Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
741 am EDT Sat Jul 26 2014
a warm front lifts north across the area today with winds shifting
to the west. A complex of thunderstorms will develop along this
frontal boundary in the Ohio Valley by evening and spread
southeast toward our area by Sunday. A strong cold front should
move across the middle Atlantic and southern Appalachians by
Monday...with cooler weather for the middle of next week.
Near term /through tonight/...
as of 715 am Saturday...
Updated to allow for more clearing per Sat loop and web cams
showing more sunshine. No other changes were made with this
As of 530 am Saturday...
Updated forecast through middle morning reflects limited precipitation
chances...and less sky cover to the west of Interstate 81. Showers
faded as they moved east toward Martinsville. Cannot rule out a
lone shower developing in the higher Theta-E air in the Piedmont
through 8 am...but overall should be dry.
As of 320 am EDT Saturday...
Moderate to heavy showers have plagued Patrick County early this
morning per weak southeast flow and higher Theta-E ridge into the southern Blue
Ridge. The 00z local WRF picked up on this pretty well. Radar trends
are finally showing some weakening as the showers shift east toward
Henry County. Only expecting isolated coverage east of the Blue
Ridge through about 8 am. Otherwise...we are looking at patchy fog
developing when the middle/high clouds shift east.
The models are in reasonable agreement today of lifting a frontal
boundary north through our County Warning Area. Models place the best lift/Theta-E
convergence well east of US this afternoon so not expecting any
showers today. Skies will become mostly sunny this afternoon and
temperatures warm considerably to Friday thanks to an increasing west flow
aloft...and rising heights/800 mb temperatures. Looking at highs from the lower
to middle 80s over the mountains and adjacent valleys...to around 90
Roanoke and Piedmont.
Tonight...will be watching upstream as mesoscale convective system is expected for the Ohio
Valley and models shift this area east and south toward southeast WV/far SW
Virginia by Sunday morning. Models in decent agreement through 12z Sunday
in hitting southern Ohio to eastern Kentucky and western WV with best coverage...with
higher Theta-E ridge situated across this region. Given that systems
tend to be faster than model forecast will have probability of precipitation up to likely in
the far west by morning tapering to slight chance to the Blue Ridge
and north of lyh. Convection is likely to be weakening toward
morning given lack of thermal instability but wind fields are
favorable for a few strong gusts. The Storm Prediction Center is
keeping the main threat out of our area until after 12z
Sunday...which is covered in the next section of this discussion.
Lows tonight will be muggy as dewpoints increase...mainly in the
60s County Warning Area wide....around 70 southeast.
Short term /Sunday through Tuesday night/...
as of 240 am EDT Saturday...
Sunday into Sunday night continues to look like a very active
weather day across at least the western and northern parts of the
region...with a potential thunderstorm complex entering and moving
through the area during the afternoon and evening hours. The new day
2 convective outlook by the Storm Prediction Center has the center
of a moderate risk of potential severe weather near the Kentucky/WV
border...with the eastern edge of the moderate area just touching
Greenbrier County WV southwest into Tazewell County Virginia. The
remainder of the forecast area is within the new day 2 slight risk
area. Damaging winds and large hail are expected to be the main
threats across the region...with an isolated tornado potential
within the moderate risk area.
This system will be in advance of a shortwave trough/low that will
head southeast through the Great Lakes region Sunday night. It will
be this feature that merges with a southeast Canadian area of low
pressure that yields a longwave trough across most of the eastern
third of Continental U.S....centered over our region...by Monday afternoon. A
decent gradient will exist around the southern wave for this time of
year to produce 850 mb winds on the order of 35 to 45 kts across the
area late Sunday...Sunday night...and early Monday to allow for
surface gusts in the 20 to 30 miles per hour range at higher elevations during
Weak disturbances with in the flow of the parent longwave trough
will periodically progress trough our area Monday into Tuesday. The
first of these will allow for some scattered shower and perhaps an
isolated thunderstorm on Monday...especially across the mountains. A
Lee trough will develop thanks to strong west-northwest flow. Afternoon
development will be possible just east of this trough where low level
winds will back southwest...and yield a convergence line along the
Monday night...the active will slowly wane...but not completely end
across the mountains where lingering upper support...and a northwest
upslope flow will help maintain some isolated showers.
On Tuesday...another weak disturbance heads south through the
area...but by this time...surface high pressure will also be across
the area. Will limit precipitation coverage to isolated showers
across mainly western and southern parts of the area. This activity
is expected to end by Tuesday evening.
Temperatures during this portion of the forecast will trend cooler
each day. Temperatures by Tuesday and Tuesday night will be some 10
to 15 degrees cooler than those expected Sunday and Sunday night.
Long term /Wednesday through Friday/...
as of 1245 PM EDT Friday...
Models do bring a couple re-enforcing shortwaves around the upper
trough late in the period with these pivoting through the region
Thursday into Friday. However moisture limited under the continued
weak ridging with probability of precipitation mainly driven by cold air aloft and weak
local convergence mainly mountains. Thus including some slight
probability of precipitation later Thursday and a bit more chance coverage Friday with a
second stronger middle level wave that looks to sharpen up the trough
a little more.
Below normal temperatures the main aspect of next week with highs likely a
good 5-10 degrees below average much of the period and lows dipping
into the 40s and 50s. Coldest mornings look to be Wednesday and
Thursday when the surface ridge will be nearby and 850 mb temperatures remain
around +10c over the west. This could produce some record lows with
coop MOS data giving Deep Valley readings well down into the 40s.
Aviation /12z Saturday through Wednesday/...
as of 730 am EDT Saturday...
VFR through early overnight. Some shallow fog this morning is
going to be gone soon and not affect terminals.
Main aviation issue should be a complex of storms prognosticated to
impact the blf/lwb area by 12z Sunday. Convection may be faster so
have -shra and thunderstorms in the vicinity at blf/lwb at 10z but with no worse than MVFR
Storms will definitely impact the blf/lwb and over toward
roa/bcb/lyh through Sunday...though models are showing it weakening
before making it to the Piedmont. Still aviation interests should
be wary of potential sub VFR ceilings/visibilities with heavy rain and
The system associated with this will be moving across by
Monday...so threat of sub VFR through Monday expected at most taf
sites...but will not be continuous.
Some gusty winds behind the front Monday with drier air pushing
in for Tue-Wed...and mainly VFR. Cannot rule out late night fog
bcb/lwb but low levels may stay dry enough for less fog coverage.