Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
124 am EDT Sat Aug 1 2015
a stationary front will dissipate over the area through Sunday as
high pressure builds in early next week. An inland low pressure
trough will then develop by late next week.
Near term /until 6 am this morning/...
an upper level trough will dig farther south across the region
overnight...while ridging over the western Atlantic and The Four
Corners regions loses some of its influence. At the surface...a
late evening analysis depicts a nearly stationary front extending
from the Pee Dee...through the eastern midlands and into east-
central Georgia. This front will shift southeast toward...and
possibly even into...the area later tonight as a cold front
beneath the deepening upper trough.
The potential is there for additional redevelopment of convection
at just about any time through the remainder of the night with
deep moisture in place and the development of broad low level
convergence with the approaching front. However...it does appear
that chances might be greatest over the adjacent Atlantic waters
and right along central coastal South Carolina where upper level
divergence is most pronounced. As a result...the forecast will
indicate a slight chance of showers and thunderstorms through
tonight...with a chance near the immediate coast.
Current temperatures have been influenced by the earlier
rainfall...and should overall remain nearly steady or only slowly
fall through the overnight hours.
Short term /6 am this morning through Monday/...
Saturday...the primary concern is heavy rain and possible flooding.
The synoptic pattern will favor numerous showers/thunderstorms with the area
positioned along the base of a broad East Coast longwave trough with
a well-defined surface front becoming stationary across southeast
South Carolina and southeast Georgia. Precipitable waters are forecast to remain
2.30-2.40 inches or very near the daily maximum of 2.42 inches for 1
August at kchs. These values coupled with sufficient instability...
favorable upper difluence for convective venting and a weak mean
wind in the 925-700 hpa layer will support torrential rains with a
risk for isolated flash flooding. The European model (ecmwf) and GFS still differ
slightly on exactly where the axis of heaviest convective rains will
setup...but the differences are narrowing with each run. Odds favor
in the corridor bounded by a Metter-Hampton-downtown Charleston-
Hilton Head-Savannah-Hinesville line for the area at most risk for
excessive rains. The latest 3-hour flash flood guidance suggests most
of this area could likely handle 2-3 inches of rain...but 1-hour flash
flood guidance could be exceeded where hourly rainfall rates exceed
3 in/hour over a sustained period of time. The risk for flash flooding
does not look widespread enough to justify a Flash Flood Watch at
this time. Probability of precipitation will be increased to 60-70 percent area wide and a
rain may be heavy at times qualifier will be included in all gridded
and text forecasts. As strong thunderstorm or two will also be possible with
a risk for damaging winds. High will generally top out in the upper
80s-lower 90s. Shower/thunderstorm activity will gradually diminish during
the evening hours as the strongest low-level convergence shifts
offshore along the western wall of the Gulf Stream and instability
wanes. Lows will drop into the Lower-Middle 70s inland with upper 70s
along the beaches/downtown Charleston.
Sunday...the stationary front will gradually washout on Sunday as
high pressure to the north tries nudge in from the north. The axis
of deepest moisture is prognosticated to sink south into far southeast
Georgia. The better chances for convection will be confined to areas
south of the I-16 corridor. Probability of precipitation will range from 20 percent northern
areas to 50 percent south of I-16. The risk for heavy rain and
isolated flash flooding will continue across the south. Highs will
range from the Lower-Middle 90s with upper 80s across the far south
where convection will be more concentrated. Convection will
gradually wind down Sunday evening. Lows will range from the Lower-
Middle 70s inland with upper 70s coast/downtown Charleston.
Monday...an inland trough will setup across the area Monday
afternoon. The axis of deepest moisture will remain confined to
areas just offshore...but edging back across far southern areas near
the Altamaha river. This will support isolated-scattered
showers/thunderstorms with the greatest coverage near the Altamaha river.
Probability of precipitation 20-40 percent look fine. Highs will top out in the Lower-Middle
Long term /Monday night through Friday/...
the upper ridge briefly rebuilds on Tuesday before a weak upper
trough redevelops over the eastern United States middle to late week.
Low-level moisture will return in earnest late week into the weekend
with precipitable waters back above 2 inches. Upper shortwave energy and an
approaching cold front will bring a return to higher rain chances
during the period.
Aviation /06z Saturday through Wednesday/...
overnight...showers are possible around kchs with mainly VFR
conditions at ksav and kchs. Both terminals could see some MVFR
ceilings breaking out from near daybreak to middle morning. Showers and
thunderstorms are anticipated across the region from late morning through
this afternoon...most numerous along and S of I-16. Difficult
to pin down timing and coverage at this point but have trended
with tempo showers at ksav 15z-19z.
Extended aviation outlook...mostly VFR. Mainly afternoon/evening
showers and thunderstorms will impact the terminals. Low chances for
MVFR ceilings or shallow fog Sunday and Monday mornings around
overnight...a weak cold front inland will head east toward the
coast leading to a slight increase in southwest winds. Best
pressure gradient likely over the Georgia waters where winds should
reach upwards of 15-20 knots...especially beyond 20 nm. Seas will
build slightly to about 3 feet within 20 nm and 4 feet beyond 20
nm...possibly near 5 feet around 60 nm out.
Saturday through Wednesday...mainly south to southwest winds will
persist over the waters Saturday through the middle of next week.
The strongest winds will occur at night during nocturnal
surging...though speeds should generally 15-20 knots or less. A sea
breeze will develop along the coast each afternoon with winds
topping out 15 knots for several hours. Seas will average 2-4 feet
nearshore waters and as high as 4-5 feet at times over the Georgia
elevated high tide levels will continue through the upcoming
weekend thanks to the upcoming perigean Spring tide. Shallow
coastal flooding is likely into early next week especially during
each evening high tide. Will continue to highlight the shallow
coastal flood threat within the hazardous weather outlook. Also of
note...there is a chance that heavy rain could coincide with the
elevated high tides the next couple of evenings. If this occurs...
more serious localized flooding would be possible...particularly
in downtown Charleston.