Weather

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fxus62 krah 300011 
afdrah 


Area forecast discussion 
National Weather Service Raleigh NC 
810 PM EDT Friday Aug 29 2008 


Synopsis...a weak surface trough of low pressure will linger across 
eastern North Carolina through Saturday... as an upper level 
disturbance spins over the Carolinas. A back door cold front will 
drop south through the state on Sunday. 


&& 


Near term /through Saturday/... 


..updated at 430 PM... 


The latest surface analysis indicated a surface trough or wind shift 
line was located over the southern sections of our region... 
roughly stretching west to east from Rockingham and Laurinburg to 
Fayetteville... Smithfield and Goldsboro. The highest instability 
and surface moisture convergence was located along this boundary. 
Radar trends were showing scattered thunderstorms with heavy rain 
developing along and just north of the surface boundary. With light 
steering flow... heavy rain under the strongest storms will be the 
main weather hazard... with 1.5 to 2 inch hourly rates expected. 


We will update the forecast to place the highest pop across this 
region for the late afternoon and early evening... with isolated 
convection possible northward through the coastal plain. There was 
little support for convection to the west and north. Updated grids 
and forecasts have been sent. -Badgett 


Previous discussion... as of 230 PM Friday... 


The morning clouds have been slow to lift and break... not 
surprising given the weak wind field in the lowest 4 km as seen on 
morning soundings and amdar aircraft soundings. The 12z gso sounding 
was strongly capped with near normal precipitable water... whereas 
the 12z mhx sounding exhibited a much weaker cap and greater 
moisture with precipitable water values nearly 130% of normal... and sure enough the 
isolated showers and storms have tended to form this afternoon over 
the eastern County Warning Area where mixed layer cape has risen to 1000-1500 j/kg. 
Will hold onto a chance of a shower or storm over the east half 
through early evening... then the column should quickly stabilize 
leading to minimal rain chances after midnight. Overall moisture 
beneath the warm layer aloft should remain high with continued precipitable water 
above 1.5 inches over the central and eastern counties... so would 
not be surprised to see a shower or two overnight... but coverage 
shouldn't be enough to warrant a pop. Areas of fog are likely to 
develop overnight... especially in the high-dew point air along and 
east of Highway 1. Some of this could be locally dense. Uncertainty 
about the location of the most dense fog precludes issuing a dense 
fog advisory at this time... but we may indeed need an advisory 
tonight. Expect lows similar to last night... 68-73. 


The upper level vortex noted on water vapor imagery over upstate SC 
will wobble eastward over the region late tonight through Saturday. 
While this will serve to largely erode the warm cap aloft... the 
very weak wind field will provide little in the way of dynamic 
forcing for ascent as it approaches. The cooling temperatures aloft 
should increase instability by the afternoon... and given the 
projected precipitable water nearly 150% of normal... we should see convection 
firing earlier tomorrow than we've seen today. Will go with slight 
chance probability of precipitation west and chance probability of precipitation east. Severe weather doesn't look 
like a big threat with marginal middle level lapse rates and virtually 
zero deep layer shear... but cape could approach moderate levels in 
the east... and isolated briefly strong cells are possible. Expect 
highs of 88-92... not far from today's highs given the similar air 
mass. 


&& 


Short term /Saturday night through Sunday night/... 
as of 230 PM Friday... 
a few outflow driven convective showers could persist into the late 
evening/overnight hours Saturday night as airmass remains marginally 
moist and unstable with precipitable waters  between 1.6 to 1.8" and sbcapes 
generally between 500 to 1000 j/kg. However...low level 850 flow 
looks to be large diffluent initially across the western 
Piedmont...before models indicate some weak low-level convergence 
late as short wave trough pushes east through coastal sections. 


