501 
fxus61 krnk 072043 
afdrnk 


Area forecast discussion 
National Weather Service Blacksburg Virginia 
343 PM EST Wednesday Jan 7 2009 


Synopsis... 
a reinforcing shot of cold air...windy and snowy conditions are expected 
tonight as a trough of low pressure swings across the region. The 
snow showers and wind will subside Thursday night as high pressure 
builds over the area. 


&& 


Near term /until 6 am Thursday morning/... 
by 00z the 850 mb cold front will be just exiting the eastern County 
Warning Area with 850 winds behind the front out of the west at 45 
to 55 knots and surface winds at 10 to 20 knots for the rest of the 
night. This is not an ideal direction for upslope but the dynamics 
and lift provided by two short waves coming through will make up for 
that. Plus...6hr pressure rises through about 06z will be in the +6 
to +10 range...so no changes need to be made to current Wind 
Advisory. 


The first short wave was in eastern Kentucky and the second was 
diving into northern Iowa. The best lift on the latest RUC and local 
WRF for the southwest part of the County Warning Area with this 
second wave will be starting around 07z. 


Will be issuing Winter Weather Advisory in the west for tonight and 
Thursday morning. Temperatures drop sharply by 00z and any time 
after sunset snow could start accumulating in the west so advisory 
will begin at 9pm. The local WRF is showing some banding of 
precipitation overnight and westward guidance has 2 inches of snow 
throughout that area. There is also enough quantitative precipitation forecast to support up to 4 
inches in snow in the areas that typically get more quantitative precipitation forecast/snowfall 
with a west wind...Mt. Rogers down through western Watauga...along 
the Tazewell County/Smyth County line...and from western Greenbrier 
into northwest Mercer County. 


&& 


Short term /6 am Thursday morning through Saturday night/... 
short wave noted over Iowa is prognosticated to be moving across the County Warning Area 
between 12 and 18z Thursday. This should enhance western slope snow 
showers...and we may have some banded spill over further east into 
the rest of the mountains- mainly Thursday morning. By afternoon the bulk of 
the snow shower activity will be on the western slopes in southeast WV as 
the flow veers from west to northwest behind the short wave. Winter 
Weather Advisory issued for snow will continue through noon. 
Additional accumulations Thursday morning will be 1 to 2 inches- possibly 
more in the highest elevations. Cold advection ahead of the short 
wave brings 850 mb temperatures of -6 to -10c by 12z Thursday...and with even 
stronger cold air advection behind the short wave...I have lowered maximum temperatures a few 
degrees for all but the Piedmont sections of the County Warning Area. 
Mountains...especially the western slopes...will not rise above freezing. 
Will be a windy day...but the low level jet decreases to 35 knots or 
less...so despite increased subsidence behind the short wave Thursday 
afternoon it looks like wind gusts will remain just below Wind 
Advisory criteria. 


Thursday night...the snow showers will gradually taper off on the western 
slopes areas of southeast WV. Cold air advection peaks around 06z with 850 mb temperatures of -8 
to -13c. Will be a breezy and cold night. Have stayed close to 
guidance lows in the 20s...with some teens in the mountains 


Will have high pressure building across the area Friday...but fast 
flow aloft should quickly push the ridge southeast of the area by Friday 
afternoon with some return flow warm advection developing in the 
afternoon. Very dry air under sunshine and weak SW flow will allow 
temperatures to return closer to more normal levels- especially in the far 
west...as the upper cold pool gives way later in the day. I have 
lowered the maximum temperatures in the east a few degrees as the warm 
advection will be much later in the day out that way. 


Friday night...the next system will be approaching the region with 
warm advection/isentropic lift spreading over the area toward 
morning. I have gone closer to the quicker GFS/European model (ecmwf) in bringing 
some chance probability of precipitation into the far west toward morning. Since surface temperatures will 
likely drop to near or just below freezing overnight...this precipitation 
coming in early will have the potential to be freezing rain or 
sleet. The wave moves quickly across the County Warning Area Saturday...and I have 
raised probability of precipitation to chance all areas...with just lower than likely across 
the far west. More cold advection Saturday night...but models do 
not show a lot of low level moisture...so I have some chance probability of precipitation for 
snow showers on the western slopes through 06z Saturday night. 


