656 
fxus62 krah 051828 
afdrah 


Area forecast discussion 
National Weather Service Raleigh NC 
228 PM EDT Sat Jul 5 2008 


Synopsis... 
a moist and unsettled weather pattern will persist over central 
North Carolina through the weekend. Weak high pressure will build in 
late Monday...but moist air remaining in place keeps chances of 
mainly afternoon and evening showers and storms in the forecast for 
much of the upcoming work week. 


&& 


Near term /through Sunday/... 
as of 210 PM Saturday... 


Low stratus has been very persistent across The Triad this afternoon 
ahead of the middle level wave currently over the Tennessee/NC mountains. 
Models never did have a good grasp on this...with the NAM only 
showing some weak isentropic lift on the 300k surface. The persistent 
cloud cover has resulted in a pronounced differential heating 
boundary across the County Warning Area...extending from just west of the Triangle 
southwestward to just south of Charlotte. Areas to the east of the 
boundary have risen into the middle and upper 80s... to even 90 in 
Clinton. To the west of the boundary...temperatures are stuck in the 
middle 70s in The Triad. 


This differential heating boundary should serve as a focus for 
thunderstorm development this afternoon (beginning to see signs of 
this in the southwest piedmont) as the middle level shortwave moves in 
from the west. MLCAPES across central and eastern sections will 
climb to 1500-2000 j/kg. Still anticipating numerous showers and 
thunderstorms to develop late this afternoon and persist into this 
evening. In the northwest Piedmont...will maintain likely probability of precipitation as numerous 
showers are being detected by radar across the foothills. 


Precipitable waters climb to near 2 inches this evening...so any 
heavier thunderstorm activity will be efficient rainfall producers 
and localized flooding is certainly possible. As stated earlier... 
deep layer shear is not impressive...only on the order of 20-25 kts. 
Convection will remain pulse in nature...with locally damaging winds 
the primary severe threat across central and eastern sections of the 
region where the steepest low level lapse rates and highest dcapes 
(around 700 j/kg) are located. 


Convection likely to persist well into the evening as upper level 
divergence increases in response to the upper level trough digging 
south from the Ohio Valley into the Tennessee Valley. Likely probability of precipitation 
will be carried through about 03z...with a chance lingering through 
the night. Expect the overnight hours to be similar to last 
night....where isolated to scattered convection maintained itself 
through sunrise. Lows tonight will be mild and muggy...generally 
around 70 degrees. 


Sunday should be a repeat performance of today with numerous showers 
and thunderstorms breaking out in the afternoon hours. MLCAPES once 
again rise to 1500-2000 j/kg and precipitable waters remain around 2 
inches. Upper level divergence will persist across the region with 
the trough axis oriented north to south across the Ohio and 
Tennessee valleys. If any subtle shortwaves rotate out of the base 
of the trough...they will only serve to further enhance convective 
coverage across central North Carolina. Probability of precipitation once again well into 
the likely range. Persistence for highs... generally middle to upper 
80s. 


Severe potential will be isolated once again due to weak shear. High 
precipitation Waters Point to heavy rainfall and locally damaging wet 
microbursts as the main threats with the stronger storms. 


&& 


Short term /Sunday night through Monday night/... 
as of 130 PM Saturday... 


Sunday night: 
the upper low just to our west over the Tennessee/Ohio valleys is expected to 
begin to weaken Sunday night through Monday morning as it draws 
closer to the southern Appalachians. Even so...there are still 
indications that some shortwave energy rounding the base of the 
upper low will cross from south to north or south-southwest to north-northeast over central 
NC at some point(s) during the night. Additionally...forecast soundings 
show above normal precipitable water values (around 2.00") 
throughout the night. A very moist July atmosphere plus an upper low 
just to the west is more than enough reason to keep in a chance 
(30-40%) of showers/storms throughout the night. Coverage will 
depend on the extent of upper level lift during the night and also 
on any pre-existing convection and associated outflow boundaries 
lingering from afternoon/evening convection on Sunday. Low temperatures 
will be mild...in the upper 60s to lower 70s. 


