560 
fxus62 kchs 070748 
afdchs 


Area forecast discussion 
National Weather Service Charleston SC 
348 am EDT Tuesday Oct 7 2008 


Synopsis... 
high pressure will shift into the Atlantic Wednesday...allowing a 
potent low pressure system to move in from west late Wednesday. 
The system will slowly move across the area through Friday...then 
linger just east of the area through the weekend. High pressure 
will return from the northwest early next week. 


&& 


Near term /through today/... 
early morning surface analysis showed strong high pressure solidly 
wedged in across the Lee of the Appalachians with the southern 
extent of The Wedge axis reaching as far south as southwestern 
Georgia. The 1031mb parent high was positioned over northeast 
Pennsylvania and...while not immediate apparent in msas pressure 
fields nor buoy observations...an ill-defined coastal trough is 
likely positioned somewhere off the Georgia and Florida coasts. 
The combination of clear skies and light winds have allowed 3 am 
temperatures to drop into the middle 50s across the far interior 
while farther south and east temperatures are much warmer...ranging 
from the upper 50s to the lower to middle 70s at the beaches. Expect 
temperatures to only drop another 1-3 degrees prior to sunrise. 


Deep layered high pressure both at the surface and aloft will 
remain in place today...before slowly giving way tonight as a potent 
storm system currently traversing the central U.S. Draws closer. 
An extremely dry air airmass remains entrenched across all but the 
far southern portions of the local forecast area where a persistent 
onshore flow and diminished influences from The Wedge high as 
allowed atmospheric moisture to remain rather plentiful. This 
dramatic airmass change was captured well by the 07/00z radiosonde observations 
where precipitable waters  ranged from 0.44 inch at chs to 1.65 inches at jax. 
Models push the tight low-level Theta gradient along the Altamaha 
river to the south and southwest today as the parent high over 
Pennsylvania drifts slowly southeast to a position near the 
southern Delaware-Maryland-Virginia peninsula. Will maintain a dry forecast for all 
areas today. 


Skies are generally clear but some patches of stratocumulus are 
beginning to move onshore while cirrus blow off from ongoing 
pockets of convection firing over the Gulf of Mexico is drifting 
in from the southwest. The combination of stratocumulus moving 
onshore...thin cirrus and the development of some diurnal cumulus 
will yield a mainly partly cloudy sky today. Could see skies 
become mostly cloudy off and on at times across parts of southeast 
Georgia today as various cloud layers develop and pass through...but 
anticipate enough sun will be present to justify a partly cloudy 
forecast. 


The orientation of the surface ridge and the 850mb thermal pattern 
suggest the warmest temperatures will be found across the 
southwestern zones this afternoon. Will forecast highs in the middle 
80s in this region with lower 80s elsewhere except middle to upper 
70s at the beaches. 


&& 


Short term /tonight/... 
ridging will begin to breakdown down as a potent shortwave and 
its attendant cold front approach from the west late. The low- 
level wedge will likely hold for much of the night but as the middle- 
level high shifts offshore expect a deeper onshore flow and some 
isentropic lift atop the low-level wedge to occur. This could 
support a somewhat deeper surge of Atlantic moisture reaching the 
north Georgia and far south coastal South Carolina coast after 
midnight. The surge may take the form of isolated shower activity 
and the NAM/GFS as well as the short range ensemble mean all bring 
some spotty shower activity onshore in this area late. Will 
include a 20 percent chance for showers in the forecast after 
midnight...mainly east of I-95 in Georgia and across portions of 
Jasper and Beaufort counties. The onshore flow does not look as 
strong or deep as far north as coastal Colleton and Charleston 
counties where the influence of the low-level wedge will be the 
greatest so will keep these areas rain free. Expect dry weather to 
prevail elsewhere overnight. 


There will be a large range in overnight lows. Radiational 
influences will be the greatest across portions of Dorchester and 
Berkeley counties where temperatures could drop into the middle 50s 
while increasing cloud cover...late night warm air advection and 
the deepening onshore flow farther to the south will yield 
considerably warmer temperatures. Expect lows to range from the 
middle 50s north to the lower to middle 60s elsewhere except upper 60s 
to lower 70s along the beaches. 


&& 


Long term /Wednesday through Monday/... 
high pressure centered over the Middle Atlantic States with its southern extension 
down through the Carolinas...along with an upper level ridge axis... 
will shift offshore Wednesday morning. Meanwhile...a sharp longwave trough 
crossing the middle miss valley will produce a cutoff low over northern 
portions of the Gulf Coast states. All these features will shift east 
through Thursday...although the upper low over the southeast will become 
separated from the mean upper level northern jet stream thus slowing down 
the low and having it meander through Georgia and the Carolinas Thursday-Friday 
before it moves offshore Sat and lingers offshore through the 
weekend. Though this general scheme has pretty good agreement among 
the models...at least through Thursday night...the surface reflection details 
are still sketchy for the weekend but there is at least decent 
agreement among the GFS/Gem/ECMWF. 


