Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
721 am EST Thursday Dec 12 2013
high pressure will build today then persist through Friday. A frontal
system will then impact the region during the weekend. Another area
of high pressure will build in behind the departing system early
Near term /through tonight/...
cold air advection pattern today as the upper wave races offshore
after middle morning. H925 temperatures forecast to fall from 7-8c around
daybreak to 1-3c north and 3-6c south. Despite full sunshine...we
lowered a few high temperatures over northern South Carolina zones by a
couple degrees to account for the latest thickness forecasts. A good
average for highs today middle 50s north to around 60 south of I-16.
With 1000 mb geostrophic flow ramping up to 25-30 knots by middle
morning...some areas will see northerly wind gusts in the 15-20
miles per hour range for while middle morning into middle afternoon as the drier
air punches in.
No changes to the forecast tonight...clear skies...perhaps a few
thin high clouds nearing dawn. Freezing temperatures to around the U.S.
17 corridor South Carolina and west of the I-95 corridor in southeast
Short term /Friday through Sunday/...
Friday...after starting with the coldest temperatures of the month
so far...temperatures will begin to rebound as the surface high moves into
the Atlantic and winds turn around to the east. Temperatures will coolest
on Friday along the immediate coast where the wind will come off the
cool shelf waters...while the rest of the County Warning Area will get to a couple
of degrees either side of 60. Clouds will increase significantly
during the day as the cirrus shield from the developing low pressure
area in the Southern Plains starts rocketing eastward on the back of
a strong southern stream jet maximum. We will also see a slow increase
in lower clouds due to the low level onshore flow.
By Friday night...a coastal trough is expected to develop just
offshore. The enhanced low level convergence should cause some
showers to form over the coastal waters...but a few showers might
sneak on to the coast toward morning. Overall chances will be low
for measurable rain...however.
On Saturday...chances of rain will increase as a disorganized area
of low pressure moves into the southeast. The GFS and European model (ecmwf) are in
better agreement tonight...keeping the bulk of the heavier rain to
the west of the area during the day...but scattered showers are
still possible as broad...but weak...synoptic scale lift spreads
overhead. There might also be an isolated thunderstorm over the
coastal waters that may brush the coast as the models bring some
modest instability into the region from the south.
Southerly winds will become established during the day as the
coastal trough pushes north of the County Warning Area by early afternoon. 925 mb
temperatures are forecast to surge to 13-16c...so afternoon temperatures will rise
above normal...potentially reaching 70 again south of Interstate 16.
During Saturday night...the front associated with the complex low
pressure system moves through the area. Most if not all areas should
see rain...with some locally heavy spots in excess of one inch
possible. Instability guidance in the models again indicate the main
area for the threat of thunder would be over the coastal
waters...but there may be just enough over land to allow for an
isolated crack of thunder.
The front will be offshore by Sunday morning...but the area will be
slow to clear as we remain in the favorable right entrance region of
the upper jet for synoptic scale lift. There will still be a good
chance of showers in early in the day...with chances gradually
tapering off from west to east during the afternoon. The cold
advection behind the front is not expected to be particularly
strong...so after a mild Saturday...temperatures should back up a little to
around normal on Sunday...in the lower to middle 60s.
Long term /Sunday night through Wednesday/...
on Sunday night...the front should push further into the
Atlantic...and cloud cover will gradually diminish. Temperatures will fall
back to the middle 30s inland and lower 40s at the coast. Any remaining
chance of rain should be confined to the offshore coastal waters.
On Monday and Tuesday...the GFS is more aggressive than the European model (ecmwf) in
developing a weak low pressure trough to the east of the area. Right
now...my inclination is to keep the weather dry and cool Monday and
Tuesday...more in line with the European model (ecmwf)...as the upper flow in the GFS
looks too flat to support as much of a surface wave as it is depicting.
High pressure should continue to rule through middle week. Temperatures
will be within a few degrees of climatology...and chances for rain will be
Aviation /13z Thursday through Monday/...
VFR conditions will prevail at both kchs and ksav through 12z Friday.
Extended aviation outlook...mainly VFR conditions Friday and Friday
night...although I cant totally rule out some brief MVFR due to some
Saturday there could be some periods of MVFR ceilings...especially in
Saturday night into Sunday morning looks like there will be a high
chance of MVFR/IFR ceilings at times due to rain and low clouds with the
frontal passage. Conditions should be improving again late Sunday
into the first half of next week.
we have issued a Small Craft Advisory for near shore waters today
mainly for conditions in the outer 10 nm of our marine legs. An
increasing northerly flow is expected to frequently gust to near
25 knots from the Charleston pilot buoy southward to Grays Reef nms.
Within in 10 nm of the coast and in The Harbor/sounds...conditions
likely to remain just below criteria with occasional gusts
exceeding 20 knots. Advisory conditions over outer Georgia waters
will continue today with seas 5 to 7 feet with the north flow.
As high pressure settles over Georgia and the Carolinas tonight...
modest pressure gradient continues over the waters tonight along with
some sharp thermal gradients. This should keep NE flow in the 15
or 15-20 knots range with seas 3-4 feet near shore and 4-6 feet offshore.
Seas should subside to below 6 feet over outer Georgia waters by
daybreak on Friday...but progress overnight will be slow with the
more favorable fetch.
Northeast winds of 10 to 15 knots will veer to the east during
Friday as high pressure gradually moves into the western Atlantic.
Winds will turn more southeast on Friday night as a coastal trough
develops...then southerly and increase on Saturday as the trough moves
quickly northward ahead of an advancing cold front.
By late Saturday and during Saturday night...south and southwesterly
winds look to be just below Small Craft Advisory criteria in the neighborhood of 20
knots in the offshore waters...but seas will get up to 6 feet in at
least amz374...and possibly also amz350...so scas may have to be
lifted with a later forecast cycle.
On Sunday...behind the front...the winds will turn
northwesterly...but since the pressure gradient is not expected to
be particularly tight and the cold advection only modest...winds and
seas should drop below Small Craft Advisory criteria once again.
Monday and Tuesday winds should be north to northeasterly as weak
high pressure builds in to the southeastern states. The GFS is
advertising stronger winds Monday afternoon and evening in the
offshore Georgia waters than I am currently advertising because I
think it overly develops a weak surface wave in the western
Marine...Small Craft Advisory until 4 PM EST this afternoon for amz350-
Small Craft Advisory until 5 am EST Friday for amz374.