Weather

705 
fxus62 kchs 292327 
afdchs 


Area forecast discussion 
National Weather Service Charleston SC 
727 PM EDT Friday Aug 29 2008 


Synopsis... 
weak surface low pressure will remain inland through 
Saturday...while weak high pressure will remain offshore. A weak 
cold front will approach from the northwest Saturday and will 
dissipate over the area Sunday. Stronger high pressure will then 
build from the north for early next week. 


&& 


Near term /until 6 am Saturday morning/... 
isolated to scattered showers/thunderstorms should gradually diminish 
late this evening. Areas of stratus and patchy fog will again 
develop after midnight with low level moisture and light winds 
under mostly clear skies. Lows will range from the lower 70s well 
inland to the middle/upper 70s closer to the coast. 


&& 


Short term /6 am Saturday morning through Saturday night/... 
upper-level troughing looks to weaken over the area Sat but plenty 
of low-level moisture and the sea breeze warrants leaving 20 probability of precipitation 
in the forecast...mainly for the afternoon away from the immediate 
coast. 


Highs will generally reach the lower 90s away from the coast with 
overnight lows Sat night ranging from around 70 inland to the middle 
70s near the coast. 


&& 


Long term /Sunday through Friday/... 
Sunday...a strong anticyclone aloft building over the upper 
Midwest...while at the surface we also will find strong high 
pressure...centered over the eastern Great Lakes. These features will 
result in a steering of tc Gustav northwest through the Gulf of mex...while 
keeping tc Hanna to the east and NE of the Bahamas. Although precipitable waters  
will be between 1.5 and 2 inches on Sunday...the main forcing is a 
result of the sea breeze and low level convergence with an onshore 
synoptic flow. Thus we will go no more than 20-30 percent 
probability of precipitation...highest over southern areas where moisture is deeper and a 
weak short wave may move in from the southeast and south. Temperatures will reach 
near 90...except middle 80s closer to the coast with onshore fetch. 
Activity inland will diminish Sunday evening...but with continued 
coastal convergence overnight with easterly flow...Atlantic showers will 
redevelop and impact some southern and eastern areas after midnight. 
However...no more than slight chance probability of precipitation...while min temperatures will be 
in the upper 60s far inland to middle 70s at the shoreline. 


Monday and Tuesday...some uncertainty regarding the timing of the 
backdoor cold front...but latest indications point toward either 
late Monday or Tuesday with this feature moving into the local area. 
A large surface high pressure area will build from the north in wake of 
the front...as 593/594 anticyclone aloft builds from PA and New York. 
Moisture pooling south of the front with the interaction of the sea 
breeze will lead to slight chance and chance probability of precipitation Monday...highest 
over southern locales where precipitable waters  are close to 2 inches. 
However...moisture is on the decrease for Tuesday...but have 
maintained small probability of precipitation...mainly south...in case the front is slower. 
The large ridging to the north will continue to steer tc Gustav 
toward the northwest or north Gulf Coast...where landfall will occur early 
next week. The same ridge will prevent tc Hanna from reaching no 
further north than about 25 or 26 degrees north latitude...in a 
position east-northeast of the Bahamas. Daytime temperatures Monday and Tuesday will 
be near or slightly below climatological with onshore synoptic flow. 


Wednesday...continued dry and sinking air in wake of the backdoor 
front as high pressure settles southward toward the local area. Precipitable waters  
decrease to between 1 and 1.5 inches...thus rain chances will be no 
more than isolated coverage along the sea breeze. We will cap probability of precipitation 
at 14 percent in the grids. Temperatures will remain status quo with little 
synoptic change. Meanwhile...tc Hanna will be moving westward toward 
South Florida or northern Cuba...courtesy of the ridge over the 
eastern United States. 


