975 
fxus62 krah 200720 
afdrah 


Area forecast discussion 
National Weather Service Raleigh NC 
320 am EDT Wednesday Aug 20 2008 


Synopsis...high pressure will build into the area from north today 
and remain an influence on our weather through Friday. 


&& 


Near term /through today/... 
as of 255 am Wednesday... 


In near term...moist air mass near surface and relatively clear 
skies will allow areas of fog to develop through 12z. Do not 
anticipate widespread dense fog as boundary layer a couple of 
degrees warmer than last night and will likely see an increase in 
middle/upper level cloudiness. 


Otherwise middle level shear axis exiting off the Delaware-Maryland-Virginia at 
06z...trailing westward back into central Virginia. Associated surface front 
nearing Virginia/NC border. This feature prognosticated to wash out over the 
region later today. Surface high pressure will then nose southward into 
region from southern Ontario/New England. Appears that there will 
be enough residual moisture and weak instability along decaying 
boundary to warrant a slight chance pop over the NE counties this 
afternoon. Over the rest of the area...air mass appears too dry 
and stable to initiate/support shower activity. Low level 
thicknesses this afternoon comparable to Tuesday afternoon. 
However...expect to see temperatures a degree or two cooler as will 
experience some weak cold air advection at 850mb. Highs 89-92. 




&& 


Short term /tonight through Saturday night/... 
as of 255 am Wednesday... 


Tonight through Thursday night...dry Air Ridge strengthens over the 
region in response to building middle/upper level ridge overhead. 
Subsidence associated with this feature will dry out air mass 
through most of the column. Low level Ely flow will likely advect 
enough Atlantic moisture below middle level subsidence inversion to 
result in a scattered cumulus field. Thicknesses lower about 10m. 
Also....weak 850mb thermal trough beings to nose southeastward across 
region. This translates to maximum temperatures Thursday 3-5 degrees cooler than 
highs expected today. 


Friday...forecast uncertainty increases as models varying on the 
amount of moisture that may be potentially advected across the 
region. GFS wetter than most of the other short term models. This 
may be due to the GFS shifting ridge axis a little farther eastward. 
However a slight veer in the low/middle level wind flow (as 
advertised by gfs) would be able to tap moisture lurking off the 
southern NC and SC coast. For now...will lean a bit toward the GFS 
solution as it as been suggesting an increase in low/middle level 
moisture and weak lift the last few model runs. Better instability 
(though weak) will reside over the southern counties. Middle level 
lapse rates generally at or below 5 degree c/km . Thus...only anticipating a 
few low top showers during the heat of the day. Will graduate probability of precipitation 
from slight chance north to small chance south. 


Due to anticipated cloud coverage and surface pattern signaling 
classical cold air damming signature (though the word "cold" 
misleading this time of year)...expect temperatures to average 3-5 
degrees below normal. Potential for maximum temperatures to be a few degrees 
cooler...especially if showers more extensive than current thinking. 


For Saturday....expect drier air aloft to slowly mix out moisture 
near surface...leading to a slow erosion of cloud coverage 
Saturday. Cannot rule out an isolated shower in the far south and 
southeast counties where better moisture will reside. Thicknesses recover 
a bit. Temperatures should rebound to near normal levels. However...if 
clouds linger than forecast...maximum temperatures may be a category cooler. 


&& 


Long term /Sunday through Tuesday/... 
as of 300 am Wednesday... 
the dominance of unseasonably strong surface high across central NC 
begins to wane on Sunday...as surface shifts out into the Atlantic. 
This will allow a weak surface boundary to approach the area from 
the west Monday and into Tuesday...as the parent shortwave trough 
transverses southeast Canada and northeastern US. Kept a slight chance to 
small chance probability of precipitation with the approach of the surface trough for Monday 
and Tuesday. Temperatures will remain near climatology. 


The jury is still out concerning Fay and her eventual evolution but 
it appears more and more likely than Fay or her remnants will remain 
over the deep south/northern Gulf of Mexico through the 
weekend...well removed from the steering flow of the westerlies. 
Uncertainty increases exponentially in conjunction with the spread 
in model guidance. Based on the official NHC guidance... 
some Fay-tropical related moisture could still potentially manage to 
get pulled northeastward ahead of or along the front...resulting in 
some decent rain coverage by Tuesday. 


&& 


Aviation /06z Wednesday through Sunday/... 
as of 130 am Wednesday... 
high humidity/dewpoints continue over much of the forecast area...in 
particularly across the eastern and southern portions of the 
forecast area with dewpoints in the upper 60s to lower 70s. 
This will allow for another night of dense IFR/LIFR fog at rwi and 
Fay...as calm winds and clear skies will allow for near surface 
saturation. Drier dewpoints exist across the western and central 
Piedmont...so only marginal MVFR fog is expected at rdu...gso...and 
int. Additionally...the onset of northeast winds as surface high 
builds in from the north...should mitigate fog development as 
surface wind gradient increases. 


All fog should burn off by 13-14z. Northeast to east northeast winds 
dominate the rest of the day with the surface high centered off the New 
England coast ridging down the middle Atlantic Seaboard. Winds 
around 3 kft increase to 15-20 knots by 12z...so as daytime mixing 
commences...could see a few gusts to 15 knots or so at gso/int/rdu. 
Once the boundary layer becomes deeply mixed...expect NE to east-northeast 
winds around 10 knots at all sites for the afternoon. Forecast 
soundings indicate a layer of middle-level moisture will surge south 
into the area late this afternoon...in an area of weak isentropic 
lift on the 305k surface. These clouds could actually result in a 
broken cloud deck between 7-9 kft...as the moisture remains trapped 
underneath the middle-level subsidence inversion. An isolated shower or 
sprinkle is also possible...as negative surface lifted indice's spread west 
over the northern tier. 


Winds may be modestly gusty again Thursday afternoon and Friday 
afternoon...otherwise...VFR conditions will be the rule. In the 
persistent northeast and east flow...Atlantic moisture may be 
cause for some low clouds...particularly during the early morning 
hours. The latest GFS would suggest the best chances for this 
would be Friday and Saturday mornings. 


&& 


Rah watches/warnings/advisories... 
none. 


&& 


$$ 
Synopsis...wss 
near term...wss 
short term...wss 
long term...cbl 
aviation...cbl 

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