Weather

177 
fxus62 krah 110704 
afdrah 


Area forecast discussion 
National Weather Service Raleigh NC 
300 am EDT Sat Oct 11 2008 


Synopsis... 


An upper level low pressure system will linger near the South 
Carolina coast early today. Surface high pressure will build south 
down the Atlantic Seaboard from New England later today and 
tonight...bringing drier air to the region Saturday evening into 
Sunday. Temperatures will gradually warm to well above normal during 
the early part of next week. 


&& 


Near term /through tonight/... 
as of 245 am Saturday... 


00z radiosonde observation information showed considerable moisture at 700mb and 
850mb into central North Carolina...although the low-level 
influence of high pressure extending south from the Great Lakes 
was evident in surface dew points only in the 50s in the northwest 
Piedmont...along with an apparent reduction in low clouds on the 
first Post eclipse fog product satellite image. On the last water 
vapor image...the upper low was apparently moving back toward the 
west...and this motion should continue today as the surface high 
continues to build south...reducing the definition of the inverted 
trough off of the Carolina coast. As ridging continues to build 
south...mean drier air will continue to filter south as well... 
with precipitable water values at or below an inch by 18z along 
and north of about U.S. 64...and below an inch throughout central 
North Carolina by 06z. Good upper diffluence...best this morning 
south...gradually wanes as well although the shear axis aloft will 
remain over or very close to southern zones throughout the day. 


Probability of precipitation will be fairly consistent with the ongoing radar trends... 
from likely in the far south tapering quickly to below slight 
chance from about U.S. 64 north. While NAM bufr soundings suggest 
there may be enough middle- and lower-level moisture for a sprinkle 
or patch of drizzle early this morning north...chances for this 
should diminish quickly after sunrise due to increased ridging and 
drying from the north. Will mention a slight chance of thunder in 
the far south for a brief period this afternoon as the GFS and the 
NAM provide for lifted indices near or just below zero with the 
NAM more aggressive with cape...to around 500j/kg toward kfay and 
kctz diminishing quickly by 00z with boundary layer drying and 
middle-level warming. With even modest mixing...20-30kt 925mb winds 
should allow for a few gusts above 20mph and possibly approaching 
25mph this afternoon. With mainly high clouds northwest and a 
cooler start versus lower clouds and a warmer start southeast... 
expecting upper 70s as a rule for maxes this afternoon. 


Tonight...aside from a small chance of a shower in the very far 
south this evening...overall moisture in the atmosphere continues 
to dwindle with 850mb Theta-E troughing easily in place by 06z 
with good subsidence. NAM bufr soundings and some of the 
statistical guidance suggest there could be areas of low clouds 
late especially along and east of roughly U.S. 1. Despite drier 
air moving in...winds should stay up enough to keep low 
temperatures in the 50s in the majority of locations. 


&& 


Short term /Sunday through Monday night/... 
as of 245 am Saturday... 


High pressure continues to build in Sunday...and though the low- 
level gradient should relax a little with 925mb winds diminishing 
to 15-25kt...there still could be a few gusts to around 20mph 
during the late morning and early afternoon. 1000-850mb 
thicknesses...very similar on the NAM and GFS...suggest maxes 
about a degree or two cooler than readings expected today except 
toward kfay where there should be more sun on Sunday. The NAM bufr 
soundings exhibit a period of modest instability toward the coast 
Sunday afternoon...which with a slight increase in low-level 
moisture on the northeast and east flow could lead to an isolated 
shower mainly east and southeast of the central North Carolina 
coastal plain if this verifies. Probability of precipitation will be kept below slight 
chance in central North Carolina but non-zero particularly east of 
U.S. 1. Otherwise...as the gradient continues to relax look for at 
least a few Sunday lows in the 40s mainly north with highs near 80 
Monday under a lighter wind with high pressure directly overhead. 
Continued dry with few clouds Monday night and lows then in the 
lower to middle 50s as a rule. 


&& 


Long term /Tuesday through Friday/... 
as of 300 am Saturday... 


