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Area forecast discussion 
National Weather Service Raleigh NC 
246 PM EDT Thursday Aug 21 2008 


Synopsis... 
an unseasonably strong high pressure center to the north will extend 
into the region through the weekend...bringing dry weather to the 
area. 




&& 


Near term /through Friday night/... 
as of 245 PM Thursday... 


Temperatures have responded well across the area with breaks in the 
stratus early this afternoon. Wind gusts have increased across the 
southern County Warning Area in response to The Breaks and the tighter pressure 
gradient. Surface high pressure continue to reside over the Middle 
Atlantic States...and northeasterly surface flow continues across 
the area. A few showers have moved onshore and into the sandhills 
south of Fayetteville. This trend should continue into the early 
evening hours. 


Tonight...model soundings are not completely saturated in the lowest 
layers...but given the fact that model soundings would not have 
indicated persistence of the stratus deck that developed this 
morning....and since dew points will likely be every bit as high as 
they were going into last evening...stratus should have no problem 
developing. East to southeasterly surface to 850 flow will enhance 
upslope flow...and areas south and closer to the foothills may 
struggle to break out of the stratus again Friday morning. 
Overnight lows will respond to the stratus...lower 60s in the NE to 
upper 60s south and west. 


Friday...once the clouds to begin to break...strong ridging aloft 
with the 500 mb high parked over the mid-Atlantic...and little to no low 
level convergence should once again hinder most shower development. 
The Atlantic feed that has been present the last couple of days 
brings precipitable waters back toward 1.5" Friday afternoon...and 
spotty showers drifting onshore and through the southern Piedmont 
and sandhills are possible once again. However...because these are 
in part associated with ts Fay...shower activity should continue to 
shift southward overall. Temperatures today have responded well with 
breaks in the stratus...so temperatures in the middle 80s should work. 


Persistence should cause Friday night to be a virtual repeat of 
tonight. Shifting of ts Fay and the parent surface high do little 
alter the moist Atlantic feed over the area..although stratus may be 
focused a bit further south and west. Low temperatures again in the middle 
60s. -Smith 


&& 


Short term /Saturday through Sunday night/... 
as of 245 PM Thursday... 


Saturday and Saturday night: 
the surface and upper level ridge over the area will begin to weaken on 
Sat as a strong upper low currently over the Pacific northwest and British Columbia moves 
east along the far northern Continental U.S. And southern Canada toward the 
upper Great Lakes. Forecast soundings off the 12z GFS/NAM show precipitable water 
values across central NC below normal...between 1.00-1.25" over most 
of the area...perhaps closer to 1.50" in the southeast coastal plain. This 
dry air is due to a strong middle-level subsidence inversion in associated/west 
the upper ridge over the area. Surface dewpoints are expected to mix out 
into the upper 50s to lower 60s during the afternoon hours...except 
in the southeast coastal plain where middle 60s will remain likely. Even in 
the southeast coastal plain...forecast soundings show little or no instability 
due to the warm temperatures aloft. Will therefore keep the forecast dry across 
the entire area...with the better chance for shower activity further 
south in SC. The NAM/GFS do hint at a shortwave or weak north-south 
elongated vortmax moving west (from the atlantic) into central NC 
Sat afternoon...and while this could help spark off isolated convection 
along the coast...it should be just too dry further inland. 
Thicknesses on Sat range from 1390 meters in the morning to 1415 
meters in the afternoon...corresponding to high temperatures in the middle 
80s...assuming full sunshine. There will likely be some cloud 
cover...especially in the morning if low stratus is present. 
However...in the afternoon...only flat diurnal cumulus and some upper 
level outflow from Fay is expected...and temperatures in vicinity of 85-88f 
are expected. Expect a few upper level clouds associated/west Fay 
overnight...otherwise...very seasonable with low temperatures in the 
middle/upper 60s. 


