Area forecast discussion
National Weather Service Charleston SC
745 am EDT Tuesday may 21 2013
Synopsis...
Atlantic high pressure will prevail through Thursday. A cold front
will drop through the area on Friday with dry high pressure building
in next weekend into early next week.
&&
Near term /until 6 PM this evening/...
near dawn...another mild morning across the region as a moist
atmosphere persists over the forecast area. Water vapor satellite
imagery indicates a weak middle low level low pressure not far from
Jacksonville Florida...embedded in a 500 mb trough axis that runs
north through Charleston to Greensboro NC. Areas of layered clouds
early this morning with pockets of stratus and fog over inland
areas. Weak upper difluence and moist coastal convergence noted
on latest mesoanalysis over eastern South Carolina and an area of
heavy rainfall was easing into NE Charleston County and weakening
after dumping plenty of rain over southern Berkeley County. Radar
estimated rainfall over 4 inches of rain southeast of Cypress Gardens.
Today...the upper trough does not move much and embedded features
tend to weaken with time. The deepest moisture continues to reside
over southeast South Carolina along and east of Interstate 95 including the
Charleston and central South Carolina coastal areas. Through midday
convective rains will begin to develop as the land mass quickly
warms with the steep sun angle this time of year. There could be a
few heavy downpours along the coast north of Beaufort if recent trends
from the rap model pan out. Convective temperatures are pretty low today
and we think scattered convective rains will develop south along
the Georgia sea breeze and propagate west and SW this afternoon. 00z
models forecasts differ on the amount of instability this afternoon
but the operational 13k RUC shows decent potential for short-lived
but strong updraft potential. Our thinking for significant weather
today is similar to yesterday/S reasoning...localized downpours
more likely and only an isolated severe weather potential as best
middle level instability and low and middle level lapse rates linger west
of the area.
A couple of changes we made to our forecast today. We raised probability of precipitation
to 40-50 percent in the Charleston quadrant-County region and raising
high temperatures over inland southeast Georgia zones further into the upper
80s.
&&
Short term /6 PM this evening through Friday/...
inland convective rains will be ending early in the evening as
the sea breeze gets well inland and convergence diminishes. No
changes to the forecast tonight with clouds gradually thinning
and lows 65-70 degrees. We may need to eventually add some patchy
fog in again tonight...dependent on trends with cloud dissipation.
A fairly similar pattern will persist Wednesday into Thursday with
broad upper troughing and Atlantic high pressure. Southerly
low-level flow will maintain a fairly moist atmosphere and help push
a seabreeze inland each day. The upper troughing will create a
somewhat more favorable environment for convection to develop...
primarily driven by the seabreeze and boundary interactions. We kept
probability of precipitation in the 20-30 percent range both days.
The models are now quite a bit faster and in better agreement
regarding a cold front pushing through Friday morning. We removed
probability of precipitation on Friday and show dewpoints dropping into the upper 50s to
around 60 during the afternoon.
&&
Long term /Friday night through Monday/...
there is good model agreement that dry Canadian high pressure will
expand southeast into the forecast area over the weekend then shift
offshore. Dewpoints look to drop into the lower 50s Saturday and
Sunday...which combined with highs in the lower 80s will result in
pleasant Spring weather. Overnight lows will probably drop into the
upper 50s in spots. Low-level moisture increases early next week as
the surface high shifts offshore...but we did not see any strong
synoptic scale features to warrant adding precipitation.
&&
Aviation /12z Tuesday through Saturday/...
kchs...the morning is looking wetter and wetter with each passing
hour this morning. Patchy MVFR/IFR ceilings will linger through middle morning
or until rains diminish. Additional rains are likely with the sea
breeze this afternoon but should eventually develop west of the
terminal by later in the day. Tonight...mainly VFR but will watch
for late night low clouds and patchy fog with wet soils and light
winds.
Ksav...a few showers may develop with the initiation of the sea
breeze late morning or early afternoon along with brief morning
MVFR ceilings possible. Best chances for showers/thunderstorms will be with
afternoon convection which could be moving SW along or just west of
Interstate 95. Mainly VFR tonight but there are chances of late
night MVFR/IFR ceilings and possibly light fog...especially if skies
clear.
Extended aviation...sporadic reductions in ceilings/visibilities mainly in
afternoon/evening showers or storms through Thursday.
&&
Marine...
through tonight...the steady state synoptic pattern continues unabated.
Offshore ridge of high pressure extends west across the coastal waters...
generating southeast and S winds less than 15 knots and seas in the 2-4 foot
range...highest over outer Georgia waters beyond 20 nm.
High pressure will sit over the western Atlantic until Friday when a
Canadian high drops southeast. We may see a brief period of Small
Craft Advisory conditions Friday night into Saturday as the gradient
tightens and cold advection mixes some decent winds to the surface.
&&
Chs watches/warnings/advisories...
Georgia...none.
SC...none.
Marine...none.
&&
$$
Synopsis...
near term...
short term...jrl
long term...jrl
aviation...
marine...jrl
