265 
fxus61 krnk 220153 
afdrnk 


Area forecast discussion 
National Weather Service Blacksburg Virginia 
953 PM EDT sun Mar 21 2010 


Synopsis... 
strong low pressure will track northeast from the lower 
Mississippi Valley into the Ohio Valley by early Monday. The 
associated cold front will move east across the region Monday 
afternoon. An upper level area of low pressure will slide east 
through the area Monday night into early Tuesday...followed by 
high pressure on Wednesday. 


&& 


Near term /through Monday/... 
a line of convection with embedded thunderstorms has re-developed across 
eastern Georgia. Think this line may make it into the northern mountains of NC where 
we have already had 1.5 inches of rain. If these heavy 
showers/thunderstorms do move in...the higher rain rates would likely cause 
some flash flooding. So...will go ahead and issue a Flash Flood 
Watch for the northwest foothills and mountains in NC...and extend it into 
Grayson and Carroll counties in Virginia for now. Have decided that the 
Wind Advisory still looks good. I do not think I we will see win d 
gusts greater than 55 miles per hour overnight...but even gusts to around 50 
miles per hour will likely down a few trees. The strong gusts should only be 
in the highest elevations...except for Tazewell/Mercer counties 
where we often see mountain wave action bringing gusts to the surface on 
the northwest side of ridges. No other changes. Flash Flood Advisory will be out soon. 


Previous discussion... 
there have been bands of showers moving northward into the northern NC 
mountains past several hours...with little or no break in between. 
Upstream radar at gsp shows the bands of showers getting heavier. 
I have updated the forecast through 200 am and included some occasional 
heavy rain in southeast upslope areas of northwest NC and far SW Virginia. I also 
raised the 6 hour rainfall amounts down that way to around 1 inch 
in northwest NC mountains...and 0.75 increase far SW Virginia along the Blue Ridge. This 
looks like a good upslope event shaping up...with deep southeast flow and 
no damming. Surface dewpoints are already into the lower 50s in the 
southwestern County Warning Area along the Blue Ridge...and the satellite blended total 
pcptl water product shows precipitable waters  >1.00 streaming into the southwestern County Warning Area. 
This is already .200% of normal. Will take a close look at 00z 
upper air soundings and mesoscale models- may need a short fused 
Flash Flood Watch if it looks like the more organized convection 
close to the occluded front and just entering the extreme southern 
applns...will continue propagating northward into the southwestern County Warning Area. As for 
winds...so far the highest gust seen is at Tazewell Middle School 
of 37 miles per hour...but strongest winds are yet to come near midnight. 
Current Wind Advisory looks good. Finally...I made some 
adjustments to the evening probability of precipitation/temperatures for the rest of the County Warning Area 
based on current trends which were warmer and wetter. 


Previous discussion... 
leading band of rain showers moving into the SW zones a bit faster at this time 
so running with higher probability of precipitation to init given strong band of 
isentropic lift prognosticated through early this evening. Otherwise expecting 
bands of rainfall to spread across most of the area by 00z...then 
likely diminish a bit ahead of the main axis of upper divergence 
that will cross the area with the occluded front between 04z-12z. 
Models suggest this is the period of strongest southeast flow relative humidity 
transport as convection just offshore gets linked into the 
convergence axis. Thus thinking split potential less espcly given 
depth/strength of the southeast trajectory and some surface based 
instability along the band. Expect upslope flow into the southern 
Blue Ridge to locally enhance precipitation totals into northwest NC but for now 
quantitative precipitation forecast of up to 1.5 still not enough to hoist a Flood Watch since may 
see a break or two in between heavier bouts of rainfall. However 
if more thunderstorms and rain mixes in then localized flooding possible. 


Otherwise boosted probability of precipitation a bit faster tonight with overall categorical 
flavor. The other concern with strong winds across the west espcly 
at elevation from southeast wva into the mountain Empire and over the northwest 
NC ridges. Latest BUFKIT profiles show 45-50 kts near the ridgetops 
this evening and then possibly being mixed down with heavier 
rain showers/isolated thunderstorms and rain by midnight. Normally these speeds would stay 
aloft except blf/Tazewell vicinity but given the surface low just 
west plan to issue a Wind Advisory from this afternoon through early 
Monday for the above mentioned corridor. Stayed on the warm side 
of MOS temperatures given clouds/rain overnight plus mixing. 


Front slides through from the SW Monday followed by a very pronounced 
middle level dry slot by afternoon ahead of the upper low to the 
west. This should cut off most sig rain showers over much of the region 
during the morning with most residual rain showers across the far north/NE 
and SW through middle afternoon. Some concern that breaks will develop 
and help destabilize things espcly east where the flow should 
remain SW. However forecasts show moisture shallowing out with most 
lift taken off to the NE with the initial band...so only running 
with chance/slight probability of precipitation east while holding likelys north/west. As 
the colder air arrives aloft over the west could see isolated thunderstorms and rain 
develop espcly northwest NC ridges with some graupel potential before 
temperatures fall and ptype GOES to snow by days end at elevation. High 
temperatures quite tricky pending 12z values and then whether or not 
sunshine breaks through across the south/east. Also expect readings 
to fall under the 500 mb cold pool and associated 850 mb cold advection 
west where readings should hold in the 40s/50s. Thus followed 
closer to the met MOS which was colder west but much milder in the 
east. 


