761 
fxus62 krah 061909 
afdrah 


Area forecast discussion 
National Weather Service Raleigh NC 
309 PM EDT Monday Oct 6 2008 


Synopsis... 
strong high pressure will build into the region from the north 
tonight... pushing a dry cold front southward through the area. 
Much cooler temperatures are expected behind the front for 
Tuesday through Wednesday. A storm system will approach from the 
west and bring our next chance of rain starting Wednesday night. 


&& 


Near term /through Tuesday/... 
as of 220 PM Monday... 


The latest surface analysis confirm that the dry cold front has made 
very slow progress southward through Virginia so far today. However given 
the strength of the surface high behind it... greater than 1032 
mb... and the confluent flow aloft over the middle Atlantic coastal 
region supporting its southward extension... the front should make 
steady southward progress overnight into Tuesday morning. The 
primary effects of the front will be a slight up tick of northeast 
winds as well as scattered to broken clouds just ahead of and along 
the front late tonight into Tuesday morning. The areal extent of 
clouds and fog tonight is uncertain as the NAM/GFS bufr soundings 
show differing amounts of low level moisture. However both agree on 
a pooling of higher 1000-850 mb moisture especially over the northwest 
Piedmont overnight as boundary layer winds become more easterly with 
moistening parcels being forced up-terrain and with increasing (yet 
still weak) isentropic upglide. Will continue the earlier trend of 
increasing clouds tonight especially in the northwest County Warning Area. With these 
clouds balancing late-night cool air advection and lowering dew 
points... expect lows from 51-57. The morning clouds should be last 
to break over the northwest County Warning Area given the upslope-directed low level 
flow... and have held onto mostly cloudy skies there through Tuesday 
morning. Low level thicknesses plummet into the 1360-1365 M range by 
tomorrow... which yields highs of 69-76. -Gih 


&& 


Short term /Tuesday night through Thursday night/... 
as of 300 PM Monday... 




Difficult forecast this period with nwp guidance providing several 
possible solutions. Guidance is in decent agreement on Tuesday night 
as short wave riding builds into the Carolinas as trough deepens across 
the MS valley. Should see high clouds increase on Tuesday night with 
lows in the upper 40s across the north coastal plain and the lower 50s 
in the western Piedmont. 


On Wednesday guidance begins to diverge with GFS depicting a 
stronger wave moving across the Midwest that closes off early Wednesday 
while the European model (ecmwf) is slower and weaker...waiting until Wednesday evening before 
closing off. 12z Canadian and NAM-WRF area also slower and a bit 
weaker with the upper low. All of the guidance suggest that best 
forcing and moisture should be located over central NC late Wednesday 
night and especially Thursday. Have adjusted probability of precipitation upward and focused 
them a little further S with likely probability of precipitation now included for much of 
the County Warning Area. 


Several details are Worth noting. The 12z GFS is notably stronger 
in the low level flow especially at 850 where a southeast flow reaches 35 kts on 
Thursday morning. This enhanced flow is coincident with a bullseye of 
precipitation that moves out of the Gulf. Model generated 
convective precipitation is only responsible for around a quarter of this 
precipitation area. Have downplayed precipitation amounts during this period as 
this feature appears anomalous. NAM shows a similar feature 
although it is of a weaker magnitude. Expect to see some weak 
in situ damming as precipitation moves into a stable airmass across 
interior portions of the Piedmont. Will lower temperatures in cad favored 
areas on Thursday and hold onto cad feature through Sat am. Some 
concern exists over the potential impact of upstream convection 
across the northern Gulf Coast on Wednesday into Thursday. With a 
variety of scenarios possible it is difficult to conclude 
definitively but at least a modest Atlantic onshore flow is 
expected. Studies have shown the onshore flow can mitigate the 
impact of upstream convection. We've trimmed back quantitative precipitation forecast amounts and 
probability of precipitation slightly in The Triad. -Blaes 




&& 


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


Not a great deal of confidence in the extended forecast as well. 
Sensible weather details at the beginning of the extended period 
will largely depend on the evolution of the upper low that lifts 
across the Carolinas. The GFS and 00z Canadian are progressive 
with the upper low and move it northeast into the middle Atlantic 
away from the area. The European model (ecmwf) and the 12z Canadian move the upper 
low slowly southeast and off the Georgia/SC coast on Friday. While the 00z UK 
moves the upper low progressively and off the Georgia coast by Thursday 
night. 


HPC is leaning toward the European model (ecmwf) which would bring the best chance 
of precipitation across central NC on Wednesday night and Thursday. 
Moisture...clouds and the threat of precipitation lingers into Friday 
night. Temperatures on Friday should still be modulated by a lingering 
in situ damming event. If European model (ecmwf) verifies then a longer then 
advertised period of clouds and showers may continue into Saturday 
and Sunday. With various scenarios in play will opt to play up 
clouds on Sat but keep mention of precipitation out the forecast. Heights 
should build eventually after the system departs with some 
clearing and moderating temperatures. -Blaes 




&& 


Aviation /18z Monday through Saturday/... 
as of 200 PM Monday... 


VFR conditions will continue through this evening at all sites with 
a very light wind from the NE... few clouds if any... and no visibility 
restrictions. Then after midnight however... conditions could drop 
to MVFR to IFR... as a dry cold front drops southward into the County Warning Area. 
Forecast confidence regarding the details of ceilings and visibilities is 
low... due to the models' differences in the amount of low level 
moisture that pools near and along the front late tonight into 
Tuesday morning. The front is expected to first pass through rwi/rdu 
around midnight... gso/int shortly after midnight... and Fay toward 
sunrise. Looking upstream... patches of LIFR stratus/fog as well as 
areas of VFR stratocumulus were noted this morning over 
VA/MD/Delmarva... and these are likely to occur late tonight over 
central NC as well... although exactly what locations will see the 
low stratus/fog is difficult to determine. Current expectations are 
for MVFR ceilings to develop near 08z-09z at all sites except rwi/Fay 
where a period of IFR ceilings may occur. Aviation interests keep a 
close eye on the latest observations for inclement aviation 
conditions in and near central NC taf sites late tonight into 
Tuesday morning. Any low stratus should be shallow enough to mix out 
before 14z... when VFR conditions will return. Winds will remain 
from the NE... increasing to around 6-9 kts Tuesday morning. 


Looking beyond early Tuesday afternoon... IFR stratus is possible 
mainly at gso/int late Tuesday night into early Wednesday morning. 
Then... IFR/LIFR ceilings/visibilities are expected to develop across central 
NC starting after midnight Wednesday night... and these are likely 
to hold through much of Thursday and Friday. VFR conditions should 
return on Saturday. -Gih 
&& 


Rah watches/warnings/advisories... 
none. 


&& 


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

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