As the upper low develops...a surface low pressure system will develop just 
east of the upper low and shift east-northeast through the area. The combination 
of low pressure both at the surface and upper levels will allow for a strong 
return flow of Gulf moisture to the area. The added dynamic lift 
will produce widespread showers over the area by Wednesday night that will 
last through most of Thursday. Given continued good model agreement 
during this period...have increased probability of precipitation to the high likely / low 
categorical range. Will also continue mention of thunder due to 
moderately impressive lapse rates but am uncertain as to the degree 
of instability so will only give thunder a slight chance....although 
should the core of the low pressure system move over the area then an 
upper cold pool could support storms capable of producing hail. 


The system looks to move across the area Thursday night and take a 
position in the Atlantic Friday. The upper low begins to drag its feet 
behind the surface low starting at this point...so have trended down to 
chance probability of precipitation for Friday. By this point...the models diverge as to exactly 
what happens to the upper low. However...the general consensus is to 
have the upper low meander somewhere between the Carolina and northern 
Florida coasts. Given that thinking and the continued presence of 
deep moisture over the area...have opted against a dry forecast through 
the weekend so have added in slight chance probability of precipitation. The system looks to then 
weaken and move off to the east by Monday...allowing high pressure to build in 
from the west. 


Temperatures will become more problematic as for the end of the week 
through the weekend...particularly due to the uncertainty of where 
the upper low will end up which would impact cloud/rain 
complications versus strong warming under an incoming ridge. Below 
normal maximum temperatures and small diurnals are expected Wednesday-Thursday with the 
prevalent clouds/rain expected. For Fri-sun...if the upper low 
settles over the area then maximum temperatures will continue to run near to 
slightly below normal. However...if it settles offshore then temperatures 
may easily run above normal as a bubble ridge settles over the area. 
With model agreement of moving the low offshore...have gone with the 
latter solution at this time. 


&& 


Aviation /06z Tuesday through Saturday/... 
kchs will be VFR for the next 24 hours. Will need to watch for 
MVFR ceilings late tonight into middle morning Monday at ksav with Atlantic 
moisture streaming to the west under at strong cap around 900 mb. We 
have not indicated tempo MVFR ceilings in the 00z/07 taf for 
ksav...but at least low confidence exists at this juncture. 
Otherwise ksav should remain VFR. 


Extended aviation outlook...MVFR to IFR conditions...including 
periods of moderate/heavy rain and thunderstorms...can be expected Wednesday night 
into Thursday night as a low pressure system moves across the area. 
Occasional MVFR conds are possible Friday-Sat as the low pressure system 
lingers just offshore. 


&& 


Marine... 
the marine forecast is generally on track. Expect a slight 
increase in winds throughout the day as the gradient tightens in 
response to a strengthening of the inland high pressure wedge. 
This combined with weak cold air advection will support a rather 
nasty day over the local marine area including the Charleston 
Harbor. Will maintain small craft advisories for all zones with 
only minor tweaks to their beginning and ending times. 


Wednesday through Saturday....conditions will gradually settle 
over the waters as inland high pressure moves offshore and moves 
through the waters. This will allow for a gradual decline in wind 
speeds and a shift in direction from offshore to a parallel/onshore 
combination. With a weakening pressure gradient and loss of favorable 
fetch...seas will be below Small Craft Advisory criteria for nearshore marine zones 
by late Wednesday morning and for the outer Georgia waters by late Thursday 
morning. 


A potent area of low pressure developing over the Gulf Coast states Wednesday 
will move into the waters Thursday then settle just east of the waters Friday 
through the weekend. There is considerable model spread in the 
details of where the low will be positioned by then along with its 
strength but the predominant thinking is that conditions will remain 
below Small Craft Advisory levels through the weekend. The low pressure system looks to 
pull off to the east by Monday...allowing high pressure to return from the northwest. 


Rip currents...there were no reports of rip currents along any of 
our beaches yesterday. Conditions look marginal for a moderate rip 
current risk...mainly due the absence of any long-period swell and 
astronomical influences. It still looks to be a rather nasty day 
within the surf zone with breezy conditions and a favorable 
northeast fetch so will maintain the moderate risk through this 
evening. 


&& 


Chs watches/warnings/advisories... 
Georgia...none. 
SC...none. 
Marine...Small Craft Advisory from 11 am this morning to 10 PM EDT this 
evening for amz330. 
Small Craft Advisory from 11 am this morning to 5 am EDT 
Wednesday for amz352. 
Small Craft Advisory from 11 am this morning to 10 am EDT 
Wednesday for amz350. 
Small Craft Advisory until 11 am EDT Thursday for amz374. 
Small Craft Advisory from 2 PM this afternoon to 5 am EDT 
Wednesday for amz354. 


&& 


$$ 


St/jpc 

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