Sunday and Sunday night: 
the surface and upper level ridge over the middle-Atlantic are prognosticated to 
be significantly weakened by Sunday afternoon courtesy of the upper 
low moving east through southern Canada and a building ridge over 
The Rockies and intermountain west. Although the East Coast upper 
ridge will have weakened considerably...the middle-levels should remain 
warm/dry across central NC given weak variable middle-level flow. The 
GFS forecast soundings are in line with this thinking. Some of the NAM 
forecast soundings are a bit odd...however...with precipitable water values surging to 
nearly 2.00" in some locations...including rdu/Fay/rwi. This is 
likely tied to convective precipitation being generated by the NAM over the 
Triangle and coastal plain during the afternoon hours...and I feel 
that the overall environment should still be just too dry to support 
much in the way of convection. Cannot rule out a few isolated 
showers...primarily in the southeast coastal plain...however...coverage 
does not appear to be enough to warrant slight chance probability of precipitation...and the 
forecast will therefore remain dry on Sunday. Given thicknesses slightly 
higher than on Sat...with cloud cover in the form of diurnal cumulus and 
some cirrus...high temperatures should make it into the upper 80s to around 
90f. Sunday night...mid/upper level clouds should increase over 
central NC as a shear axis approaches the middle-Atlantic from the 
northwest and flow aloft over the state becomes more southwest... 
which would start to bring middle/upper level clouds from Fay (or 
perhaps the tropical storm formerly known as fay) toward our area. 
Low temperatures in the middle/upper 60s to around 70f. -Vincent 


&& 


Long term /Saturday through Wednesday/... 
as of 355 am Thursday... 


The middle level longwave trough that progresses eastward across 
eastern Canada through this period helps propel a cold front 
southeast toward NC during the early and middle part of next week. How 
far this front actually gets into our area... however... is all 
dependent on the amplitude of this longwave trough. The 00z/21 GFS 
has a much deeper and farther-south vortex over eastern Canada by 
Tuesday/Wednesday as compared to the 00z/21 European model (ecmwf)... and as a result 
the GFS pushes the front on through by Wednesday whereas the European model (ecmwf) 
holds it up to our north. While the GFS has been consistent the last 
couple of runs... it is still climatologically unlikely that the 
front will drop all the way to our south. Will continue a lean 
toward the European model (ecmwf) direction at this time. This will keep thicknesses 
(and hence temperatures) up near seasonal normals... with quite a 
bit of cloud cover. It still appears likely that whatever moisture 
is left from Fay will work northeastward along or just ahead of the 
front... and if so... we could see decent rain coverage by late 
Tuesday or Wednesday... but obviously the uncertainty with such a 
scenario is pretty high and will trend probability of precipitation to just above 
climatology... starting with slight chance probability of precipitation Monday increasing to 
30-40 percent chances and mostly cloudy skies by midweek. -Gih 


&& 


Aviation /19z Thursday through Monday/... 
as of 145 PM Thursday... 


This afternoon and evening...VFR ceilings and visibilities should finally 
materialize after stratus held for most of the morning across a 
majority of the area. Also...should still see some gusts before 
sunset as mixing finally kicks in. Tonight...expect more stratus 
from the Triangle south and westward. Dont expect the stratus to be 
as thick and persistent into the morning hours...as drier air 
continues to try to push into the area around a surface high off to 
the northeast. However...most terminals will still see a period of 
MVFR or IFR conditions. Once the stratus mixes out Friday 
morning...will see VFR conditions with a light breeze out of the 
northeast. Any showers that develop during the afternoon should be 
confined to the southern Piedmont and sandhills...possibly nearing 
the Fayetteville (fay) terminal. 


Looking ahead to the weekend...some stratus possible again Saturday 
and Sunday morning...although the trend should be for less coverage 
each day. Otherwise...VFR conditions. Moist easterly flow will 
continue...and a few isolated showers may be possible...mainly 
across the southeast. -Smith 


&& 


Rah watches/warnings/advisories... 
none. 


&& 


$$ 


Synopsis...Vincent 
near term...Smith 
short term...Vincent 
long term...Hartfield 
aviation...Smith 

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