464 
fxus62 krah 081914 
afdrah 


Area forecast discussion 
National Weather Service Raleigh NC 
313 PM EDT Friday Aug 8 2008 


Synopsis... 
high pressure will bring drier and cooler air to the region through 
the weekend in the wake of a cold front that has moved south of the 
region. 


&& 


Near term /through Saturday/... 
as of 130 PM Friday... 


The cold front has cleared central North Carolina early this 
afternoon. Northerly winds and deep mixing are contributing to a 
drying out of the airmass with dewpoints ranging from the low 50s in 
The Triad to low 60s in the southeast coastal plain. Dewpoints will 
continue to lower this afternoon and most areas should see dewpoints 
in the 50s by late afternoon/early evening. With precipitable waters 
of only three quarters of an inch...low dewpoints...light 
winds...and clear skies...the stage is set for just about as 
comfortable a night as one could hope for in early August. Lows 
should range from about 60 in the north to middle 60s in the sandhills 
and southern coastal plain. A few upper 50s are also possible in the 
typically cooler rural locations. 


Precipitable waters stay under one inch through tomorrow as the deep 
trough over the east continues to provide a west northwest middle and 
upper level flow. Low level thicknesses are prognosticated to be a few 
meters lower than today...suggesting highs one or two degrees 
cooler. Dry adiabatic profiles also support this...therefore will go 
with highs of 87-90 which is very close to climatology. A well mixed 
and dry boundary layer should also help to keep dewpoints in the low 
to middle 50s Saturday afternoon...producing relative humidity values of 30-35 percent. 


The models do advertise another shortwave rounding the base of the 
trough and moving through our area tomorrow afternoon. However...the 
lack of moisture should mean the shortwave only produces some middle 
and high level clouds along with some diurnal cumulus. 


&& 


Short term /Saturday night through Monday night/... 
as of 245 PM Friday... 


This period will be dominated by cyclonic flow aloft in associated/west an 
unusually deep upper level trough over the eastern Continental U.S.. although 
the 500 mb heights in associated/west this trough are 3-4 Standard 
deviations below normal...our high temperatures are expected to remain near 
normal on Sunday/Monday (near normal 1000-850mb thicknesses)...with 
the only noticeable cooling in associated/west low temperatures...which will remain 
in the Lower/Middle 60s due to the drier airmass in place and limited 
cloud cover...particularly on Sun morning. 


Sunday and Sunday night precipitation chances: 
will continue to show a slight chance of showers/storms across the 
entire area on Sunday afternoon in associated/west increasing moisture and 
instability from the south and east as southerly flow strengthens. 
Although precipitable water and surface dewpoint values will be drier than 
normal...the fact that we have cooler-than-normal middle-levels in 
associated/west the deep upper level trough will help to compensate. As for 
lift...it is certainly possible that a shortwave disturbance could 
rotate into the area in deep cyclonic flow aloft...and both the 
GFS/NAM hint at this...in addition to developing a weak surface low or 
modest surface trough across the Carolinas. The better probability of 
seeing convection on sun after/evening will be in the coastal plain...on 
the east side of the surface trough axis or weak surface low. Will also keep 
a slight chance of showers/storms through the overnight period in 
the eastern portion of the area where better elevated instability is 
prognosticated to be and where there will be lift in the form of weak surface 
convergence and potential upper level help in the form of weak 
disturbances in cyclonic flow aloft. 


Monday and Monday night precipitation chances: 
the deep upper trough is prognosticated to begin shifting east/NE during 
the day afternoon...and how far east it shifts will determine 
whether or not we have a chance for convection Monday afternoon. The 
GFS suggests that the upper trough will shift east/NE enough that 
synoptic subsidence on the backside will drop precipitable water values to near 1" 
with warming mid-levels...and no chance for convection. The NAM 
hangs onto the upper trough over the area a bit longer than the 
GFS...and forecast soundings are moist/unstable enough to support a 
slight chance of showers/storms in the afternoon. At this time... 
will try to compromise and will show a slight chance of 
showers/storms confined to the coastal plain only. No convection is 
anticipated Monday night with synoptic subsidence overhead. -Vincent 


&& 


Long term /Tuesday through Friday/... 
as of 300 PM Friday... 


The long term period is expected to be dominated by periods of weak 
cyclonic flow aloft across the mid-Atlantic. Whether or not we have 
significantly increased chances for precipitation by middle-week will be 
determined by the evolution of upper level energy entering the 
Pacific northwest tonight. The GFS/European model (ecmwf) differ in how they handle this 
energy as it breaks down the deep ridge over the upper Midwest and 
central Canada. The GFS/European model (ecmwf) both show a shortwave dropping through 
the central Continental U.S. Toward the middle-Atlantic or southeast states by 
mid-week...and this could definitely help increase our chances for 
precipitation in the Tuesday night through Wednesday night timeframe...depending on 
where the wave tracks. There is currently enough uncertainty that 
will generally skew the forecast towards climatology...with highs in 
the upper 80s to lower 90s and lows in the upper 60s to lower 70s. 
Will also keep precipitation chances near climatology (slight or low chance 
of diurnal showers/storms)...although middle chance probability of precipitation may be 
warranted on Wednesday when the GFS/European model (ecmwf) are showing a shortwave dropping 
very close to the area. -Vincent 


&& 


Aviation /18z Friday through Wednesday/... 
as of 130 PM Friday... 


VFR conditions are expected to prevail through the taf period as 
weak high pressure builds into the area in the wake of the front. 
Northerly winds will become light overnight and veer to the 
northeast tomorrow but remaining just around 5 knots. Scattered diurnal 
cumulus will dissipate this evening leaving clear skies tonight. 
More diurnal cumulus is possible tomorrow. 


VFR conditions will prevail through early next week as a deep trough 
continues to encompass the eastern states. Several disturbances will 
likely move through the trough but with limited moisture across 
central North Carolina...the only impact will likely be periods of 
higher cloud cover. 


&& 


Rah watches/warnings/advisories... 
none. 


&& 


$$ 


Synopsis...jfb 
near term...jfb 
short term...Vincent 
long term...Vincent 
aviation...jfb 

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