For Tuesday/wednesday: strong middle level ridging holds over the 
interior southeast into midweek with a quickly modifying surface 
ridging overhead. The warm spell is likely to peak during this time 
frame... as low level thicknesses climb to roughly 20-25 M above 
normal. Global ensemble forecasts show 850 mb temperatures around 14c 
Tuesday... 1-2 Standard deviations above normal... and these values 
correlate very closely to conditions on this date in 1985... when 
record highs were set (see climate section below). Will hold highs 
Tuesday about a category below records however... as guidance could 
waver considerably one way or the other this far out in the 
forecast. But highs will be nudged upward... to 81-85... with 
minimal cloud cover and no probability of precipitation. The middle level ridge begins to break 
down a bit Wednesday as the powerful storm system now over the 
western U.S. Begins to lift out. Gefs 850 mb temperatures slip to 12-13c by 
Wednesday... although this is still quite warm... and European model (ecmwf)/GFS 
consensus thicknesses remain much above normal. We should see a 
minor increase in clouds Wednesday which should take the edge off 
temperatures just a bit... and will have highs 80-84... still a few degrees 
below record territory. 


And for Thursday/friday: as the North American mean flow transitions 
to a less amplified... flatter pattern... timing of forecast 
features becomes more difficult and prone to error. The European model (ecmwf)/GFS 
differ on the speed and significance of a front (the reflection of 
the aforementioned system lifting through the northern states) that 
approaches from the west. The GFS is faster with a generally dry 
Wednesday night frontal passage ... whereas the European model (ecmwf) keeps US very 
warm and holds this front to our northwest in favor of a stronger wave 
zipping through the Pacific northwest Wednesday night which amplifies in 
the eastern U.S. By next Saturday. With so much ambiguity... will 
keep the forecast largely as is... with above normal temperatures and probability of precipitation 
held below 15%. -Gih 




&& 


Aviation /06z Saturday through Wednesday/... 
as of 130 am Saturday... 


The low pressure trough along the Carolina coastline... associated 
with an upper level closed low just east of sav... is helping to 
keep a steady influx of low level moisture from the Atlantic into 
NC. This is particularly evident across eastern and southern NC... 
including rwi and Fay... where patchy sprinkles and areas of rain 
persist... along with MVFR to IFR ceilings. These conditions will 
continue at these sites well through Saturday morning... extending 
to the lunch hour... as this moist Atlantic inflow holds. Farther to 
the northwest... based on the latest pre-eclipse satellite imagery... rdu 
is at the outer edge of the shield of low ceilings... so sporadic MVFR 
ceilings are expected there into the morning. At int/gso however... due 
to the high pressure area nosing in from the north and the resultant 
drier air pushing in... the MVFR clouds have greatly eroded in 
recent hours and should continue to do so... thus only patchy clouds 
at 1800-3000 feet are expected at int/gso with no ceilings through 
morning. As the northeast breeze at roughly 5-10 kts keeps the low 
levels mixed... visibilities restrictions should not be a big problem... 
and only periodic brief MVFR fog within sprinkles/light rain is 
expected at rwi/Fay up to sunrise. But of concern at rdu/Fay/rwi 
remains the potential for low level wind shear with east-northeast winds at 
30-35 kts as low as 1500 feet above ground level... while surface winds hold at 7-13 
kts. Will hold onto a low level wind shear risk in these taf sites through sunrise. 


Starting at middle morning the low levels will mix sufficiently to end 
the low level wind shear threat... but this will result in periodic northeasterly 
gusts that will last through the afternoon. The prevailing winds and 
gusts will be strongest at Fay/rwi/rdu... and lesser so at int/gso 
which will be farther removed from the coastal trough. As the high 
pressure area slowly builds southward into Virginia and NC... any clouds 
below 3000 feet will dissipate at int/gso/rdu by middle morning and at 
rwi by late morning... however improvement will be slower at Fay 
where MVFR ceilings should hold into middle afternoon. VFR conditions will 
then hold at all central NC taf sites through the end of the valid 
period. However there will be a renewed risk of low level wind shear 
beginning near the end of this valid period tonight at all sites but 
especially rwi/Fay/rdu. 


Looking beyond late tonight... shallow MVFR-IFR fog is possible late 
tonight into Sunday morning and again early Monday morning. 
Otherwise... VFR conditions will prevail. -Gih 


&& 
climate... 


Record highs for Tuesday October 14: 
rdu - 86 (1985) 
gso - 85 (1975) 


Record highs for Wednesday October 15: 
rdu - 88 (1985) 
gso - 87 (1985) 


&& 


Rah watches/warnings/advisories... 
none. 


&& 


$$ 
Synopsis...Hartfield 
near term...djf 
short term...djf 
long term...Hartfield 
aviation...Hartfield 
climate...Hartfield 

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