658 
fxus62 kchs 220356 
afdchs 


Area forecast discussion 
National Weather Service Charleston SC 
1156 PM EDT sun Mar 21 2010 


Synopsis... 
a cold front will approaching from the west will advance through 
the region tonight. Deep low pressure to the north will dominate 
conditions Monday and Tuesday...before high pressure expands over 
the region middle week. Another cold front will likely pass through 
the area Friday or early Saturday. 


&& 


Near term /until 6 am Monday morning/... 
grids and text products have been issued to reflect going short 
term trends. Convection is having a hard time overcoming middle-level 
dry air that is in place. Atmos has destabilized over much of 
southeast South Carolina ahead of the occluded front as expected... 
so will need to keep at least 30 percent probability of precipitation in place there for a 
few more hours. Still can not rule out an isolated to severe 
thunderstorms...but the risk does appear to be diminishing. 


&& 


Short term /6 am Monday morning through Wednesday/... 
Monday...the upper level low will move east-northeast through the southern 
Appalachians. There may be enough moisture and upper-level forcing 
for some showers...mainly across southern SC and interior southeast Georgia. 
Grauple is possible with these showers due to the low freezing 
levels /around 4-5k ft/. Highs should top out in the Lower/Middle 60s 
under mostly cloudy skies. It will also be breezy...with gusts up 
to 30 miles per hour possible from late morning on. 


Monday night...the showers may linger into the early evening 
around the Charleston tri-County area before conditions begin to 
improve from the SW. Lows will drop into the lower 40s for most 
locales...possibly slightly cooler well inland and slightly warmer 
along the coast. 


Tuesday...deep low pressure will slowly exit the middle Atlantic 
coast. After a chilly start we should see temperatures rebound 
into the middle or maybe even upper 60s with breezy afternoon 
conditions. 


Tuesday night and Wednesday...high pressure will build in Tuesday 
night and Wednesday with temperatures moderating above normal 
Wednesday with highs in the middle 70s away from the coast where an onshore 
flow will keep highs no warmer than the middle 60s. 


&& 


Long term /Wednesday night through Sunday/... 
Wednesday night and Thursday...surface high pressure over the area will maintain 
dry weather. A moderating temperature trend with highs returning 
to the middle/upper 70s Thursday. 


Thursday night through sun...uncertainty begins Thursday night as the 12z/21 
model guidance is in disagreement with respect to the next cold 
front/low pressure system Friday into Sat and another cold front sun. 
Did not make any changes to this portion of the forecast through 
this period as slight chance probability of precipitation should cover it for now. 


&& 


Aviation /04z Monday through Friday/... 
expect conditions to continue to deteriorate this evening and 
overnight for kchs and ksav as a deep low pressure system affects 
the area. Have initiated ksav with MVFR conditions as majority of 
the surrounding observation continue to show prevailing MVFR ceilings. As 
more showers develop this evening and move across the area...expect 
ceilings to quickly lower to IFR levels at ksav. Expect a one to two 
hour lag in deteriorating conditions at kchs due to the positioning 
of the low pressure and its associated cold front...but VFR 
conditions should quickly lower a couple hours after sunset as well. 
Have maintained a threat of thunderstorms and rain for both sites as some organized 
convection could still move through the area this evening. May need 
amendments as timing for thunder could be slightly off...especially 
at ksav where the thunder risk could pass just north of the terminal. 


00z kchs sounding showed marginal low level wind shear conditions for kchs which 
appears on track with what most model sounding are showing. 
Therefore...have included low level wind shear for both sites as models show 40 to 
45 knot winds around 2k feet...especially at kchs where potential for 
slightly stronger winds are possible. After 05z/06z...winds aloft 
should start to diminish ending the risk of low level wind shear conditions. 


Expect ceilings to lift and scatter out prior to daybreak Monday 
morning. VFR conditions should then prevail along with gusty 
southwesterly winds in the afternoon due to a tight pressure 
gradient. 


Extended aviation outlook...VFR through at least Thursday before MVFR 
conds or lower possibly affect the area with the approach of the 
next frontal system. Gusty winds Monday and Tuesday afternoon. 


&& 


Marine... 
no major changes. Within the coastal waters...winds out of the southeast 
through late this evening and becoming S late will be the 
favorable fetch needed to steadily increase seas such that all 
marine zones will have seas in excess of 6 feet overnight. Will 
keep the remaining Small Craft Advisory headlines in place as they are delineated. 


Monday through Friday...the relatively tight pressure gradient and 
strong low-level winds are expected over the coastal waters 
through Tuesday before diminishing Tuesday night as the upper level low 
pressure system pushes farther off the coast and well NE of the area. 
Small Craft Advisory conds...mainly for seas...will linger through Monday for the SC 
coastal waters north of Edisto Beach and the Georgia waters beyond 20 nm 
from shore. High pressure will then provide a much more tranquil 
marine environment Wednesday into Thursday before conds possibly begin to go 
downhill as a cold front/low pressure approach from the west. 


&& 


Chs watches/warnings/advisories... 
Georgia...none. 
SC...none. 
Marine...Small Craft Advisory until 6 am EDT Monday for amz330-354. 
Small Craft Advisory until 11 am EDT Monday for amz352. 
Small Craft Advisory until 11 PM EDT Monday for amz350. 
Small Craft Advisory until 6 am EDT Tuesday for amz374. 


&& 


$$