126 
fxus62 krah 181903 
afdrah 


Area forecast discussion 
National Weather Service Raleigh NC 
301 PM EDT Friday Jul 18 2008 


Synopsis... 
surface high pressure centered over southern Virginia will drift 
east and offshore through tonight. Meanwhile... an area of low 
pressure just off the Georgia coast... which has the potential to 
become a tropical or sub-tropical depression... will drift slowly up 
along the southeast coast through the weekend. 


&& 


Near term /through Saturday/... 
as of 215 PM Friday... 


Tonight: most shower and thunderstorm coverage should diminish this 
evening with loss of daytime heating. However... continued southeast 
flow and resultant frictional convergence along the coast... coupled 
with precipitable water values rising to between 1.5 and 2 inches... 
could result in the development of a few additional showers which 
would be advected our way overnight. Low stratus and fog will also 
develop northwestward later tonight as strengthening and 
increasingly moist low level easterly flow off the Atlantic develops 
ahead of the approaching low. Lows in the 66 to 72 from northwest to southeast. 


Saturday: the low pressure system off the Georgia coast is expected 
to drift slowly north or northeastward along the coastal Carolinas. 
Favor a somewhat eastward solution of the NAM/WRF... since a weak 
upper level trough/shear axis which settled across central and 
eastern NC this afternoon should gradually turn the low toward a 
more northeasterly direction. However... increasing low and deep 
layer moisture combined with the approach of an inverted surface to 
850 mb trough into the central Piedmont should support scattered to 
numerous showers and isolated thunderstorms with tropical downpours 
Saturday afternoon... with the greatest coverage east of Interstate 
95. A residual Continental air mass associated with the dry air 
surface ridge that extends into western NC this afternoon... as well 
as large scale subsidence on the northwest side of the low may keep 
The Triad and extreme western Piedmont relatively sunny and dry. 
High temperatures should range from about 90 west to middle 80 east 
(where clouds and precipitation coverage will be greatest). 


&& 


Short term /Saturday night through Monday night/... 
as of 220 PM Friday... 


For Saturday night through sunday: the characteristics... 
evolution... and track of the low pressure center east of Savannah 
are the big question Marks as we head through the latter half of the 
weekend. Please see latest guidance from the National Hurricane 
Center concerning any possible tropical development of this system. 
With the westerlies well displaced to the north... and no clear 
trend with markedly rising or falling heights with the middle level 
ridging over the offshore western Atlantic... the steering of this 
low is liable to be very weak and subject to the whims of subtle 
features that are difficult to detect at this time range. The latest 
NAM... which tracks the low up the warm waters of the Gulf Stream... 
seems more reasonable than the GFS which tracks it up through 
eastern NC Sunday... and which seems to absorb the low too quickly 
into the northern stream synoptic storm system tracking along I-80. 
The 00z/18 European model (ecmwf) supports a track just off the coast as well. But... 
it must be noted that if the Atlantic ridge does strengthen a bit... 
it could easily shove the low to a more westward track. Either 
way... the precipitation pattern noted on the models and that typically 
seen with such systems with nearshore tracks would favor a chance of 
moderate showers and isolated storms over the eastern sections of 
the County Warning Area Saturday night into Sunday... with lesser to no chance well 
inland over the far western Piedmont... where precipitable water 
values are likely to be near or below normal. Also factoring against 
precipitation chances in the western County Warning Area are the middle level ridging nosing 
in from the west-southwest as well as light diffluent and downslope 850 mb 
flow... likely to steepen low level lapse rates yet dry the boundary 
layer. Expect mild lows of 69-74... then highs Sunday of 91-96 with 
the warmer readings in the western County Warning Area beneath subsidence and plenty 
of sunshine. 


