Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
1110 am EDT Wednesday Mar 12 2014
deepening low pressure will pass north of the area this afternoon
and swing a very strong cold front across the region by early this
evening. This will bring showers and possibly a few strong
thunderstorms during the day...followed by high winds and much
colder air overnight into Thursday. High pressure builds in with
drier and warmer weather by the end of the week.
Near term /through tonight/...
as of 1030 am EDT Wednesday...
Latest forecast update reflect adjustments in the coverage of
shower activity this morning...as well as the timing of the cold
frontal passage this afternoon. Weak upper level disturbances
passing across the area continue to generate light rain
showers...which are in turn keeping a lid on daytime heating. Have
therefore increased rainfall coverage for the morning and early
afternoon. Not as many breaks in the cloud cover as anticipated
earlier per latest visible satellite imagery. Therefore have also
lowered afternoon highs by a few degrees. The cooler temperatures
will translate to lower instability as the cold front passes
across the region later this afternoon...but still see some potential
for thunderstorms with locally strong winds just ahead of the
boundary. Still expect the cold front to enter our southeast West
Virginia counties by around 3 PM...and shift rapidly eastward...
exiting the area for the coastal plain by around 8 PM. Frontal
passage will bring a very rapid drop in temperatures...falling 8
to 10 degrees per hour for the first few hours after frontal
As of 240 am EDT Wednesday...
Very dynamic event to unfold over the next 24 hours as low
pressure rounding through the lower Ohio Valley phases with a
southern stream wave and quickly deepens heading to the middle
Atlantic coast this evening. This will propel a very strong cold
front east across the region by early this evening as rapid
deepening of the 500 mb trough drives Arctic air back in from the northwest in
a matter of a few hours or less.
Initially appears southern/eastern areas may get scraped by rain showers
heading up with the southern low with afternoon focus just ahead
of the front crossing the west. This makes coverage of rain showers iffy
and perhaps more widely scattered elsewhere with even some sunny breaks
around midday. This potential heating the main concern as the
front arrives driven by a very strong middle level wave on the
leading edge of the cold air aloft. Models suggest enough
instability to support a developing line west of the Blue Ridge by
early afternoon with this feature quickly heading east reaching
the Piedmont late in the day. Strong jet aloft and degree of shear
per forecast hodographs over the east suggest a better severe
threat pending just how much low level instability can get going
espcly if early clouds/rain showers are a bit more. Thus adjusted probability of precipitation
higher early on with a late morning break before including more
convection with gusty winds from west to east this afternoon.
Bumped highs up espcly east where should get enough heating to
push temperatures back into the 70s with 60s west.
Other perhaps bigger widespread concern for later this afternoon
and overnight is with the Post frontal northwest winds that will surge in
as 850 mb temperatures tumble a good 20-25 degree/c in a 6/12 hour window this
evening. This combo with a very strong pressure rise couplet...a
45-50 knots 850 mb jet and a rather low inversion supports possible
damaging winds across most of the mountains and even in spots over
the east early this evening. Therefore have upgraded the high wind
watch to a warning from noon today through noon Thursday and added in an
advisory elsewhere out east for late today into the overnight.
Any rain showers should exit the east by sunset this evening with rain
over the upslope areas quickly changing to snow in a short period
after any convection exits. However much of the deeper moisture
will be exiting about the time the switch occurs with limited
leftover low level moisture given the dry advection. Think 1-3
inches likely northwest with an inch or so farther south given warm
ground to init and blowing around nature of the snow with more of
a flash freeze to take shape. This may need to be covered with an
advisory later but will only address in the severe weather potential statement for now. Elsewhere
should be clearing with downslope east of the Blue Ridge with
leftover clouds/flurries just east of the upslope areas so cut
back probability of precipitation sooner this evening. Temperatures will quickly fall from the
50s to below freezing far west late today and be in the teens to
low 20s mountains to middle 20s east by morning. Winds and cold at
elevation suggest some wind chills at or below advisory criteria which
will address in the npw for now.
Short term /Thursday through Saturday/...
as of 330 am EDT Wednesday...
