015 
fxus62 kchs 160118 
afdchs 


Area forecast discussion 
National Weather Service Charleston SC 
918 PM EDT Wednesday Oct 15 2008 


Synopsis... 
high pressure will remain over the region for the remainder of 
the work week. A cold front will move through the area Friday 
night. High pressure will build into the region for the latter 
half of the weekend through early next week. 


&& 


Near term /until 6 am Thursday morning/... 
deep layered ridging will persist across the area again 
tonight...resulting in clear skies...little to no wind and 
seasonable temperatures with excellent radiational cooling to occur. 


Current temperatures are similar to this time 24 hours ago...and with a 
similar synoptic set up we look for min temperatures tonight to be close 
to what they were last night. However...there will be just enough 
mixing in the boundary layer to keep some of our northern 
communities from getting as cool as last night...as they will be 
to the north and further removed from the center of the low level 
ridge. Readings will range from 50-55 far western zones to 55-60 
to the east...except lower to middle 60s along the coast. 


Main issue for tonight again looks to be the fog. Similar to last 
evening we are already experiencing a little fog forming near the 
County warning forecast area /1sm at kbqk and 6sm at kcre/. It is even drier aloft this 
evening as opposed to last evening as shown on water vapor imagery 
and data from Charleston and Jacksonville soundings. 
However...while the onshore low level flow is still evident over 
southern areas...low level flow across northern areas has shifted 
from onshore to offshore since yesterday. This along with the 
southern areas that will reach their cross-over temperatures while to the 
north they will be hard pressed to see readings that low...fog 
potential remains highest across the central and southern 
counties. However...given fsi down below 10 even in the 
northernmost zones...there will be at least patchy fog and will 
add to the forecast. We will increase coverage to areas of fog 
across southern zones...mainly south of a line from khxd to ksav 
to ktbr. 


&& 


Short term /6 am Thursday morning through 6 PM Thursday/... 
deep layer ridging will begin to break down across the region as a 
cold front begins to approach the region from the northwest. Model 
relative humidity time sections indicate an increase in upper level moisture 
through the day...but Don/T anticipate anything more than some 
scattered cirrus moving into the region...primarily during the 
afternoon hours. With plenty of sunshine expected...high 
temperatures should top out in the middle to upper 80s away from the 
coast as suggested by low level thickness values. 


&& 


Long term /Thursday night through Wednesday/... 
have seen generally good agreement among the models...particularly 
the NAM/GFS/ECMWF/UKMET. Meanwhile...the Canadian Gem is highly 
unreasonable and the NOGAPS is about 24-36 hours slower than the 
favored conglomerate so have opted for the aforementioned consensus. 


Deep layered ridging centered over the southeast states going into Thursday 
night will shift southward late Thursday night through Friday afternoon...allowing a 
cold front to approach from the west Friday evening. This front looks to push 
across the coastal plain Friday night and through the coastal waters 
Sat morning. Deeper moisture and strong lift along the front will make 
for scattered showers/thunderstorms to develop along and ahead of the front as it 
moves east. Have shown an increasing trend in probability of precipitation during the day Friday 
along with the movement of precipitation from west to east across the County warning forecast area. Have 
confined mention of thunder to only the afternoon hours...with best chances 
for thunder during the Friday period in the late afternoon hours. 


Given enough sunshine ahead of the front...there is a small 
possibility of organized thunderstorm development which could lead 
to isolated heavy rain and/or severe thunderstorms. Confidence is quite 
low in this regard so will need to see how later model runs evolve 
the front. 


Showers/thunderstorms will pick up in intensity and coverage Friday evening into Friday 
night...mainly for northern portions of the County warning forecast area in closest proximity to 
an upper level shortwave rounding the base of the longwave trough 
ushering the front east. Thus...have increased probability of precipitation to likely for the 
Charleston tri-County area with high and middle chance probability of precipitation going S. Probability of precipitation 
have then been trended down further during the early morning hours Sat 
as the front moves through the area. 


By late Sat morning...the front will be offshore followed shortly 
behind by the digging shortwave trough. Have maintained slight-low chance 
probability of precipitation...mainly for eastern sections of the County warning forecast area...during the day on Sat. 


High pressure then makes good inroads from the north starting Sat night and 
remains through Monday...followed by a reinforcing wedge of high pressure 
for Tue-Wed. This will make for a dry and noticeably cooler weekend 
continuing into the first half of next week...including min temperatures 
reaching the upper 40s well inland over the weekend. 


&& 


Aviation /01z Thursday through Monday/... 
we have maintained a VFR forecast through most of the taf period 
with the 00z issuance...but similar to last night and this morning 
there is again a probability of light fog. Cross-over temperatures are 
more likely to be achieved at ksav than at kchs...thus we have 
shown slightly worse conditions at ksav. As a result our forecast 
indicates MVFR visibilities from 08-12z at both terminals. However...potential 
does exist for a brief period of IFR conditions given that fsi is 
down in the single digits and excellent radiational conditions 
will occur tonight. 


Only concern regarding winds is the timing of a weak sea 
breeze...which looks to occur late Thursday afternoon...especially 
at kchs. 


Extended aviation outlook...VFR conditions will prevail through 
Friday morning. An approaching cold front will bring scattered showers/thunderstorms 
to the terminals Friday afternoon through early Sat morning...which could 
produce periods of MVFR to IFR conditions. VFR conditions then 
return Sat through Monday as the cold front exits and high pressure 
returns. 


&& 


Marine... 
light winds across the marine area this evening will continue 
through the night...as winds veer to an offshore direction by 
morning resulting from both the land breeze circulation and the 
synoptic pattern with high pressure over the southeast states. Seas will 
continue to be the combination of a small wind wave atop a 2 to 3 
foot swell wave. 


Fairly benign weather conditions are expected over the waters 
Thursday...with winds and seas remaining well below Small Craft 
Advisory criteria. 


High pressure will prevail through Friday morning which will keep winds 
and seas within relatively tranquil levels. However...a cold front 
will approach from the west Friday afternoon and push through the waters late 
Friday night through Sat morning. The first noticeable change will be 
a wind shift from SW to north Friday night...then a strong Post-frontal 
pressure gradient will increases wind speeds to Small Craft Advisory levels for all marine 
zones over the weekend. There is an small chance that gusts could 
reach gale force over the outer Georgia waters over the weekend...but 
confidence is not high enough at this time to forecast as such but it 
cannot be ruled out. 


Conditions then settle down during the first half of next week as 
high pressure returns from the north. There is the chance that a prolonged 
Small Craft Advisory may be needed for the outer Georgia waters due to slowly receding 
seas but all nearshore waters should no longer need any headlines 
beyond Sun night. 


&& 


Tides/coastal flooding... 
recent tidal departures have been near predicted levels at both 
Charleston and fort Pulaski...and with an offshore flow developing 
overnight and Thursday it will be hard pressed to see much change. 
However...the next high tide during middle morning Thursday will bear 
close scrutiny as the forecast in Charleston is just under 7.0 feet 
MLLW. A coastal flood advisory is required at 7.0 feet MLLW in 
Charleston...but even if levels fall just short there will still 
be at least minor inconvenience flooding in some of the typical 
parts of the Charleston metropolitan area. 


Astronomical high tides will persist into at least the 
weekend...and we will keep watch for possible coastal flood 
advisories the next several days. 




&& 


Climate... 
record highs for Thursday 11/16... 
chs 86f / 1973 
sav 90f / 1941 
downtown Charleston 85f / 1992. 


&& 


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


&& 


$$ 


33 

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