378 
fxus62 kchs 211506 
afdchs 


Area forecast discussion 
National Weather Service Charleston SC 
1006 am 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. This system 
will then move NE along the eastern Seaboard...as high pressure holds 
in place inland. High pressure will slowly become more dominant from 
middle-late week. 


&& 


Near term /until 6 PM this evening/... 
middle and high clouds will continue to increase from the SW today 
ahead of an approaching upper trough. Meanwhile the low stratus 
from this morning should slowly burn off...but low-level clouds 
will probably increase late in the day as isentropic lift begins. 
There will be little in the way of forcing for precipitation except just 
off the Charleston County coast where an isolated shower could 
develop in the coastal trough. Ongoing forecast of highs in the 
middle to upper 60s looked good based on latest trends so we made 
only minor tweaks to today/S graphics. 


&& 


Short term /6 PM this evening through 6 am Sunday/... 
the vigorous southern stream system will lift from southeast Texas today 
into the lower Mississippi Valley region by 12z on Sunday. Dynamic 
influences will guide significant rains into western and north Georgia 
by dawn with isentropic lift and deep moisture gradually increasing 
overnight over our forecast area. The GFS is now a faster outlier 
in spreading rains through our entire forecast area after midnight 
tonight. The baroclinic zone will likely persist off the coast and 
deep layered convergence and upper forcing will likely incite 
rainfall development SW of the Savannah River shortly before 
daybreak on Sunday. Coastal South Carolina appears to be between 
the approaching dynamics and the offshore frontal zone and we 
trimmed probability of precipitation back to a low end chance late tonight. Clouds and 
a light/steady NE flow should keep temperatures in the lower to 
middle 50s coastal areas with some upper 40s closer to the csra and 
midlands where the surface wedge will be entrenched. 


&& 


Long term /Sunday through Friday/... 
medium range guidance is in good agreement through early next 
week...with solutions then diverging during the last several days of 
the extended forecast period. As a result...confidence is near 
normal through Monday...then drops below normal Tuesday through 
Friday. 


Guidance has come into good agreement bringing a weakening wave of 
low pressure across the northern Gulf Coast states Sunday and 
tracking across the Georgia/Florida border Sunday night. As this low 
passes by to the south of the forecast area...a wedge of high 
pressure will hold in place across much of the eastern third of the 
country. The area of low pressure will then track northeastward off 
the eastern sea board and into the southern New England Monday 
through Tuesday...helping to re-enforce the existing high pressure 
wedge across the forecast area. 


Model solutions then remain split on degree of phasing between the 
northern and southern stream Wednesday through Thursday...with the 
GFS and Canadian solutions showing little phasing. As a 
result...these models keep the forecast area under the influence of 
high pressure with little chance for rain. On the other-hand...the 
European model (ecmwf) and UKMET models depict the two streams phasing...which 
results in a wave of low pressure tracking through the southeast 
states along with rain chances Wednesday through Thursday. Guidance 
then comes into agreement building high pressure into the area 
Thursday night and Friday. 


As for forecast details...will carry likely/categorical probability of precipitation Sunday 
and Sunday night as good forcing for ascent along with deep moisture 
and isentropic upglide overspread the forecast area. Model solutions 
depict some decent middle-level lapse rates on the order of 6.5 c/km 
moving through Sunday afternoon through Sunday evening. Although a 
strong surface high pressure wedge will be in place...given the 
degree of forcing moving overhead...can/T rule out elevated 
convection...so will carry thunder mention in the forecast as well. 


As surface high pressure wedge is re-enforced Monday through Tuesday 
as the surface area of low pressure lifts northward toward New 
England...not anticipating much if any clearing across the forecast 
area. Model time height relative humidity cross sections indicate 
that the 1000-850 mb layer will remain nearly saturated through this 
period...so will carry slight chance/low chance probability of precipitation and mention of 
drizzle across the forecast area. Have significantly undercut MOS 
guidance maximum temperatures as well...as light wind fields combined 
with the low sun angle should lead to primarily overcast conditions 
and small diurnal temperature swings. 


Forecast confidence GOES downhill quickly Wednesday and Thursday 
with guidance solutions split on eventual pattern evolution. For now 
will stick with a solution close to the GFS and Canadian solutions 
which maintains good forecast continuity and await better model 
agreement before making any sweeping changes. This results...in a 
rain free forecast Wednesday through Friday. 


&& 


Aviation /15z Saturday through Wednesday/... 
kchs...LIFR ceilings expanded overnight and blanket the Charleston 
area at dawn along with visibility in the 1-3nm range just under 100 
feet ceilings. The latest GFS lamp guidance and 06z WRF BUFKIT soundings 
indicate the low clouds will take some time to disperse again this 
morning. By early afternoon...low clouds likely out of the picture 
and only a mix of layered clouds for the remainder of the taf cycle. 
Tonight...increasing higher clouds expected to inhibit widespread 
fog/stratus development and winds should stay up a bit stronger 
tonight with slightly lower surface dew points possible. 


Ksav...stratus shield has advanced into southern Beaufort and 
Jasper counties and appears to have a slightly better chance 
than yesterday morning to reach the Savannah Airport. Cloud 
pattern trends have become difficult to follow in the past hour 
as high clouds block the vsbl progress of the lower clouds on 
satellite imagery. With the latest observation carrying a few 
stratus...we carried a couple hour tempo of IFR ceilings at ksav to 
be on the safe side. Otherwise...VFR conditions anticipated 
through tonight with a steady NE flow and plenty of middle and high 
level clouds on tap. 


Extended aviation outlook...MVFR or lower ceilings and visibilities likely 
Sunday through Tuesday. VFR conditions may return on Wednesday. 


&& 


Marine... 
the pressure gradient was pinching pre-dawn from around buoy 41004 up 
to the buoy 41013. North-northeast flow averaging 10 to 15 knots over our waters 
this morning...except coastward from Grays Reef where speeds were 
fairly light in a weaker gradient. Seas were elevated beyond 40 
nm off the coast according to steady wavewatch output and fetch 
length and persistence forecasts. We will continue with marginal 
Small Craft Advisory conditions...mainly for seas over our outer 
Georgia waters. The coastal Carolinas pressure gradient will tighten 
tonight and with steady NE flow...it appears seas will build into 
Small Craft Advisory range off at least the Charleston County coast overnight. The 
models could be underdone with the pinching and forecast conditions 
may turn out somewhat optimistic. Outer Georgia waters will remain 
in Small Craft Advisory conditions tonight. 


An area of low pressure will track eastward along the northern Gulf 
Coast and Georgia/Florida border region Sunday and Sunday 
night...then northeastward along the eastern Seaboard and into 
southern New England Monday through Tuesday. High pressure will then 
build in from the west on Wednesday. 


Expect a fairly strong pinched gradient across the waters Sunday and 
Sunday night...with small craft conditions continuing across 
portions of the waters. As low pressure pulls away from the region 
Monday...expect improving conditions with winds and seas remaining 
below small craft thresholds through Wednesday. 


&& 


Chs watches/warnings/advisories... 
Georgia...none. 
SC...none. 
Marine...Small Craft Advisory until 5 am EST Monday for amz374. 
Small Craft Advisory from 10 PM this evening to 5 am EST 
Monday for amz350. 


&& 


$$ 


Jrl 

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