664 
fxus62 kchs 231740 
afdchs 


Area forecast discussion 
National Weather Service Charleston SC 
1240 PM EST Monday Nov 23 2009 


Synopsis... 
an area of low pressure off the SC coast will lift NE of our 
region today. Meanwhile...a high pressure wedge will remain firmly 
entrenched over South Carolina and southeast Georgia through Wednesday night. A 
cold front will then push through the area Thanksgiving day... 
followed by high pressure building back into the region from the west 
through the weekend. 


&& 


Near term /until 6 PM this evening/... 
the surface low off the NE SC coast will gradually shift NE away from 
the area today...reaching the Outer Banks by early this evening. Most 
areas of light rain have shifted away...but some patches of 
drizzle barely showing up on kclx radar will continue to drift in 
from the west through the early afternoon hours...especially over the northern half of 
the chs County warning forecast area. In addition...areas of fog will persist through 
early afternoon but should slowly erode as weak late day boundary layer 
downslope will bring down some of the drier air showing up on WV 
imagery. 


As the coastal low pressure center pulls away...the inland high pressure 
wedge may become more north-S oriented rather than NE-SW oriented. 
This will...in turn...allow northern portions to remain under low 
clouds and have temperatures remain in the 50s while southern areas break out 
of the clouds a bit and have temperatures reach the lower 60s. 


Because the measurable precipitation has moved away...have removed probability of precipitation 
from the forecast but did keep in both the -dz and fog. 


&& 


Short term /6 PM this evening through 6 am Tuesday/... 
plenty of low clouds...patchy drizzle and areas of fog will be 
the rule. Diurnal temperature variations should be fairly small 
with The Wedge solidly in place. Low level geostrophic flow will 
be light tonight and a build-down fog situation may pose a risk 
for dense fog development. We have continued mentions of fog in 
our forecast but dense fog advisories are certainly possible over 
parts of our forecast area. 


&& 


Long term /Tuesday through Monday/... 
medium range guidance is in fairly good agreement throughout the 
extended periods this morning. Model solutions continue to depict 
high pressure wedging entrenched in the Lee of the Appalachians 
Tuesday through Wednesday night...before a broad upper level 
trough dives southeastward into the region for Thanksgiving 
day...bringing an end to the rather cool and dreary conditions 
across the low country. 


Before the stagnant air mass gets kicked eastward and out of 
the region on Thanksgiving day...expect mainly cloudy skies and 
cool temperatures to prevail. Model solutions are in fairly good 
agreement showing an area of low pressure tracking northeastward 
off the Georgia and Carolina coast Wednesday and Wednesday 
night...so have expanded probability of precipitation to include the entire forecast area. 
Will keep probability of precipitation in the slight chance range though...as most 
activity will likely remain in closer proximity to the Gulf 
Stream. 


Thanksgiving day a surface cold front swings through in advance of 
the broader upper level trough...allowing boundary layer winds to 
increase out of the west and breaking down the stubborn high 
pressure wedge. High pressure will then slowly build into the 
region from the west Thursday night through the upcoming 
weekend...with a couple dry re-enforcing cold fronts moving 
through the area Thursday night through Friday night. The coldest 
air mass of the season is likely to settle over the low country 
Friday through Saturday...and although maximum temperatures won't 
be terribly cold...overnight lows Friday night and Saturday night 
will dip into the 30s across most locales...and frost/freeze 
headlines may eventually be needed. 


&& 


Aviation /18z Monday through Saturday/... 
conditions are expected to be no better than IFR throughout the 
18z tafs. Low stratus is hanging tough with the inland high 
pressure wedge building in behind the departing coastal low. 
Isolated breaks in the overcast and some thinning is apparent on 
visibile satellite over southeast Georgia...thus ksav may see a few brief 
periods of improving conditions but confidence is not high in this 
scenario and trying to determine any improvement in flight 
conditions is highly problematic and low probability. Will 
maintain the low stratus through tonight but as the nightly 
inversion sets in...the clouds will lower to the surface quickly thus 
making IFR visibilities develop. Have capped visibility at 1/2sm but would not 
be surprised to see visibility down to 1/4sm and vv001 instead of 
ovc001. 


Conditions will not improve quickly after daybreak tmrw as The 
Wedge will be slow to erode to the north. Have maintained IFR or lower 
conditions through late morning tmrw. Some model guidance depicts 
improving conditions quicker than the tafs indicate...but 
climatology dictates the degraded conditions hanging on longer. 


Extended aviation outlook...IFR/MVFR likely through Wednesday. VFR 
conditions should become prevalent Thursday through Friday. 


&& 


Marine... 
as a surface low pressure area off the northern SC coast pulls NE 
today...winds should become north or north-northwest most locations with seas 
coming down to 3-4 feet near shore and 4-5 feet well offshore this 
afternoon. This lines up well with the latest model guidance for 
both winds and seas...thus am dropping the Small Craft Advisory headlines with the 
update this morning and will show the diminishing trend. 


Tonight...a bit of a late night north surge possible and speeds may 
reach 15 knots off the Georgia coast after midnight. 


A wedge of high pressure inland will keep a northerly flow in 
place over the coastal waters Tuesday through Wednesday night 
with winds less than 15 knots. A cold front will shift offshore 
Thursday...with strong cold air advection developing in its wake. 
This could result in the development of Small Craft Advisory 
conditions...especially over the outer Georgia waters...by late in 
the work week. 


&& 


Chs watches/warnings/advisories... 
Georgia...none. 
SC...none. 
Marine...none. 


&& 


$$ 

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