940 
fxus61 krnk 121144 
afdrnk 


Area forecast discussion 
National Weather Service Blacksburg Virginia 
644 am EST Friday Mar 12 2010 


Synopsis... 
a large low pressure system centered over Iowa will move east 
today and tonight. A second low developing off the Carolina coast 
will merge with the western low over the middle Atlantic region by 
Saturday night. 


&& 


Near term /until noon today/... 
several stations still reporting visibilities 1/4 mile or less. 
Area just on the edge of the lower clouds has the best coverage of 
fog this morning. In coordination with adjacent National Weather Service offices will 
issue a dense fog advisory for southwest counties. 


&& 


Short term /noon today through Monday night/... 
winds at low levels back this morning and by the end of the day 
BUFKIT is showing an easterly component to the winds from the 
surface through 10kft. Forecast area will be in a strong 
dynamics/strong upslope scenario for 12 to 18 hours starting by 00z 
tonight. Duration of the best lift...especially the very impressive 
upslope will depend on how fast the surface and 850 low now off the 
Carolina coast moves north into Virginia. But until the center of 
this low reaches the County Warning Area 850 mb winds off the 
Atlantic increase into the 30 to 50 knot range. Will be posting a 
Flood Watch for much of the forecast area for tonight where local 
climatology suggests heaviest rainfall along and just east of the 
Blue Ridge. 


Lifted index values on the fringe of The Wedge area are low enough 
this afternoon to keep isolated thunderstorms in the forecast. As 
surface front lifts into the area late tonight there may still be 
an axis of instability along with the convergence of the front so 
have kept thunder in for the evening. 


Little diurnal change in temperatures in expected today or 
tonight. Models are showing some dry air may come into the far 
western counties this afternoon. If the clouds become scattered 
then high temperatures will have to be increased. 


Saturday will be a time of transition from the current S-southeast flow in 
advance of the approaching upper low...to one of a west-northwest flow across 
the area on the back side of the exiting low. During this time...the 
focus for the best chances of precipitation will be along the 
northern portions of the region as the deformation zone on the north 
side of the track of the low pivots from a west-east orientation to 
one of a SW-NE orientation. 


By Saturday night...a prevailing west-northwest flow is expected across the 
area along with the onset of cold air advection. Precipitation from 
this point through Sunday will become more confined to the mountains 
where upslope showers are expected. Low level temperature profiles 
support a few areas of snow showers at higher elevations of 
southeast West Virginia and the northern mountains of North Carolina 
both Saturday night...Sunday night and Monday night...with just 
slightly better coverage Sunday night. Little if any accumulations 
are expected given the limited time period for any snow. 


By Monday into Monday night...the low level flow will trend more 
northerly...helping to reduce the coverage of upslope rain/snow 
showers in the mountains. 


&& 


Long term /Tuesday through Thursday/... 
for the medium range leaned towards hpcguide which favored the European model (ecmwf) 
over the slower UKMET...GFS and Canadian. 


A large area of low pressure off the East Coast will continue to 
move northeast into the Atlantic Ocean Monday. As the storm moves 
away...northwest flow with upslope rain/snow showers in the west 
will continue Monday into Tuesday. A vorticity lobe will rotate around 
the main low and enhance upslope component on Monday. Then...it 
appears that the available moisture starts to thin around 12z 
Tuesday. High pressure will build across our area from the Ohio 
Valley Tuesday night into Thursday. In general...kept temperatures 
on the cooler side early next week then moderate a little by the end 
of the week. 


&& 


Aviation /12z Friday through Tuesday/... 
confidence is low about how long LIFR to vlifr conditions will 
persist this morning. Blf is the most likely taf site to improve 
first since it is on the edge of the lower clouds and models have 
a pocket of drier air moving into southeast West Virginia this 
morning. 


With easterly low level winds and a fairly strong wedge developing 
across the area today many locations may not even lift above 1000 
foot ceilings. 


There is high confidence of widespread moderate to heavy rainfall 
tonight. This should keep ceilings and visibilities in the IFR to LIFR range 
from 00z through Saturday morning. Some low topped elevated 
thunderstorms are possible at Dan and blf tonight. Instability is 
limited but the lift from the approaching short wave and upslope 
winds is so strong heavy rain is still likely. 


The heavy rain should end Saturday morning with a return to 
MVFR/VFR conditions expected...however scattered convective 
showers are likely Saturday through Sunday...with ceilings and 
visibilities falling back into the MVFR/IFR range during these 
showers. Looks like low clouds and spotty -ra may keep at least 
MVFR conditions going into Monday with some slow improvement to 
VFR on Tuesday. 


&& 


Rnk watches/warnings/advisories... 
Virginia...Flood Watch from 4 PM EST this afternoon through Saturday 
afternoon for vaz018>020-022>024-033>035-043>047-058-059. 
Flood Watch from 4 PM EST this afternoon through Saturday 
morning for vaz007-009>017-032. 
Dense fog advisory until 11 am EST this morning for vaz009- 
012>017-032. 
NC...Flood Watch from 4 PM EST this afternoon through Saturday 
afternoon for ncz005-006. 
Flood Watch from 4 PM EST this afternoon through Saturday 
morning for ncz001>004-018>020. 
Dense fog advisory until 11 am EST this morning for ncz001>003- 
018>020. 
WV...Flood Watch from 4 PM EST this afternoon through Saturday 
morning for wvz042. 
Flood Watch through Saturday afternoon for wvz043>045. 


&& 


$$ 
Synopsis...air mass 
near term...air mass 
short term...ds 
long term...kk 
aviation...ams/jh