901 
fxus62 kchs 200534 
afdchs 


Area forecast discussion 
National Weather Service Charleston SC 
134 am EDT Wednesday Aug 20 2008 


Synopsis... 
surface high pressure will push into the Middle Atlantic States and 
stall north of the area through Friday. Tropical cyclone Fay will 
stall near the northeast Florida coast Wednesday...then push onshore 
just south of the region around Thursday. On the southern 
periphery of the high pressure...the remnants of Fay could meander 
over the southeast late this week into early next week. 


&& 


Near term /until 6 am this morning/... 
surface high pressure continues to build from the north...while 
Tropical Storm Fay gradually moves north-northeast along the 
Florida Peninsula. These two features have set up a notable 
northwest-southeast moisture gradient across the forecast area 
this evening. Scattered light showers persist across the southeast 
Georgia coastal counties and across the coastal waters...where 
moisture and convergence are enhanced by a lingering baroclinic 
zone...onshore flow and the northern periphery of Fay. 


Water vapor imagery continues to show dry air aloft across upstate 
South Carolina and Georgia...associated with negative vorticity 
advection pushing into the inland zones tonight. As a 
result...will decrease probability of precipitation from west to east through the early 
morning hours. Have maintained highest probability of precipitation around 30 percent 
across the southeastern third of the forecast area and along the 
coast...closest to the flux of deep moisture and the approaching 
bands of precipitation from Tropical Storm Fay. 


Confidence is low in fog development overnight. Despite clearing 
skies away from the coast...increasing wind profiles ahead of 
Tropical Storm Fay suggests that fog development could be 
suppressed. Any trapped low level moisture could evolve into low 
stratus instead of fog. 


Made a few minor tweaks to the temperature forecast for tonight 
to incorporate latest trends. Low temperatures are expected to 
range from around 70 degrees far inland to the middle 70s near the 
coast. 


&& 


Short term /6 am this morning through 6 PM Thursday/... 
Tropical Storm Fay will continue to move across Florida and emerge into the 
Atlantic Wednesday morning...making a left turn through the day on Wednesday. With 
tropical influence expanding north...showers will be on the increase 
from south to north as the day progresses. The details are difficult 
to pin down in terms of rainfall timing and amounts...since much 
uncertainty still remains in the track and intensity of the storm 
as it near the forecast area. 


Winds will also be on the increase...with boundary layer winds 
increasing into the 20 to 30 knots range. Strongest winds will again 
be to the south...with wind fields diminishing further north. 
Expect surface winds to eventually increase to the 15-25 miles per hour range. 
As for temperatures...maximum temperatures in the low to middle 80s per met guidance 
look good. 


&& 


Long term /Thursday night through Tuesday/... 
based on recent model trends...including 12z guidance...confidence 
the evolution of tropical cyclone Fay has improved over the past 24 
hours. Strengthening high pressure to the north of the region will 
slow then halt the northward progress of the system midweek... 
perhaps just east of the northeast Florida coast...then will steer 
the system toward the west for the latter half of the week. The 
latitude/intensity of the system when the turn to the west commences 
remains uncertain. However...the probability of a landfall on the 
South Carolina coast has become extremely low...and the potential 
for landfall within southeast Georgia north of the Altamaha 
river...while not yet eliminated...has diminished during the past 
24 hours. 


Wednesday night...as high pressure expands and strengthens north of 
the region...Fay should meander south of the area. Then...the high 
pressure should push Fay west and back into the coast...somewhere 
along the NE Florida or extreme southern Georgia coast. 


Based on 12z guidance trends regarding the expected track of 
Fay and subsidence/dry air produced by the Northern Ridge...adjusted 
probability of precipitation Wednesday night into Thursday night...graduated from likely 
S/southeast to chance N/NW. 


Once Fay pushes west late this week into this weekend...the high 
pressure could expand into the region and may limit probability of precipitation to slight 
chance/chance through early next week. However...due to a weak 
steering flow the remnants of Fay may meander over the southeast US 
through early next week...perhaps enhancing the potential for 
showers/thunderstorms...especially during each afternoon/evening. 
Based on this potential...raised probability of precipitation to chance in most 
areas...trended toward GFS MOS/HPC guidance...Saturday through 
Tuesday. 


&& 


Aviation /06z Wednesday through Sunday/... 
increasing high clouds over southeast South Carolina and southeast 
Georgia along the northern periphery of Tropical Storm Fay 
suggests a low chance of fog development at both terminals during 
the early morning hours. Have thus maintained VFR conditions at 
kchs and ksav through the taf period. 


