Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
923 PM EDT Monday Aug 3 2015
a weak area of low pressure will move northeast up the coast
overnight before moving away from the forecast area on Tuesday. High
pressure will briefly build in from the northwest into Wednesday.
An inland trough of low pressure will develop by Thursday and a
cold front will approach the area Friday into Saturday.
Near term /until 6 am Tuesday morning/...
04/01z satellite...radar and surface analysis placed 1010 hpa low
pressure over Sapelo Island...Georgia. The low remains embedded
within a broad surface trough which curves northwest through
interior southeast Georgia into the southern South Carolina
midlands and southwest into north-central flordia. The low is
forecast to meander northeast along the Georgia and lower South
Carolina coast tonight...becoming situated very near or just
northeast of the Charleston area by sunrise Tuesday.
Inland convection has dissipated with the loss of insolation while
marine-based showers/thunderstorms continue to expand over the lower South
Carolina coastal waters within an axis of surface moisture flux
convergence >55 g/kg/12hr. Expect to see a continued slowly uptick
in convection along the Lower-Middle South Carolina coast through
midnight-1 am with the highest concentration of rain likely
becoming centered between Edisto Beach and McClellanville
area...including downtown Charleston...where the strongest low-
level convergence is forecast to occur. Will have to watch
downtown Charleston very carefully as heavy rains could fall
during the high astronomical tide late this evening. Given tide
levels are projected to be in excess of 6.7 feet MLLW...any heavy
rain that falls within an hour either side of high tide could
cause some problems. Probability of precipitation will be increased to 60 percent across
Charleston...southern Dorchester and southern Berkeley counties
with layered with 20-50 percent probability of precipitation farther inland and to the
southwest. Expect dry conditions to prevail for the remainder of
the night across much of the interior outside of the Charleston
tri-County area. Lows will range from the Lower-Middle 70s inland
with upper 70s/near 80 at the beaches and downtown Charleston.
There are signals in the various high resolution guidance that
fog/stratus may become an issue across far interior southeast
Georgia late tonight as winds diminish and skies clear. 1000mb
condensation pressure deficit forecasts support favorable fog
formation as skies clear from the west. Some of which could
become locally dense with visibilities 1/4 mile or less. Will include
areas of fog across much of interior southeast Georgia.
Short term /6 am Tuesday morning through Thursday/...
Tuesday and Tuesday night...shortwave energy aloft will pull away
from the area to the northeast as the region remains on the southern
periphery of the longwave trough that encompasses much of eastern
Canada and the northeastern Continental U.S.. at the surface...the slowly
meandering weak area of low pressure will move along the forecast
area coastline and eventually become an open wave and lift out to
the northeast. In the morning...it appears the eastern reaches of
the forecast area will still be dealing with the deep Atlantic
moisture feed and corresponding shower activity. Have increased probability of precipitation
across eastern portions of Charleston and Berkeley counties to 40-
50. The rest of the day...drier air will work its way into the area
and rain chances will be quite minimal. Overnight...the forecast is
dry as high pressure briefly builds in. Highs will be in the low 90s
east...ranging to the middle/upper 90s for interior southeast Georgia.
Lows will fall into the middle/upper 70s.
Wednesday through Thursday...Wednesday looks like the most benign
day with limited convective potential with just a weak Lee trough.
Drier low levels will help result in very little surface based
instability. Just a slight chance pop is in place for diurnal
convection. By Thursday...moisture is a bit better and the inland
trough becomes better defined as a well defined shortwave aloft
approaches from the west. Diurnal convection chances will be
better...with 30 probability of precipitation in place for land areas. The warmest day will
be Wednesday with highs in the middle/upper 90s...and low/middle 90s on
Long term /Thursday night through Monday/...
a persistent upper trough Thursday night through Saturday will
maintain an inland surface trough and abundant low level moisture
across the forecast area. Greater coverage of showers and thunderstorms
expected through Saturday. The upper ridge may attempt to rebuild
Sunday into Monday which could decrease convective coverage somewhat.
Aviation /00z Tuesday through Saturday/...
kchs...showers could impact the terminal at just about any time
through 08z. Timing when meaningful impacts will occur is proving
difficult...but high resolution models are pinning in on the
03-06z time frame. Will show a tempo group for MVFR visibilities/ceilings
during this time. VFR otherwise.
Ksav...VFR. Could see brief showers impact the terminal
00-02z...but durations will be very short. There is evidence
fog/stratus will develop across far interior southeast Georgia
after midnight as clouds clear. Some of this could bleed into the
terminal after 09z...but confidence is quite low at this time. Will hold
conditions at VFR for now...but some lower conditions may need to
be introduced later tonight depending on observational and later
Extended aviation outlook...main concerns are occasional reductions
in visibility and/or ceiling due to showers and thunderstorms. Although
impacts cannot be ruled out any day...greater coverage is expected
Thursday and Friday.
through tonight...a Small Craft Advisory will remain in effect
through tonight for the offshore Georgia waters as low pressure moves
up the southeast coast. Seas in the 4-6 feet range and gusts over
25 kts...especially seaward of the greys reef buoy where the
nocturnal jet will be the highest...can be expected in this area.
Showers and thunderstorms with heavy downpours are possible
throughout the evening and into the late night hours...especially
Tuesday through Saturday...southwest winds will prevail across the
local waters into the weekend. There will be periods of enhanced
flow...primarily with nocturnal surges and the afternoon sea breeze
along the land/sea interface. The Small Craft Advisory for the outer
Georgia waters will come down Tuesday morning as seas subside to
less than 6 feet. Otherwise...winds will generally top out in the 15-
20 knots range during the periods of surging. Seas will be highest
through Tuesday night...up to 5 feet around 20 nm and beyond. From
Wednesday Onward seas will be in the 2-3 feet range for the nearshore
waters and the 3-4 feet range for the outer waters.
Marine...Small Craft Advisory until 8 am EDT Tuesday for amz374.