up to 4 feet.
Monday through Friday...a weak area of low pressure will move
northeastward along the coast through Tuesday. High pressure will
then return with a trough of low pressure inland...with another
front reaching the area late in the week. This setup will result in
a prevailing southwest flow across the waters through the
week...with various periods of enhanced flow as the pressure
gradient tightens. Winds will generally top out in the 15-20 knots
range with some potential for gusts to around 25 knots at
times...mainly over the outer Georgia waters. Seas will be elevated
through much of the week...especially Monday through Tuesday night.
A Small Craft Advisory will be in effect for the outer Georgia
waters through midday Tuesday to account for 6 feet seas.
Otherwise...seas will be 2-4 feet out to 20 nm...and 5 feet beyond.
current data suggest tides will remain below critical thresholds
for Charleston Harbor with this evening/S high tide cycle.
Marine...Small Craft Advisory until noon EDT Tuesday for amz374.