Low pressure off the New England coast will push into the Atlantic
Ocean Thursday. 500 mb trough axis crosses through our region and
reaches the coast by Thursday night. Weak upper ridging expected
Gusty northwest winds continue Thursday morning especially across
the higher terrain. Trimmed back probability of precipitation Thursday morning to just the
western mountains. By Thursday afternoon winds will begin to
diminish and the low level moisture wrapping around the low will
erode. Surface high pressure moving across the southeast United
States will be centered over the Carolinas and Georgia by Friday
With Post-frontal cold advection and the core of the 850mb thermal
trough moving overhead...played high temperatures Thursday cold from
around 20 degrees in the west to the lower 40s in the southeast.
Lows Thursday night will range from the teens in the northwest
mountains to the lower 20s in the Piedmont. Temperatures moderate on
Friday as the high center slips off the coast and southwest flow
kicks in. Highs on Friday will vary from the upper 40s in the
mountains to the upper 50s in the Piedmont.
A frontal boundary will drop south across the Ohio Valley Friday
night. One northern stream shortwave will rotates through the trough
north of the forecast area on Friday night and Saturday.
Intersecting to note this morning GFS also showed a weak vorticity
with light quantitative precipitation forecast pivoting to our south. Lows Friday night will
generally be from around 30 degrees to the middle 30s. Allowed for
some light probability of precipitation in the west on Saturday as cold front slides
south. Highs for Saturday warm into the middle 50s in the west to the
middle 60s in the east.
Long term /Saturday night through Tuesday/...
as of 330 am EDT Wednesday...
Models were showing a more amplified trough in the east by Monday
but were still different on the timing. Similarly...the
differences in the models show up on the surface forecasts.
Canadian has a low over Virginia by Sunday morning while the 00z
European model (ecmwf) has it in the Gulf of Mexico. Compromise from wpc has an
elongated surface low from Georgia to the Carolina coast.
850 mb temperatures drop back down just below zero behind the cold
front Saturday night. Forecast area remains in that air mass through
Monday. Still the potential for a stripe of snow or winter
precipitation on the northern edge of wherever the precipitation
shield ends up Sunday night and Monday.
Warm air advection picks up again on Tuesday and bulk of deep
moisture moves well off the East Coast.
Aviation /15z Wednesday through Sunday/...
as of 650 am EDT Wednesday...
VFR conditions will give way to lowering ceilings this morning as
broken bands of rain showers head NE ahead of the main storm system to the
northwest. However likely to see quite a bit of variability in cloud
bases into early afternoon with lower MVFR across kblf/klwb and
perhaps over the southeast around kdan where moisture will be a bit
deeper. May see all except the southeast wva locations return to a period
of VFR early this afternoon ahead of a broken line of shra/tsra.
Ceilings will drop into the low to Middle Range MVFR range all sites with
IFR- MVFR visibilities in -shra/shra. Overall deep convective threat
appears limited...except from the Blue Ridge east across the
Piedmont where better heating will yield higher instability and
better potential for thunderstorms and rain. However...the biggest concern will be
wind. In advance of the front...expect SW mainly 10-22kts with
gusts to 40 kts possible in a few spots...becoming northwest 20-30kts
with gusts 35-55kts behind the front. Such winds will also be
possible with any convection as momentum Transfer on BUFKIT is in
the 40-60kt range.
Northwest winds will stay strong overnight espcly across the mountain
locations where continued gusts to 40-50 kts appears possible.
Ceilings should improve to VFR from kroa east this evening while
remaining MVFR at kbcb and MVFR to IFR in -shsn at kblf/klwb into
the overnight. Moisture finally decreases Thursday morning over the far
west with all sites improving back to VFR on Thursday. Westerly winds
will still remain strong espcly during the morning when gusts to
40-45 kts could occur before diminishing to 25-30 kts in the
By Friday...high pressure will settle over the area and bring a
decrease in wind speed and a return to VFR conditions for the
entire area. Another weak cold front will drop through the region
during the weekend with sub-VFR possible over the south and west
as deeper moisture with yet another low pressure system spreads
east along the front by Sunday.
Virginia...High Wind Warning until noon EDT Thursday for vaz007-009>020-
Wind Advisory from 4 PM this afternoon to 4 am EDT Thursday
NC...High Wind Warning until noon EDT Thursday for ncz001>003-018-
Wind Advisory from 4 PM this afternoon to 4 am EDT Thursday
WV...High Wind Warning until noon EDT Thursday for wvz042>045.
long term...air mass