778 
fxus62 kchs 220218 
afdchs 


Area forecast discussion 
National Weather Service Charleston SC 
918 PM EST Friday Nov 21 2008 


Synopsis... 
high pressure will build over the region tonight and persist 
through the weekend. The high will move off the coast 
Monday...followed by a cold frontal passage Monday night. High 
pressure will rebuild Tuesday and continue into late week. 


&& 


Near term /until 6 am Saturday morning/... 
latest msas analysis shows pressure rises up to 4 mb over the past 
few hours...as high pressure continues to build eastward into the 
region tonight. Expect clear skies overnight and dewpoints 
remaining in the teens to 20s within a very dry air mass. The 
tightened pressure gradient ahead of the building high pressure 
will linger across the coastal counties and the marine 
zones...preventing winds from decoupling overnight. Further inland 
and closer to the center of the high...winds will diminish and 
ideal radiational cooling conditions will set up. Expect another 
hard freeze as temperatures fall into the low to middle 20s inland 
and upper 20s to lower 30s closer to the coast. Although 
temperatures will approach record lows...especially in downtown 
Charleston...do not anticipate any records to be tied or broken at 
this Point. 


Lake winds... 
a cold air mass advecting over Lake Moultrie will keep winds up 
around 20 to 25 knots with potentially higher gusts through the 
overnight hours and into middle morning Saturday. As the cold air 
advection wanes Saturday morning...expect winds to diminish below 
lake Wind Advisory levels. 


&& 


Short term /6 am Saturday morning through 6 PM Saturday/... 
high pressure will build over the middle-Atlantic and southeastern 
United States Saturday...resulting in sunny skies. Low level 
thickness values suggest highs only warming into the upper 40s 
across northern zones and around 50 to the lower 50s across southern 
zones. With lack of any good gradient as high pressure moves 
overhead...expect a very inverted airmass over the region...so will 
forecast highs similar to those suggested by low level thicknesses 
in the upper 40s north to lower 50s far south. 


&& 


Long term /Saturday night through Friday/... 
another solid freeze in store Sunday morning for all areas away from 
the beaches as high pressure settles right over the region making for 
excellent radiational cooling conditions. We painted lows from a few 
upper teens in colder places like Allendale to the middle 20s from 
Charleston to Savannah. On the barrier islands and beaches...it will 
hold closer to 40 degrees thanks to a proximity Atlantic flux. 


A weak coastal trough is expected to take shape over NE Florida... 
but for nearly all of our forecast area...very dry air will persist 
on Sunday/Sunday night. Scattered stratocumulus may work into coastal southeast 
Georgia mostly S of Savannah...but risk of showers quite low with 
shallow moisture fields. We lowered high temperatures many areas Sunday 
given the cold start to the day...high pressure location and very slow 
moderation of low level thickness values during the day. 


A fast paced short wave in the still amplified upper flow will 
pass through the region late Monday and Monday evening. A weak moisture 
return flow may be enough to bring a band of light showers across 
the area during this time frame. We maintained persistence with 
mainly slight chance probability of precipitation during these time frames. High temperatures 
on Monday have a good shot at reaching the lower to middle 60s but 
the cloud cover forecast is somewhat tricky in this fast pattern. 


Decent agreement in the global ensembles middle week and beyond 
with another cool down Wednesday into Thursday with high pressure in control. 
While the next batch of cold air will keep temperatures several 
degrees below climatology...it should not be as cold as the current cold 
air outbreak we are experiencing. 


Toward the end of next week...it seems like the onset of a warm 
advection pattern with a potential warm frontal zone over our the 
southeast states on Friday. We have shown a gradual increase in clouds with 
slightly warmer overnight low temperatures as this pattern evolves. 


&& 


Aviation /02z Saturday through Wednesday/... 
VFR conditions are expected through the taf forecast period. 


Northwest winds will likely remain around 12 knots at kchs for 
another few hours...as the pressure gradient is slightly tighter 
across the coastal South Carolina counties this evening. Winds at 
ksav have already diminished to around 5 knots and will remain light 
overnight. 


Latest model soundings suggest that wind shear will be just under 
criteria...so will not mention in the 00z taf issuance. Will need 
to closely monitor wind speed and direction trends at ksav...where 
lighter winds tonight could allow conditions to approach wind 
shear thresholds if directions become different than forecasted. 


Expect a more north-northeast direction to develop Saturday 
morning. Wind speeds will remain less than 12 knots due to a 
gradually weakening pressure gradient. 


Extended aviation outlook... VFR conditions expected to prevail 
most of the period. A cold front may produce brief MVFR conditions 
in showers Monday night. 


&& 


Marine... 
unseasonably cold air will continue to move over the relatively 
warm Atlantic waters tonight into Saturday...allowing strong 
winds aloft to mix down to the ocean surface. Latest buoy 
observations indicate strong northwest flow over the coastal 
waters this evening...with 20 to 25 knots winds being consistently 
reported in the nearshore and offshore marine zones. Only the 
advisory in Charleston Harbor is somewhat marginal according to 
observations at downtown and Folly Beach. However The Pilot boat 
reported 20 to 22 knots at 8 PM. Will therefore maintain ongoing 
small craft advisories through tonight. 


As high pressure becomes more centered over South Carolina and 
Georgia by late Saturday morning and Saturday afternoon...expect 
winds to diminish and seas to subside over the waters. Will keep 
small craft advisories up through Saturday morning for all coastal 
waters...while extending the advisory through the afternoon across 
the outer Georgia waters where seas will be slow to subside. 


The main concern for the latter part of the weekend is whether 
a pinched NE gradient sets in over southern parts of our Georgia 
waters with the position of the coastal trough. We have indicated 
some NE winds near 15 knots over the Grays Reef/sabsoon network area 
but will need to monitor for higher speed potential. Seas similar 
with locally higher seas possible if winds become enhanced. 


Eventually on Monday...surface winds should veer to a more S direction 
while mixing profiles on cool waters keep speeds mostly below 15 knots. 
After a frontal passage Monday night...winds should veer more west and 
increase with a building 850 mb flow then surge and gust in a cold 
air advection pattern on Tuesday in pure northwest flow. Some small timing 
differences with the models this afternoon...but overall persistence 
of our marine forecast is very good heading toward the middle of 
next week. 


&& 


Climate... 
record low temperatures for Saturday November 22nd... 
Charleston Airport.... 24 degrees set in 2000. 
Downtown Charleston... 32 degrees set in 2000. 
Savannah.............. 20 degrees set in 1973. 


Record low maximum temperatures for Saturday November 22nd... 
Charleston Airport.... 43 degrees set in 1937. 
Downtown Charleston... 48 degrees set in 1984. 
Savannah.............. 46 degrees set in 1888. 


&& 


Chs watches/warnings/advisories... 
Georgia...none. 
SC...lake Wind Advisory until 10 am EST Saturday for scz045. 
Marine...Small Craft Advisory until 10 am EST Saturday for amz330-350- 
352-354. 
Small Craft Advisory until 7 PM EST Saturday for amz374. 


&& 


$$ 

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