Extensive middle to high level ceilings will affect the area during 
the next 24 hours...especially at ksav with its closer proximity 
to the northward progressing tropical cyclone. Have indicated 
scattered low clouds pushing into ksav by late morning...while 
also including a cumulonimbus to account for convection associated with the 
approaching tropical rain bands. Will need to monitor for possible 
MVFR ceilings at ksav during the afternoon hours. 


Less confidence in convective coverage at kchs...considering the 
rather substantial northwest to southeast moisture gradient 
resulting from high pressure to the north and Fay to the south. 
Have thus not included cumulonimbus at kchs with the 06z taf issuance...as 
subsequent forecasts should be able to better determine convective 
trends for South Carolina zones. 


Tightening pressure gradient ahead of the approaching storm system 
will allow east to northeast winds to steadily increase from south 
to north through the taf period. Gusty winds are expected at ksav 
by late morning and at kchs around noontime. 


Extended aviation outlook...depending upon the evolution of 
Tropical Storm Fay...the terminals could experience more prolonged 
periods of MVFR to IFR conditions Thursday through Saturday. 
Conditions look to improve to prevailing VFR levels by Sunday. 


&& 


Marine... 
marine conditions will steadily deteriorate tonight and tomorrow...as 
tropical cyclone Fay makes northeast progress through Florida and into 
the Atlantic off the NE Florida coast. Conditions will generally remain 
below Small Craft Advisory levels for all marine zones tonight. Offshore Georgia 
zones will then increase to 20 to 25 knots with seas building to 5 to 7 
feet Wednesday morning. Winds will gradually increase to 10 to 15 knots 
late tonight for the nearshore marine zones and The Harbor...then 
15 to 20 knots gusting to 25 knots Wednesday. 


At the very least...the tightening surface pressure gradient between 
the northern high pressure and tropical cyclone Fay will support 
widespread Small Craft Advisory conditions across the coastal waters 
Wednesday night through at least Friday...and elevated seas could 
persist into this weekend. The greatest potential for tropical storm 
force winds remains over the Georgia coastal waters as Fay turns 
west Thursday/Thursday night. Of course...if Fay pushes onshore 
farther north and/or becomes stronger...conditions could deteriorate 
further than expected. 


&& 


Hydrology... 
there remains much uncertainty regarding the potential for excessive 
rainfall. Subsidence/drying provided by the ridge to the north will 
likely produce a rather sharp north to south moisture gradient. 
Within the drier air...showers/thunderstorms will develop along 
mesoscale boundaries and should not become sufficiently widespread 
to produce a significant threat for flooding. Farther south...within 
the shield of rain associated with the tropical cyclone...excessive 
rainfall will become common...supporting a significant threat for 
flooding. Due to uncertainty regarding the exact path and intensity 
of Fay around Thursday...the northward extent of persistent widespread 
torrential rainfall remains unclear. As of Tuesday afternoon...the 
potential for excessive rainfall appears highest across southeast 
Georgia...increasing as one heads south toward the Altamaha river 
Thursday and Thursday night. 


&& 


Tides/coastal flooding... 
the GFS extratropical surge model suggests tides will run about 
one foot above predicted levels Thursday...remaining below coastal 
flood advisory thresholds. However...the GFS holds Fay well south 
of the region...with the onshore flow between the northern high 
and Fay providing most of the elevated water levels. However...if 
Fay comes ashore farther north...and/or becomes stronger than 
expected...tide departures could increase to support at some 
degree of coastal flooding...especially across southeast Georgia. 
Further...beach erosion appears likely with the persistent/strong 
onshore flow...and high surf may impact beaches. We will continue 
to highlight these threats within the hazardous weather outlook. 


Rip currents...an enhanced rip current risk will continue through at 
least late this week. We will continue to highlight this risk within 
the hazardous weather outlook. 


&& 


Chs watches/warnings/advisories... 
Georgia...Flash Flood Watch from this evening through Friday evening for 
gaz114>119-137>141. 
Tropical storm watch for gaz116>119-138>141. 
Tropical storm wind watch through late Thursday night for 
gaz114-115-137. 
SC...none. 
Marine...Small Craft Advisory from 4 PM this afternoon to 4 PM EDT 
Saturday for amz350-352. 
Tropical storm watch for amz354-374. 
&& 


$$ 

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