764 
fxus62 kchs 212336 
afdchs 


Area forecast discussion 
National Weather Service Charleston SC 
636 PM EST Sat Nov 21 2009 


Synopsis... 
high pressure will remain north of the region through tonight. An 
area of low pressure will track along the Gulf Coast states and 
into the Atlantic off the southeast Georgia coast Sunday and 
Sunday night...then move northeast along the eastern Seaboard. A 
high pressure wedge will then dominate into middle week. Low pressure 
will pass off the southeast U.S. Coast on Thursday...then will be 
followed by high pressure from the west into the weekend. 


&& 


Near term /until 6 am Sunday morning/... 
a vertically-stacked low pressure system exists over southern 
Louisiana early this afternoon. As the surface low continues to 
occlude and weaken...only slow movement is expected to the east-northeast 
tonight. The primary driver for our weather will be secondary 
cyclogenesis off the Georgia/SC coast late tonight into Sunday as a 
baroclinic zone strengthens ahead of the upper trough. Quite a bit 
of dry air will persist in the lower levels as high pressure 
continues to wedge down the eastern Seaboard. Mostly middle and high 
clouds will stream across from the SW this evening before lower 
level cloudiness and some rain showers move in late. We continued 
the trend of a later arrival time for precipitation with probability of precipitation 
beginning at 5 am Sunday. Temperatures will drop no lower than the 50s 
overnight due to the cloudy skies and 3-5 kts of wind. 


&& 


Short term /6 am Sunday morning through 6 PM Sunday/... 
a coastal trough will further strengthen off the NC/SC coast 
Sunday morning...increasing isentropic ascent from the east. 
Meanwhile the residual Gulf low will move into the Florida 
Panhandle...advecting quite a bit of low-level moisture into 
southeast Georgia and central SC. Rain should overspread the entire 
area by late morning. We left isolated thunderstorms across far 
southeastern portions in the afternoon due to a slug of 6c 
700-500 mb lapse rates prognosticated to trek through. Cold advection and 
evaporational cooling will keep temperatures from increasing very 
much after daybreak. Highs will probably not get above 60f. 


&& 


Long term /Sunday night through Saturday/... 
guidance is in good agreement ejecting weakening surface low 
pressure off the Georgia coast Sunday night...then tracking it 
northeast along the Carolina coast into Monday. As this 
occurs...overrunning precipitation will allow a high pressure wedge 
to become solidly entrenched to the Lee of the Appalachians as the 
parent high strengthens over New England. There is some uncertainty 
as to how quickly the widespread rain will taper off from the 
southwest Sunday night...and some consideration needs to be given to 
the NAM which shows a dry slot developing over the area. As a 
result...the forecast will show categorical rain chances toward the 
Santee river decreasing to only a chance of rain toward the Altamaha 
river Sunday evening. Rain chances late Sunday night will continue 
to diminish as the deepest moisture shifts north and east of the 
area. As for Monday...deep moisture will be lacking but plenty of 
low level moisture within The Wedge could support a slight chance of 
light rain and/or drizzle across the area. Temperatures will also be 
severely impacted by The Wedge...and guidance appears much too warm 
given the expected scenario...therefore will forecast highs in the 
middle 50s well inland to around 60 toward the coast on Monday. 


The high pressure wedge should persist into Tuesday...before 
gradually eroding as the parent high shifts off the New England 
coast. Abundant low level moisture will keep considerable cloudiness 
in place...along with the potential for at least patchy drizzle. 
High temperatures Tuesday may moderate some...but should remain 
below normal and well below what guidance suggests. 


Wednesday will be a transition day...with dry conditions and near 
normal temperatures expected. Low pressure is expected to track 
offshore along the southeast U.S. Coast Wednesday night into 
Thursday...but there is some model disagreement as to how far 
offshore. Therefore...will show some isolated showers in the 
forecast for Wednesday night. Cool and dry high pressure will then 
build from the west late in the work week through the first part of 
the weekend. 


&& 


Aviation /00z Sunday through Thursday/... 
kchs...high pressure wedge holds in place tonight with 
anticyclonic flow...and only limited moisture to be found in 
place. Thus we have maintained VFR weather through Sunday morning. 
However...short wave now moving into the NE Gulf of Mexico will 
move through after daybreak and result in some light rains 
developing around 14z...and also for a lowering of the ceilings to 
around 5k feet. The combination of increasing low level 
convergence...isentropic lift and short waves aloft will produce a 
heavier and steadier rain overspreading the area during the 
afternoon...and with it our conditions will drop to MVFR at 18z 
and to IFR at 22z. The risk of wind shear is marginal but does 
bear watching Sunday afternoon. 


Ksav...the high pressure wedge and its resulting anticyclonic flow 
early tonight will give way to a short wave and more cyclonic 
curvature of the isobars late tonight. This allows for a steady 
moistening of the lower levels and eventually light rains and a 
lowering of the ceilings to occur...falling to MVFR at 14z. A more 
substantial increase in overall moisture and low level convergence 
and isentropic lift forms for the afternoon...and IFR weather will 
prevail from 18z onwards. Later shifts will need to keep watch for 
a little elevated convection that will attempt to develop late 
Sunday as showalter indices drop to -1 or -2 and middle level lapse 
rates approach 6c/km. Also...the risk of wind shear is marginal but 
does bear watching during Sunday. 


Extended aviation outlook...MVFR or lower conditions are likely 
Sunday night through Tuesday. VFR conditions may return on 
Wednesday. 


&& 


Marine... 
the pressure gradient should increase substantially tonight into 
Sunday as a wedge of high pressure strengthens from the north 
while an approaching upper trough strengthens a trough at the 
surface. 20-25 knots winds may develop across the SC nearshore waters 
late tonight into Sunday while 6 feet seas could push into outer 
portions of these zones as well. We hoisted a Small Craft Advisory 
for the SC nearshore waters from edisto southward to join the 
existing advisories for the northern leg and the offshore Georgia leg. 


Conditions will begin to improve over the waters Sunday night due 
to a weakening surface pressure gradient as low pressure ejects 
off the Georgia coast and tracks just off the coast of South 
Carolina. Winds and seas should fall below Small Craft Advisory 
criteria over the far southern South Carolina near shore waters 
Sunday evening...then over the remaining South Carolina near shore 
waters and outer Georgia waters by late Sunday night. A northerly 
flow will then persist through middle week...with speeds 15 knots or 
less. An strengthening offshore flow is expected on Thursday as 
strong cold air advection develops over the waters. 


&& 


Chs watches/warnings/advisories... 
Georgia...none. 
SC...none. 
Marine...Small Craft Advisory until midnight EST Sunday night for 
amz352. 
Small Craft Advisory until 5 am EST Monday for amz350-374. 


&& 


$$ 


33 

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