Model depicted lifted indice's indicate back-door cold front passage across the 
forecast area between 18 to 00z Sunday...as stable NE low-level 
surge spreads south. Favorable diurnal timing as baroclinic zone 
moves through the area combined with weak upper jet entrance 
divergence along the southern tail of exiting long wave trough off 
the northeast coast should provide sufficient lift as moisture 
remains slight above normal and MLCAPES hover around 1000 to 1500 
j/kg across all of central NC. Will keep a 20 to 30 percent probability of precipitation for 
widely scattered showers and isolated thunder. 


Extensive cloudiness Saturday night will make for mild min 
temperatures...lows in the upper 60s to lower 70s. Highs Sunday in 
the middle 80s north to around 90 south. Cooler Sunday night as drier 
air spreads into the area. Mins Monday morning in the middle to upper 
60s...with some lowers 60s along the NC/Virginia line. 


&& 


Long term /Monday through Friday/... 
as 230 PM Friday... 
sprawling subtropical ridge over the Middle-Atlantic States will 
dominate the long term period. The period will be characterized 
by below normal precipitable waters  with limited instability as middle-levels remain 
warm and dry. Thus...dry through the period. The above normal 
heights translate to 3 to 5 degrees above the 30 year climatology 
..with daytime highs in the middle to upper 80s and overnight lows in 
the lower to middle 60s. 


As far as the tropics...little has changed since yesterday 
concerning Gustav and Hanna. There is greater confidence today that 
Hanna will most likely not get picked up by the long wave 
trough/closed low exiting the northeast coast on Sunday. 
So...the eventual paths/tracks of both tropical cyclone Gustav and 
Hanna...will be greatly influenced by the aforementioned strong 
subtropical ridge...including it's southern/southwestern extent and 
eventual demise. Due to discrepancies in these details... 
uncertainty increases after 48 hours with both storms. 
All interests along the northern Gulf...from Texas to the Florida 
coast will need to closely monitor the latest official forecasts 
from the NHC at www.NHC.NOAA.Gov. 


&& 


Aviation /00z Saturday through Wednesday/... 
as of 800 PM Friday... 


A lingering surface trough and inland moving sea breeze in the low 
levels and the proximity of an upper low centered near kgsp 
triggered showers and thunderstorms over the coastal plain and 
sandhills of NC this afternoon. Some of these showers and storms will 
continue to affect mainly the Fayetteville vicinity through about 
02z this evening. Aviators will may have to bypass more isolated 
thunderstorms on approach to krdu and krwi... but none are expected 
to occur within a 10 mile radius of either of those airports at this 
time. A Stout middle level subsidence inversion should suppress any 
convective development over the western half of the state... 
including the kgso and kint vicinity. 


Then expect visibilities to lower to IFR/LIFR in fog after 
midnight... lowest from krdu eastward where surface dewpoints are 
higher. Confidence in dense fog is lower at int/gso where dew points 
are lower... but wet soil from recent rains and calm winds favor at 
least a brief period of IFR visibilities even there around daybreak 
Saturday. Ceilings may also become IFR-LIFR due mainly to the fog. The 
fog will begin to lift and burn off in the 13z-15z time frame... 
which may result in a period of scattered to broken 
stratus/stratocumulus at that time as the fog layer lifts. A shear 
axis aloft associated with the upper low centered near gsp this 
evening will drift across our area Saturday afternoon. Weak forcing 
for ascent from this feature... in conjunction with surface convergence 
along a lingering surface trough... will promote the development of 
isolated to scattered showers and storms Saturday afternoon... 
mainly at rdu/rwi/Fay. Fog and low clouds could bring LIFR 
conditions Sunday morning and again Monday morning... and a few more 
storms are expected mainly at Fay Sunday afternoon. Expect VFR 
conditions midday Monday through Wednesday as surface high pressure 
noses in from the north. -Mws/gih 


&& 


Rah watches/warnings/advisories... 
none. 


&& 


$$ 
Synopsis...cbl 
near term...Hartfield/Badgett 
short term...cbl 
long term...cbl 
aviation...mws/Hartfield 

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