Weather

969 
fxus61 krnk 060539 
afdrnk 


Area forecast discussion 
National Weather Service Blacksburg Virginia 
1239 am EST Tuesday Jan 6 2009 


Synopsis... 
high pressure will move into the middle Atlantic region. An area of 
low pressure developing across the south central United States 
will move northeast along the front and bring periods of rain and 
freezing rain tonight and Tuesday...with rain expected Tuesday 
night into Wednesday. 


&& 


Near term /through today/... 
some shallow instability ahead of the cold front was resulting in 
convection over the Piedmont this evening and even a few lightning 
strikes. Instability will sink south and east overnight as colder 
air advects in from the north. 


Biggest weather concern tonight and Tuesday is the chance for 
freezing rain. Strong isentropic lift develops after 00z...and 
especially after 06z tonight ahead of a wave of low pressure 
moving into the Tennessee vally by 12z Tuesday. Meanwhile...high 
pressure will build into the middle-Atlantic region which will bring 
our winds around from the northeast...and usher in colder air. 


The big question is whether the air will be cold enough to support 
freezing precipitation. Model soundings (with the GFS coldest) 
indicate potential for some freezing rain in the higher elevations 
late tonight and Tuesday...although these soundings are warmer than 
previous runs. 


As a result...will replace the watch with a Freezing Rain Advisory 
for northeastern third of the forecast area. Later shifts will need 
to monitor whether this advisory should be expanded further south 
and west along The Spine of the Blue Ridge. 


Leaned toward cooler NAM MOS guidance for highs Tuesday. Another 
concern is the potential for heavy rain...especially beyond the 
short term period...with prolonged period of rain. 


&& 


Short term /tonight through Thursday night/... 
expecting only marginal temperatures for any freezing rain in the advisory 
counties early Tuesday night. Again...warm surface temperatures and lack 
of cold air should keep most ice on trees and elevated 
surfaces...perhaps bridges and overpasses. Temperatures will be rising 
overnight as weak high pressure is shoved north and east. Focus 
will then shift to possible heavy rain event for the rest of Tuesday 
night into Wednesday...especially along the New River valley 
through Halifax County. Model quantitative precipitation forecast has been atrocious with the past 
couple isentropic events...so hesitant to believe model 
forecast...but did go with HPC quantitative precipitation forecast. If these values are realized 
South Boston could approach flood stage Wednesday or Wednesday 
night. 


Precipitation becomes lighter and more scattered from west to east on 
Wednesday...before cold front arrives Wednesday night. A 
changeover to snow will occur in southeast West Virginia...with some 
accumulations possible along the higher slopes through Thursday. 
However...mainly west wind and questionable moisture depth may 
keep accumulations only along the western slopes of Greenbrier. 
Perhaps an advisory will be needed. Coldest air should fianlly 
arrive by 12z Friday...just before it does...downsloping winds 
could gust to 35 miles per hour along the favored spots most of Thursday. 


&& 


Long term /Friday through Monday/... 
on Friday...high pressure will move fairly quickly across the 
region. The result will be a precipitation free day. By Friday 
evening...the high will be east of the region...and we will start to 
get on return flow southerly trajectory side of the feature. Helping 
to fuel a return to southerly and moist flow will be an area of 
low pressure advancing towards the region from the middle Mississippi 
River valley. Towards daybreak Saturday...isentropic flow associated 
with the systems associated warm front will be moving into western 
parts of the region. Coverage will continue to increase during the 
morning on Saturday as the surface low passes to our north and its 
associated cold front moves through the area. With cold air at the 
surface in place...and the warm front across the area Friday 
night...we will need to watch for the potential of a wintry mix in 
the west late Friday night into early Saturday morning. With 
Saturday afternoon temperatures expected to be in the upper 30s to 
near 50 degrees...most areas will transition to rain showers. 


Saturday night into Sunday a reinforcing shot of upstream energy 
heads into the region along with reinforcing colder air. The result 
will be a continuation of scattered snow/rain showers across the 
area...with decreasing coverage during the day Sunday...with the southeast 
WV mountains keeping the snow showers the longest. 


The cold air at the surface will stay in place Sunday night into 
Monday. We will start to see some warm air advection aloft by the 
afternoon Monday as surface low pressure moves into the lower Ohio 
Valley and winds ahead of it become more southwest and start to tap 
into some Gulf Coast moisture to stream our way. Light rain or light 
snow will be possible in the west Monday afternoon thanks to this 
moisture advection. 


&& 


Aviation /06z Tuesday through Saturday/... 
the approach of the next significant weather system tonight 
through Tuesday night will result in another prolonged period of 
IFR to LIFR conditions...along with widespread continuous 
rain...with freezing possible northeast of klwb and northwest of 
klyh in the Blue Ridge/Highlands. Taf sites are expected to see 
only liquid rain through the event. 


Will keep the taf sites down...although cannot rule out some 
improvements in the lower ceilings...especially at blf late in the day 
today. 


Arrival of strong cold front will allow for conditions to improve 
back to VFR east of the Blue Ridge on Wednesday as strong downslope 
winds take over. Further west...upslope clouds and Post frontal snow 
showers should assure that MVFR to IFR weather continue across 
southeastern West Virginia terminal forecast points well into 
Thursday night. 


&& 


Rnk watches/warnings/advisories... 
Virginia...Freezing Rain Advisory until 9 PM EST this evening for vaz019- 
020-024-035. 
NC...none. 
WV...Freezing Rain Advisory until 9 PM EST this evening for wvz045. 


&& 


$$ 
Synopsis...ph 
near term...js/ph 
short term...km 
long term...ds 
aviation...js/wp