Weather

236 
fxus62 kchs 212343 
afdchs 


Area forecast discussion 
National Weather Service Charleston SC 
743 PM EDT Thursday Aug 21 2008 


Synopsis... 
through this weekend...tropical cyclone Fay will remain south of 
the region and will drift toward the west...gradually weakening... 
while strong high pressure will remain anchored northeast of the 
region. The remnants of Fay could meander over the southeast well 
into next week. 


&& 


Near term /until 6 am Friday morning/... 
early Thursday evening...Tropical Storm Fay was drifting inland 
over northeast Florida....pushed inland by high pressure 
centered over the NE US. On the northern periphery of this 
system...bands of showers producing torrential rainfall and strong 
gusty winds will continue to rush onshore through tonight. The 
ongoing forecast...featuring probability of precipitation tapered from chance well inland 
to categorical south/ southeast...remains on track and has required 
only minor adjustments this evening. 


While any showers will remain capable of producing torrential 
rainfall and localized flooding...greatest coverage/persistence of 
rain bands will enhance the potential for excessive rainfall 
across the southern 1/2 of the forecast area...where precipitable waters  should 
approach 2.5 inches tonight. As Fay drifts inland...the northern 
periphery of the shield of widespread heavy rain may brush extreme 
southern Georgia counties...south of Interstate 16...enhancing 
rainfall totals and the associated flash flood threat there. Will 
maintain the ongoing Flash Flood Watch. 


Due to drier air across far northern/inland counties...as depicted 
by the 12z kchs sounding...expect somewhat less coverage/persistence 
of showers...will not expand the Flash Flood Watch within the 
early evening forecast package. 


Meanwhile...wind fields on the northern periphery of Fay will 
enhance storm relative helicity...supporting a slight chance for 
low topped supercells and brief tropical tornadoes tonight... 
especially across southern counties. 


Along the South Carolina coast...north of the Tropical Storm 
Warning area...the tight pressure gradient between Fay and the 
high pressure to the north will continue to produce strong/gusty 
onshore winds through tonight. The low level wind field will 
support periods of sustained winds around 30 miles per hour...even outside 
showers which will also produce strong/gusty winds. Thus... 
issued a Wind Advisory for South Carolina coastal counties for 
tonight. 


Lake winds...pressure gradient remains tight with boundary layer 
winds of 30-35 knots above Lake Moultrie. Sustained winds have been 
difficult to Sample with Pineville wind equipment due to the east-northeast 
direction. Upper Lake Marion wind sensor has been showing 15-20 knots 
winds with gusts to 25 knots becoming increasingly frequent as mixing 
is maximized during peak heating. Similar conditions likely 
occurring on Lake Moultrie and should continue through early 
evening before mixing profiles weaken with loss of sunshine. Will 
keep Current Lake Wind Advisory running until 11 PM. 


&& 


Short term /6 am Friday morning through 6 PM Friday/... 
the region will remain within the tight pressure gradient and deep 
layer/onshore tropical flow regime between Fay...drifting across 
northern Florida...and the strong high pressure centered 
north/northeast of the region. As a result...showers producing 
torrential rainfall and gusty winds will continue to push onshore. 
The potential for excessive rainfall/flash flooding should focus 
across the southern 1/2 of the region. 12z guidance...especially 
this GFS...suggests that an axis of low level convergence/deeper 
moisture may spread NE along the South Carolina coast...perhaps 
pushing the potential for excessive rainfall into northern counties. 
However...given the proximity of subsidence/drier air produced by 
the ridge...which could hold more widespread/heavier rainfall across 
the south...confidence remains insufficient to expand the Flash 
Flood Watch within this package. 


Meanwhile...wind fields on the northern periphery of Fay will 
enhance storm relative helicity...supporting lingering/slight 
chance for low topped supercells and brief tropical tornadoes 
Friday...especially across southern counties. 


Lake winds...another breezy day is on tap for Friday as the overall 
pressure gradient remains quite tight between high pressure 
building in from the north and ts Fay to the south. 925 mb winds 
could be a bit stronger on Friday than today with 35-40 knots over much 
of the area. Similar temperatures expected with perhaps a slightly better 
chance of seeing more sun over northern Berkeley County...thus 
another gusty day anticipated on the lakes. Thus...issued another 
lake Wind Advisory for Friday...running from 9 am to 9 PM. 


