246 
fxus61 krnk 030832 
afdrnk 


Area forecast discussion 
National Weather Service Blacksburg Virginia 
332 am EST Wednesday Dec 3 2008 


Synopsis... 
high pressure will move east and off the coast this afternoon. A 
strong cold front will then move through the area on Thursday. 
Cold high pressure will build back into the area Friday. 


&& 


Near term /through tonight/... 
500 mb ridge will move across the region today. Surface high center will 
move east and exit off the coast by this afternoon. This will 
result in a southwest flow across our County Warning Area. A cold front will 
approach from the west this afternoon and tighten the gradient. 
Play high temperatures close to mav guidance with sunshine. 


The cold front will be entering the western Ohio Valley 
tonight and marching east on Thursday. Both GFS and NAM were slower 
than previous model runs so delayed the timing of probability of precipitation tonight. Moisture 
from the Gulf is expected to surge up toward the Tennessee Valley ahead of 
this system and approach the mountains by daybreak. Warm air advection 
will be crossing the area tonight ahead of the front. With 
increasing clouds and southwest flow...played lows closer to warm 
MOS values. If winds decouple and clouds are slower to arrive 
some of the colder valley could dip below freezing...creating a 
small potential for freezing rain in the coldest valleys around 
daybreak. 


&& 


Short term /Thursday through Saturday night/... 
12z Thursday...surface cold front is prognosticated to be near an heights-Lex 
line across eastern Kentucky. Models still show strong jet dynamics 
remaining in the Ohio- tenn valley. The Gulf is open ahead of the 
front...and the strong moisture surge would argue for very high 
probability of precipitation in the mountains Thursday morning. However...instability along the Gulf 
Coast and likely thunderstorms may cut off the moisture flow right about 
the time the front is moving into the applns. In any event..I have 
gone with the high probability of precipitation...likely to categorical...thinking that 
there will at least be measurable precipitation in the morning in the 
mountains by 18z Thursday...the northern stream short wave is lifting northeast 
and the front is becoming parallel to the upper flow...so slows 
down as it moves into the foothills and Piedmont. In 
addition...there is still some weak southern stream energy moving into 
the southern applns between 18 and 00z...so I think the southern end of the 
front may still have a lot of precipitation even as it moves into the 
foothills. I have slowed down the decrease in probability of precipitation in the 
afternoon...especially in the southwest. Finally...the colder air 
does not work into the western sections of the County Warning Area until late 
morning/early afternoon...so I have slowed down any changeover to 
snow in the higher elevations. Still could have some snow 
accumulation sin the afternoon- mainly higher elevations of southeast WV. 
Will be a large spread in high temperatures Thursday...with clouds and precipitation 
in the mountains and warm air/southwesterly flow in the Piedmont. 


Does not look like there will be much in the way of upslope snow 
showers behind the front Thursday night. The trajectory of the low 
level flow behind the front is not off the Great Lakes...and 
models do not show much low level moisture. Cold advection is also 
a little weaker than earlier forecast...so I have raised the min temperatures 
a few degrees. 


Upper trough slowly deepens and sags southward Friday...but with 
no discrete short waves in the upper flow- just channeled 
vorticity. In addition...the low level flow backs more to the west 
and there is still no low level moisture. There may be some 
stratocu in the mountains...but not much chance of snow showers. Will have 
cold advection all day Friday...so will be a chilly 
day...especially in the mountains where I keep highs in the middle 30s or 
lower. This dry cold advection pattern continues Friday night. I 
stayed close to guidance for low temperatures. 


Models have a clipper short wave moving rapidly through the trough 
into the southern applns by Sat night. This has the potential to spread 
light snow into the mountains by Sat morning...and even further east 
Sat afternoon or evening. Strong cold advection behind this 
clipper late Sat night into Sunday...with a good trajectory off 
the Great Lakes...should be good for upslope snow showers in the 
mountains 


&& 


Long term /Sunday through Tuesday/... 
fast moving clipper system slips through the area during the 
weekend with limited precipitation...mainly over the west. This will be 
followed by another cool shot as high pressure drops south into 
the Midwest. This high will move east overhead Monday with 
moderating temperatures under a zonal flow aloft. Next system poised to 
bring a chance of wintry precipitation to the area Tuesday. 


&& 


Aviation /08z Wednesday through Sunday/... 
VFR conditions expected today at all taf sites. Winds will come 
around to the south this afternoon once the surface high 
pressure slides east. A few gusts of 15-20 knots are possible along 
the ridges and even the Piedmont. 


Strong cold front will move through the region Thursday morning 
into early afternoon. This will likely bring poor flight 
conditions to blf and lwb late Wednesday night and Thursday. 
Further east...there is a lower chance of poor flight conditions. 
Much colder air coming in behind this front will result in upslope 
-shsn for southeast West Virginia Thursday night. VFR should 
return Friday into Saturday...as high pressure builds in from the 
west and most moisture lifts back to the north. 


&& 


Rnk watches/warnings/advisories... 
Virginia...none. 
NC...none. 
WV...none. 


&& 


$$ 
Synopsis...jj 
near term...kk 
short term...jj 
long term...wp 
aviation...js/kk