For Sunday night through Monday night: shortwave troughing drops 
southeast through the northern Great Lakes into the St Lawrence 
Valley Sunday night/Monday... which should provide just enough 
impetus to kick the low to the east-northeast away from NC. With a 
strengthening downslope low level component and the 500 mb ridging 
still nosing in from the west-southwest... we should see precipitation chances end 
rather quickly in the east Sunday evening... with decreasing clouds. 
Lows should be very warm... 70-75. Thicknesses rise considerably 
over NC Monday in the wake of the exiting storm... climbing from the 
1420s in the morning into the 1440s... with 850 mb temperatures of 19-20c. 
Model consensus indicates precipitable water near or just below 
normal in central NC (slightly above toward the coast)... with deep 
northwest flow... diffluent in the middle levels to our northwest. Forecast soundings 
show very steep low level lapse rates and a deep mixed layer likely 
to exceed 10000 feet. While this seems to indicate dry and hot weather 
for central NC Monday... the concern here is for any strong 
mesoscale convective system that develops upstream along a front 
across Iowa/IL/in/srn Ohio late Sunday/Monday... and the path of any 
resultant mcvs. The NAM indicates probable mesoscale convective system development over the 
Ohio Valley and the middle level steering winds support a path into NC... 
although the incipient dry air and warm middle levels could kill any 
mesoscale convective vortex that approaches... and forecast surface convective available potential energy struggle to reach 
1000 j/kg east of I-95. However... considering the potential for 
enhanced surface gusts with any convection that can get past the middle 
level cap and potential mesoscale triggers... will leave in an 
isolated storm risk... higher in the southeast County Warning Area compared to the northwest 
County Warning Area... very late in the day into the evening. Highs 95-99 are in 
line with projected thicknesses and statistical guidance. -Gih 


&& 


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


The westerlies have their closest brush with NC early in the medium 
range. Shortwave energy tracks along the southern Great Lakes 
Tuesday... around the south side of the vortex over central Quebec. 
The main surface front will hold to our north... along the zone of 
stronger middle level west winds across the Ohio Valley and Virginia... but a 
leeside surface trough will persist through central NC. Forecast 
soundings show better instability compared to Monday with cooler middle 
level temperatures... but surface convective available potential energy are still only marginally unstable 
along/east of I-95... likely due to low surface dew points in the 
western County Warning Area. Will hold onto chance probability of precipitation... mainly afternoon/evening 
and mainly east... Tuesday through Wednesday. The GFS takes the 
Quebec low east and eases the trailing trough axis slightly to the 
southeast... enough to push the surface front even closer to the 
NC/Virginia border by late week... but at the same time the western 
Atlantic ridge builds into the southeast coast Wednesday/Thursday... 
which would favor holding the front north. Will continue mainly 
diurnal shower/storm chances Wednesday/Thursday with better chances 
in the north and West County warning area... closer to the front and higher terrain. 
Low level thicknesses are likely to be just above normal... yielding 
temperatures about a category above normal each day. -Gih 


&& 


Aviation /18z Friday through Wednesday/... 
as of 145 PM Friday... 


VFR conditions should prevail through early tonight at all sites but 
kfay and perhaps krwi. At those two sites... a more tropical-like 
air mass has spread onshore and will promote continued development 
of scattered heavy showers and isolated thunderstorms with gusty 
winds and brief MVFR/IFR ceilings and visibilities. A more 
Continental and capped air mass resides across the NC Piedmont... 
and this should maintain dry conditions from krdu westward for the 
remainder of the afternoon. By this evening... however... a leftover 
shower from the the activity to the southeast could make it as far 
west as the krdu vicinity. 


Then... strengthening and increasingly moist low level easterly flow 
off the Atlantic will develop tonight... as a low pressure system 
just offshore the Georgia coast drifts northward. This will promote 
the inland development of mostly IFR/MVFR stratus and fog from 
southeast to northwest late tonight. The low stratus and fog will 
gradually lift and erode Saturday morning... but MVFR cumulus cloud 
bases could persist well into Saturday afternoon at eastern 
terminals. More scattered showers and thunderstorms with tropical 
downpours are expected Saturday afternoon... mainly from krdu 
eastward... as the low pressure system to our south spreads moist 
and unstable air inland. 


Looking ahead... additional morning low stratus/fog and afternoon 
showers and thunderstorms are expected to develop at eastern 
terminals Sunday... where rich low level moisture is expected to 
linger. Generally VFR conditions are then expected Sunday night 
through the middle of the upcoming week... as the offshore low moves 
out to sea and high pressure ridges into our region from the west. 
-Mws 


&& 


Rah watches/warnings/advisories... 
none. 


&& 


$$ 
Synopsis...mws 
near term...mws 
short term...Hartfield 
long term...Hartfield 
aviation...mws 

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