&& 


Long term /Friday night through Thursday/... 
expect a slow...subtle trend toward drier conditions this weekend. 
However...the circulation associated with Fay...forecast to weaken 
to a depression...and associated with the strong high pressure north/NE 
of the region will continue to push moisture onshore and into the 
region...still supporting scattered to numerous showers/ 
thunderstorms Saturday and lesser coverage Sunday. The 12z NAM 
suggests that a band of persistent heavy rain may develop near the 
Savannah River...but prefer to await later model runs before 
adjusting the forecast to specifically address such a scenario. 


Beyond Sunday...a moist onshore flow between the remnants of Fay and 
high pressure will continue to support isolated to scattered 
showers/thunderstorms...especially during the afternoons/evenings. 
A developing upper trough may push a cold front into the southeast 
late in the forecast period. If this boundary stalls north of the 
region as expected...deeper moisture may supporting increased 
coverage of showers/thunderstorms. 


&& 


Aviation /00z Friday through Tuesday/... 
tropical showers with gusty winds and rapid transitions between 
VFR/MVFR conditions to IFR or lower conditions will continue tonight 
through Friday. Outside showers...occasional VFR conditions are 
possible...this evening...especially at kchs...but 0z tafs forecast 
prevailing MVFR ceilings through the night. 


Meanwhile...early evening surface wind trends suggest some degree 
of boundary layer decoupling...and by 04z low level wind shear 
will develop at both terminals. 


Expect amendments as appropriate to address short term convective 
trends. 


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


&& 


Marine... 
surface analysis shows the pressure gradient over the area slowly 
tightening as high pressure to the north slowly sinks south while ts 
Fay maintains strength. Sustained winds remain just below tropical 
storm force along the Georgia coast with the strongest winds at fort 
Pulaski. However winds have been quite gusty along the SC coast with 
frequent gusts to gale force in Charleston Harbor and at Folly 
Beach. The climatological nocturnal tightening of the gradient is 
expected tonight and the 12z guidance seems to be picking up on 
this. Both the GFS and NAM marine MOS are showing sustained winds of 
30-35 knots at Folly Beach overnight...indicating the favored tight 
pressure gradient along the SC coast. 


925 mb winds actually increase by about 5 knots to 35-40 knots during 
the day Friday as high pressure to the north sinks south and ts Fay 
maintains a closed circulation. Winds should be decreasing across 
the southern waters as Fay weakens but overall marine winds will 
remain strong. Squalls will also likely continue to move across the 
waters mixing down stronger winds. We plan to extend the Gale 
Warning through Friday afternoon for SC nearshore and The Harbor. 
This will also preclude the need for frequent special marine 
warnings for 34 knots wind gusts in squalls. 


Sustained winds right along the coast have hovered just below Wind 
Advisory criteria for a few hours this afternoon. For the most 
part...gusts remain far short of criteria. With the nocturnal surge 
this evening through early morning hours...we expect sustained winds 
to increase considerably...thus a Wind Advisory has been posted from 
package issuance through 5 am. 


Upon the lowering of any tropical headlines tonight or Friday 
morning...gale warnings will likely be needed at least through Friday 
afternoon/evening. Thereafter...winds and seas will slowly subside 
as high pressure builds in from the north. However seas will probably 
remain above 6 feet over outer portions of the nearshore waters and 
the Georgia offshore through Sat night or sun. 


&& 


Tides/coastal flooding... 
tides came within a few hundredths of a foot of coastal flood 
advisory criteria at downtown Charleston today with an anomaly of 
+1.1 feet. The high tide late this evening is predicted at 5.3 feet MLLW 
which should stay just below even if the tidal anomalies increase by 
another few tenths of a foot. Shallow coastal flooding is likely 
during the midday Friday high tide due to a continued strong east-northeast 
flow and expected increasing tidal departures as water remains 
trapped during low tide. Will let the middle shift make the final 
decision for a coastal flood advisory after watching the evening 
high tide and those departures. 


&& 


Chs watches/warnings/advisories... 
Georgia...Flash Flood Watch through Friday evening for gaz099>101- 
114>119-137>141. 
Tropical Storm Warning for gaz116>119-138>141. 
SC...Flash Flood Watch through Friday evening for scz047>049-051. 
Lake Wind Advisory until 11 PM EDT this evening for scz045. 
Lake Wind Advisory from 9 am to 9 PM EDT Friday for scz045. 
Wind Advisory until 5 am EDT Friday for scz048>051. 
High surf advisory until 6 PM EDT Friday for scz048>051. 
Marine...Tropical Storm Warning for amz354-374. 
Gale Warning until 6 PM EDT Friday for amz330-350-352. 


&& 


$$ 

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