&& 


Short term /Monday night through Tuesday night/... 
the upper level low to track across NC Monday night with 
precipitation filling in across the area overnight. Most of the 
heavier showers will be across the mountains with a change over to 
snow through the evening but mainly after midnight. As cold pool and 
the air column cools...accumulating snow across the mountains is 
likely with 1-3 inches possible Monday night into Tuesday morning. 
The higher end of the range will be elevations above 3 kft. Majority 
of the snow to stay on elevated-grassy surfaces and could possibly 
sticking on roads in shelter areas and northerly facing slopes. 
Accumulations likely to continue into Tuesday morning as the upper 
level low tracks to the Virginia coast and wrap-a-round moisture and more 
colder air works into the region. Deformation zone will remain over 
the area through early Tuesday but mountain accumulations will be 
fighting higher sun angle. Would not be surprised that Monday night 
snowfall is gone by sunset Tuesday evening. Despite increasing 
northwest flow Tuesday...the upper level low will be in the vicinity 
to carry rainfall probability of precipitation east of the Blue Ridge. Tuesday night...the 
disturbance tracks off the northeast coast and high pressure resides 
over the Gulf. With a west wind...precipitation will end across the 
area from east to west with lingering light showers/drizzle on 
western slopes Tuesday night. If we can manage some breaks in the 
clouds Tuesday...pressure rises and an increasing northwest jet will 
bring windy/gusty conditions to the area Tuesday and Tuesday night. 
The majority of the higher winds will be along the ridges of the 
Blue Ridge. 


Temperatures will dip to near 30f across the mountains Monday night 
then rebound into the 40s Tuesday afternoon. Conditions do not 
support any wide spread frost from forming. East of the Blue Ridge 
likely to see upper 30s Monday night then moderate into the middle 50s 
to lower 60s on Tuesday. 


&& 


Long term /Wednesday through Sunday/... 
zonal flow will keep the area dry Wednesday and Thursday. There will 
be a slow moving weak cold front across the Ohio Valley during this 
time to at least have thickening cloud cover across the north 
portions of the County Warning Area. Showers are possible with boundary near the area 
and afternoon heating but zonal flow will keep moisture limited. 
Will carry 20-30 probability of precipitation Thursday afternoon for measurable precipitation 
under 0.05 of an inch. Chance for rain will increase on Friday as 
the weak front moves south of the area. Precipitation will be light 
with convection along the Gulf. High pressure wedges down the East 
Coast over the weekend ushering in cooler air. With a southeast flow 
and moisture in the area...overcast skies and light rain is likely. 
Precipitation may only be a light drizzle so do not change next 
weekends plans. 


The global wind oscillation (gwo) does support a mean Southeast 
Ridge in the east. Therefore...any precipitation falling during the 
period will be light and not lead to hydrological concerns. 


The global atmospheric angular momentum anomaly (glaam) does not 
display any significant temperature changes through next weekend. 
Temperatures will be cooler during the latter half of next week with 
clouds and rain possible but do not foresee a early Spring Arctic 
shot. Compared to this weekend...next weekend temperatures will 
remind all not to put the winter gear in the attic just yet. 
Temperatures Wednesday and Thursday will run above normal then drop 
below normal next weekend. 


&& 


Aviation /02z Monday through Friday/... 
lowering ceilings will prevail through the rest of the evening with a 
period of light showers working north across the taf sites by 
22-00z. Once things moisten will see conditions quickly 
deteriorate to MVFR this evening while lowering to IFR at times 
espcly across the south by midnight and elsewhere between 06-12z. Looks 
like axis of heavier rain showers and possible thunderstorms and rain will arrive between 
02-05z across the kblf-kroa-kdan corridor before spreading NE through 
the early morning hours. Since thunderstorms and rain iffy wont include mention but 
will lower visibilities into low end MVFR/occasional IFR within rain showers overnight. 


Southeast winds may gust to 25 kts at blf into early this evening and 
above 30-35 kts within rain showers overnight as the gradient tightens 
ahead of surface low pressure passing along the Ohio River toward 
morning. Elsewhere low level wind shear will be an issue by midnight as the south/southeast 
strengthens so including a period of wind shear at all sites 
except kblf later tonight. 


Rainfall looks to taper off from the south Monday morning as a dry slot 
works in behind the passing precipitation axis and ahead of the upper 
low to the west. This should allow visibilities to return to high end 
MVFR/VFR by middle morning outside of lingering fog. However clouds 
remain the low confidence factor on Monday with enough breaks 
possible for VFR south. At this point plan to leave all areas in 
at least MVFR ceilings given depth of moisture with most VFR likely 
after the valid taf period Monday afternoon when the SW flow helps 
mix things out espcly east of the Blue Ridge. 


Low ceilings...sub VFR on the backside of this system is expected 
Monday night into Tuesday...especially in blf/lwb on northwest 
flow...along with potential for rain or snow showers. 


A return to VFR conditions is expected Wednesday-Thursday in southeast WV before 
another system brings more clouds/precipitation and possible MVFR for 
weeks end. 


&& 


Rnk watches/warnings/advisories... 
Virginia...Wind Advisory until 6 am EDT Monday for vaz007-009-010-015. 
Flash Flood Watch until 6 am EDT Monday for vaz015-016. 
NC...Wind Advisory until 6 am EDT Monday for ncz001-002-018. 
Flash Flood Watch until 6 am EDT Monday for ncz001>003-018-019. 
WV...Wind Advisory until 6 am EDT Monday for wvz042. 


&& 


$$ 
Synopsis...jh 
near term...jh/jj 
short term...rcs 
long term...rcs 
aviation...jh/km 

National Weather Service Glossary of Abbreviations

Non-Expanded Version (with abbreviations)