&& 


Long term /Sunday through Wednesday/... 
high pressure will stretch east along the Gulf Coast through Monday 
blocking any significant moisture from advancing towards the region. 
Meanwhile...a low pressure system will slowly work across the Great 
Lakes. By late Monday afternoon...the associated cold front will 
enter the mountains and pass across the area Monday night. This cold 
front should track across the region virtually dry with some 
mountain snow showers on the back side. With a west wind and limited 
moisture...snow accumulations will be light. 


Tuesday through Wednesday forecast will depend on the track of an 
upper level low. European model (ecmwf) has this upper level low tracking across 
southeastern Canada with a second surge of cold air entering the 
area Tuesday night. Some moisture may accompany this cold air and 
with 850 mb temperatures -8c to -12c...snow could fall across the 
mountains and into the east. 12z GFS has the upper level low 
tracking farther south and across the Great Lakes. 850 mb 
temperatures are dramatically colder at -26c to -28c 12z 
Wednesday. If the GFS verifies...mountain temperatures Wednesday 
morning will be below zero with single digits in the east. 
Wednesday afternoon temperatures will not make it above 10f and 
the east may struggle to see 20f. Looking at extremes for January 
14th...temperatures this cold are possible but rare for this time 
of year. However...this anomaly is being seen at the end of the 
forecast period. I do not have the confidence to run with these 
extreme temperatures at this time. With keep close to HPC and 
trend in the colder direction. 


&& 


Aviation /21z Wednesday through Monday/... 
LIFR clouds and fog at lyh will scour out quickly this afternoon 
as wedge erodes and winds come around to the southwest with 
increasing wind speeds. Scattered showers will continue across the 
region this afternoon with MVFR ceilings and VFR to MVFR 
visibilities. Some snow may be mixed in with the rain...especially 
west of the Blue Ridge before 00z. 


Once the sun sets the precipitation will end east of the Blue 
Ridge and change over to snow in the west. Snow showers with MVFR 
to IFR ceilings and visibilities will continue at blf and lwb 
through at least 12z. After that the moisture becomes more 
shallow and the precipitation will end. 


Winds will stay gusty through the forecast period. On Thursday 
night and Friday high pressure over the area will result in much 
lighter winds. 


Another cold front comes through Sat which could bring brief MVFR 
conditions in light mixed winter precipitation on Sat. High 
pressure builds in on Sunday resulting in VFR conditions for the 
end of the weekend. 


&& 


Hydrology... 
Flood Watch for Pittsylvania and Halifax counties will be continued 
since we are still seeing a strong rise in the headwaters of the Dan 
River. Current forecast is for the Dan to reach near flood stage at 
South Boston. If there is any flooding...it should be minor. Will 
update with another special river statement this evening...or a Flood 
Warning if the river is expected be higher than the flood stage of 19 
feet at South Boston. Other area rivers will see moderate 
rises...with the Roanoke...James and new rivers forecast to be half 
bankfull or less...the Greenbrier to three quarters bankfull...the 
Yadkin...Holston and Clinch rivers have all crested around three 
quarters bankfull or less. 


&& 


Rnk watches/warnings/advisories... 
Virginia...Wind Advisory until 6 am EST Thursday for vaz007-009>020- 
022>024-032>035-043. 
Winter Weather Advisory from 9 PM this evening to 12 PM EST 
Thursday for vaz007-009-010-012-015. 
Flood Watch through Thursday afternoon for vaz044-058. 
NC...Wind Advisory until 6 am EST Thursday for ncz001>003-018>020. 
Winter Weather Advisory from 9 PM this evening to 12 PM EST 
Thursday for ncz001-018. 
WV...Wind Advisory until 6 am EST Thursday for wvz042. 
Winter Weather Advisory from 9 PM this evening to 12 PM EST 
Thursday for wvz042-043-045. 


&& 


$$ 
Synopsis...air mass 
near term...air mass 
short term...jj 
long term...rcs 
aviation...jh 
hydrology...jj 

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