Thursday and Friday...eventually the ridge in the east will weaken 
as it shifts eastward...and this will play a major role in 
determining when and where Hanna begins to make her turn toward the 
northwest. For now we will keep a dry forecast on Thursday...but increase 
rain chances on Friday as moisture associated with Hanna looks to 
reach southern SC and southeast Georgia. 


&& 


Aviation /00z Saturday through Wednesday/... 
cumulonimbi north of the terminals will dissipate late this evening. 
Otherwise...VFR conditions will give way to MVFR visibility after 
midnight...with periods of IFR or lower ceilings and/or 
visibilities possible around daybreak. VFR conditions will then 
return Saturday morning. Cumulonimbi will develop along the sea breeze 
Saturday afternoon. 


Extended aviation outlook...VFR conditions expected to prevail 
outside of afternoon convection and possible early morning 
stratus/fog. 


&& 


Marine... 
a weak pressure pattern over the area will persist through Sat 
night. Winds will be capped at 15 knots with seas no higher than 3 
feet...except building to 4 feet across the Georgia waters late Sat night 
due to a combination of increasing NE winds and swells from Hanna. 


Sunday through Wednesday...a strong 1020-1025 mb high centered over 
eastern Great Lakes on Sunday will shift into the northern and 
central Appalachians Monday and Tuesday...and will force a 
backdoor cold front into the forecast area late Monday or Tuesday. 
The front becomes diffuse on Wednesday as the high shifts south 
into the Carolinas. Meanwhile...tc Gustav will move northwest through the 
Gulf of Mexico during the early part of next week allowing the 
ridge to nose southward into South Florida or Cuba in its wake. Tc 
Hanna will have its northwest progression impacted by the ridge at least 
through the early and middle parts of next week...keeping it far 
to the south near the Bahamas/South Florida or Cuba during that 
time. 


A large pressure difference will develop across the local waters Sunday 
through Wednesday...with the high to the north and the tropical 
systems to the south and SW. This will cause an occasional pinching of 
the pressure gradient...enough so that 20 or 25 knots winds are certainly 
possible by Sunday night or Monday...and continuing through Tuesday 
or Tuesday night. Seas will build significantly...reaching 4-5 feet 
nearshore to 5-7 feet on the outer Georgia waters by Sunday night or early 
Monday...then climbing to as high as 5-7 feet within 20 nm later 
Monday and Tuesday...and up to at least 7-10 feet further offshore. 
Thus Small Craft Advisory/S seem a good bet...if not by Sunday night than most 
assuredly by Monday and persisting through at least middle week. 


Rip currents...area buoys are indicating around a 2 feet swell wave 
about every 10 seconds. Given the upcoming new moon...am opting to 
keep the moderate risk of rip currents going through Sat. 


Strong high pressure will remain centered north of the forecast area 
with tc Hanna to the south through Friday of next week. As a 
result...coastal areas will have several days of a moderate or 
strong onshore wind next week. This in turn will result in elevated 
wind waves and swells. Thus the beaches will likely experience an 
enhanced risk of rip currents through most if not all of the 
upcoming week. 


&& 


Tides/coastal flooding... 
due to astronomical factors...tide levels will remain elevated 
through this weekend. Continued the ongoing coastal flood 
advisory for the SC coast this evening...as tide levels are 
expected to reach at least 7.0 feet MLLW on Charleston Harbor. 
Saturday evening...the high tide may again approach 7 feet MLLW on 
The Harbor. 


Even as we pass the influence of the new moon...tides will remain 
elevated during next week as well...with a persistent onshore flow 
along with increased surf action as a result of strong high pressure 
to the north and tc Hanna to the south. Due to these expected 
conditions...our coastal locations will likely see at least minor 
beach erosion and perhaps shallow coastal flooding concerns from 
time to time from Sunday through Friday...especially near the 
times of high tide. 


&& 


Chs watches/warnings/advisories... 
Georgia...none. 
SC...coastal flood advisory until 10 PM EDT this evening for 
scz048>051. 
Marine...none. 


&& 


$$ 

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