Monday: 
the upper low is expected to have weakened to an open wave by this 
time...and should be located somewhere along the southern 
Appalachians (western NC/Virginia and WV mountains). Both the 12z NAM/GFS 
show the upper wave entering the foothills or far western Piedmont 
by 00-06z Tuesday morning. Forecast soundings are continuing to show a very 
moist airmass across the entire area...with precipitable water 
values around 1... the lower values in the western 
Piedmont. Given the proximity of the upper level wave to the western 
Piedmont...the better upper level lift during the day Monday may be 
shifted a bit east toward the Triangle/Fay area and coastal plain. 
For this reason (in addition to better moisture/instability in the 
east)...will show the highest chances for showers/storms in the 
eastern half of the area...with relatively lower chances in the 
western portion. Have decided to go with likely probability of precipitation (60%) in the 
coastal plain...tapering back to low/middle chance (30-40%) in the far 
western Piedmont. Low/middle level flow (0-3 km) is 20 knots or less 
Monday afternoon...generally not enough to organize convection...and 
therefore any severe threat should be confined to isolated cells 
with a primary threat of strong gusty winds in downbursts due to 
water loading and evaporative cooling. 


Monday night: 
the upper level wave is expected to further deamplify over the 
middle-Atlantic Monday night as it progresses a bit further east into 
central NC/VA. Tough to say whether or not there will be convection 
after midnight toward sunrise Tuesday...and this will likely depend on 
the location/coverage of convection and any boundaries after sunset 
Monday evening...and the exact position of the upper level wave during 
this time. Will need to carry at least a slight chance of showers 
and thunderstorms across the entire area...and may go with low 
chance (30%) in the eastern half of the area which is more likely to 
be in a better position west/respect to the upper level wave. Will go 
with persistence for low temperatures...ranging from the upper 60s to lower 
70s. -Vincent 


&& 


Long term /Tuesday through Saturday/... 
as of 230 PM Saturday... 


The upper level pattern becomes more unidirectional for the rest of 
the upcoming work week...with very broad troughing across the 
eastern Continental U.S....primarily from the middle-Atlantic / Tennessee Valley and 
points north. Both the GFS/NAM forecast weak Lee surface troughing 
each day across central NC...and forecast soundings show a typical 
early/middle July atmosphere with enough moisture/instability that a 
low chance (30%) of showers/storms each day during the peak-heating 
afternoon/evening hours looks pretty reasonable. Better chances may 
be possible from Wednesday evening through Friday morning when a weak front 
(associated with an upper level low around the northern Great Lakes 
region and a surface low moving into new england) makes its way into the 
Lower Middle-Atlantic and Carolinas. Will advertise 40% probability of precipitation during 
that general time frame. Otherwise...high/low temperatures should be 
near-normal for this time of year. -Vincent 


&& 


Aviation /18z Saturday through Thursday/... 
as of 210 PM Saturday... 


MVFR stratus still hanging on across The Triad terminals. The 
backedge of the low stratus looks to clear the int/gso in the next 
few hours...so likely to see lifting ceilings by 21z. Elsewhere... 
daytime heating has mixed the boundary layer sufficiently to lift 
all ceilings to VFR or at the case at Fay/rwi... scatter them out. As 
the atmosphere destabilizes ahead of a middle level shortwave...expect 
numerous showers and thunderstorms to develop late this afternoon. 
Instead of bogging down the terminals with many hours of thunder in 
tempo groups...will simply go with thunderstorms in the vicinity from this afternoon through 
about 06z. As thunderstorms evolve on the radar scope...will be able 
to fine tune the forecasts at the terminals to better show the 
expected impacts. Any stronger storms will be capable of frequent 
lightning...very heavy rain which will significantly reduce 
visibilities...and locally strong wind gusts. 


Have included MVFR fog in the predawn hours given the expected 
rainfall and very humid conditions. Fog could be even 
worse...depending on how much rainfall each terminal receives. 


Numerous showers and thunderstorms are once again expected Sunday 
afternoon/evening. Have introduced cb's to all terminals beginning 
at 16z tomorrow. 


The threat for diurnal storms will persist into early next week. 


&& 


Rah watches/warnings/advisories... 
none. 


&& 


$$ 


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

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