Area forecast discussion
National Weather Service Charleston SC
354 PM EDT Tuesday may 21 2013
Synopsis...
Atlantic high pressure will prevail through Thursday. A
cold front will move through the area on Friday with dry high
pressure building in next weekend into early next week.
&&
Near term /until 6 am Wednesday morning/...
isolated to scattered showers and thunderstorms have developed
generally along and west of Interstate 95 by late
afternoon...while the persistent convection over the Charleston
tri County area has steadily dissipated into light showers.
Earlier heavy rain and thunderstorms worked over the environment
north of Beaufort County fairly well...and latest thinking is
that strongest convection will remain west and south of the tri
County. North/northwest low to middle level flow will continue to
steer storms farther south as the afternoon progresses.
Mesoanalysis reveals that best instability still persists across
the southwest portion of the forecast area...and will thus need to
monitor ongoing convection across inland areas for brief
intensification...especially where mesoscale boundary interactions
occur. The overall severe weather potential is low considering
that best lapse rates are confined west of the area and the wind
profile is rather weak. However...an isolated severe thunderstorm
cannot be ruled out where boundaries merge within the more
unstable air west of Interstate 95. Otherwise...heavy rain due to
slow storm motions still seems to be the main hazard into early
evening.
Thunderstorm activity is expected to gradually dissipate and
become further confined to the western zones within waning
heating/instability around sunset. Have maintained a slight
chance of rain most areas by around 00z...with rain potential
diminishing from east to west later in the evening into the
overnight period. The meandering baggy upper trough will steadily
weaken and become absorbed into a larger trough to the west
through the period. Considering some shortwave energy will linger
over the area to maintain a ribbon of deep moisture...onshore flow
on the periphery of the Atlantic surface high could allow isolated
marine showers to push locally onshore after midnight.
Fog potential is a bit difficult to determine...as much depends on
the persistence and location of any cloud cover overnight. Sref
and NAM model solutions do not indicate significant fog formation
despite wet soil where rain occurred this afternoon. Will keep
mention of fog out of the forecast at this point and allow
subsequent forecast updates to reassess the environment later this
evening. Expect low temperatures to range from the middle 60s to near
70 along the coast.
&&
Short term /6 am Wednesday morning through Friday/...
a fairly similar pattern will persist Wednesday and Thursday with
broad upper troughing inland and high pressure over the Atlantic.
This pattern will maintain deep moisture over the area with decent
rain chances through the period...mostly confined inland during the
day and over the Atlantic at night. The risk for severe weather
appears low given the lack of organization potential...although
heavy rainfall could result in localized flooding. High/low
temperatures will ultimately depend on cloud cover/rainfall but
generally should be close to slightly above normal.
A cold front should push through Friday morning...although the
northwest downslope flow will help to offset the cold advection.
This should lead to temperatures close to normal in the middle 80s. The
bigger change will be the drier air which will make it feel a lot
more comfortable.
&&
Long term /Friday night through Tuesday/...
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 /19z Tuesday through Sunday/...
showers and thunderstorms will continue to develop near the
terminals through the afternoon...with reduced visibilities and ceilings
likely within heavier rainfall. Amendments will likely be needed
to account for radar trends during the next several hours...with
most thunderstorm activity diminishing after sunset. Given prior
rainfall and light winds...MVFR or lower ceilings/visibilities will be
possible overnight. However...due to low confidence in timing and
duration...have opted to not include mention in the tafs at this
time and allow subsequent issuances to address this potential.
Extended aviation...sporadic reductions in ceilings/visibilities mainly in
afternoon/evening showers or storms through Thursday.
&&
Marine...
tonight...the offshore ridge of high pressure will continue to
extend west across the coastal waters...supporting southeast to
south winds less than 15 knots and seas in the 2 to 4 feet
range...highest over outer Georgia waters beyond 20 nm.
Wednesday through Sunday...Atlantic high pressure will dominate
with southerly flow until a cold front pushes through Friday. The
much cooler and drier air mass behind the front will move in and
lead to increasing winds and building seas...possibly reaching
advisory levels...at least across the waters beyond 20 nm.
&&
Tides/coastal flooding...
high astronomical tides and favorable north/northeast winds could
lead to minor saltwater inundation near the coast during the high
tide Friday and Saturday evening. By the time of the high tide
Sunday evening as well as Monday evening...winds will not be as
conducive but minor saltwater flooding will still be possible.
&&
Chs watches/warnings/advisories...
Georgia...none.
SC...none.
Marine...none.
&&
$$
Near term...wms
short term...rjb
long term...jrl
aviation...ect/rjb
marine...rjb/wms
