Area Forecast Discussion
Issued by NWS Blacksburg, VA-- Remove Highlighting --
-- Discussion containing changed information from previous version are highlighted. --
FXUS61 KRNK 250139
Area Forecast Discussion
National Weather Service Blacksburg VA
839 PM EST Fri Feb 24 2017
-- Changed Discussion --A strong cold front will approach from the west overnight before
sliding across the mountains early Saturday, and east of the
region Saturday afternoon. High pressure follows the front
Saturday night into Sunday bringing cooler air on gusty
northwest winds for the remainder of the weekend.
-- End Changed Discussion --
.NEAR TERM /THROUGH SATURDAY/...
-- Changed Discussion --As of 825 PM EST Friday...
Cold front now crossing east of the Mississippi River will
slide east overnight, likely reaching the far western mountains
around 12z Saturday. Evening soundings show quite a bit of dry
air aloft ahead of the preceding pre-frontal bands of convection
that look to reach the far western zones around daybreak. Guidance
does show some weak instability late as well as moistening ahead
of the showers under increasing southwest flow as forecast PWATS
increase to around an inch or so by morning. Thus appears some
of this upstream convection may hold together long enough to
reach the I-77 corridor Saturday morning per steep lapses and
latest HRRR. Otherwise slowed down clouds/pops a little given
latest trends with overall clear to partly cloudy overnight
until the leading edge of the solid cloud deck to the west
arrives late. Also bumped up southwest winds along the western
ridges just ahead of the boundary as expect decent mixing to
persist as the gradient just ahead of the shra/tsra tightens.
This may keep lows from dropping much at elevation far west so
bumped up temps on the ridges, while leaving elsewhere similar
to before, mainly 50s.
Surface front should pivot east through the region by early to
mid afternoon Saturday with the actual 850 mb boundary lagging
a few hours behind. This along with the best dynamics passing
north and more westerly flow, as perhaps the old outflow zips
east, should clip off coverage nearing the Blue Ridge Saturday
morning. However guidance shows some increase in instability
east/southeast sections by early afternoon where likely to see
showers redevelop along the previous outflow. Timing and fast
movement would suggest a very limited severe threat unless
things slow down more at this point. Otherwise other concern
with post frontal winds as much colder air behind the 850 mb
front arrives late Saturday when subsidence increases and the
jet aloft remains marginal. Latest local guidance suggests some
potential for a period of wind advisory criteria mainly Blue
Ridge from ROA to TNB so included mention mountains in the
earlier updated HWO.
Previous discussion as of 320 PM EST Friday...
High pressure and upper level ridge along with abundant sunshine
pushed the temperatures into the 70`s across most of the area today.
Warmer than normal temperatures will continue into tonight as
southerly flow is maintain ahead of an approaching cold front.
Overnight lows will generally be in the 50s with mid to upper 40s
Showers associated with the cold front will enter southeastern West
Virginia and southwestern Virginia around sunrise tomorrow, then
move to the Blue Ridge by 10 AM. Some gusty winds and a rumble of
thunder may accompany these showers. These showers will move over
the foothills and piedmont counties during the early afternoon.
Increase instabilities from daytime heating may allow some of these
showers over the piedmont to intensify during the afternoon. The
main threat from these storms will be straight-line winds. Latest
models are showing the line of showers redeveloping and intensifying
east of route 15 (Farmville Road). If this is the case, the wind
threat for the piedmont will be greatly reduced. The best threat and
higher confidence area for strong storms Saturday afternoon and
evening will be north of I64 and moreso north of the Mason-Dixon
line. Rainfall amounts from showers Saturday will generally be up to
a quarter of an inch (0.25).
The upper level trough of this system will track across the Ohio
Valley Saturday. This trough will bring a round of light
precipitation to the mountains during the late afternoon and
overnight hours to western slopes of southeastern West Virginia and
southwestern Virginia. These showers will start out as rain, then
transition over to snow as colder air filters in behind the Theta-E
boundary late in the day. Trajectories from the Great Lakes are
brief and along with the warm ground, only light accumulations of
snow are possible at elevations above 3000 feet into the evening.
With the slow arrival of cold air, Saturday`s temperatures will warm
above normal with readings ranging from the mid to upper 50s west to
the 60s east of the Blue Ridge. Southside Virginia may peak in the
lower 70s Saturday afternoon.
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.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/...
As of 330 PM EST Friday...
Shot of cold air will cross the forecast area Saturday night
into Sunday with temperatures tumbling back to near seasonal
norms with highs in the 40s/50s and lows in the 20s/30s. Blustery
northwest winds Saturday night will gradually diminish Sunday.
Model soundings indicate some very shallow moisture across the
Mountains west of the Blue Ridge Saturday night, so can`t rule
out a few flurries, but the overall trend will be for drying and
clearing with Sunday turning out to be Mostly Sunny.
Zonal flow will develop across the CONUS by Monday, cold shot
of air over our forecast area short lived, and replaced with
warm air advection from the southwest. Southwest winds will also
bring a return of increasing moisture and threat for showers.
Short wave trough within the southern stream will bring
increasing cloudiness on Monday, with potential for scattered
rain showers Monday night. With moderating temperatures, not
expecting any p-type issues...temperatures warming back above
freezing and above normal.
.LONG TERM /TUESDAY THROUGH FRIDAY/...
As of 345 PM EST Friday...
Zonal flow will promote warming temperatures for mid week with
main focus for precip centered around noisy southern stream
short wave energy which models are having difficulty timing.
Through Wednesday, the region is expected to remain in a deep
southwest flow in advance of a developing trough across the
Central Plains states. The GFS continues to be more progressive
in depicting pieces of energy streaming across the area, each
with the potential for additional rounds of precipitation,
especially on Wednesday. CAPE is also forecast to peak on
Wednesday, and with trough generally forecast to come through
during the day this suggest potential for thunderstorms. Since
models have not quite nailed down the timing will list the
threat as slight chance attm.
Temperatures will also peak Wednesday followed by lowering
numbers for Thursday behind the trough, dry weather and near
seasonal temperatures returning Thursday night into Friday.
.AVIATION /01Z SATURDAY THROUGH WEDNESDAY/...
As of 725 PM EST Friday...
Guidance is depicting a scenario similar to last night which was
a bit too aggressive with development of low clouds/fog so will
shade conditions away from this and go more optimistic with
generally VFR all sites tonight. The biggest challenge will be
timing the cold frontal passage west to east across the region
tomorrow. Expect fropa early morning at KLWB and KBLF, mid/late
morning at KBCB and KROA, then early afternoon KLYH and KDAN.
while best dynamic forcing will be shearing off to our north,
expect there will still be thunder with the front. Winds behind
the front become west to northwest and quite gusty as cold air
advection generates good downward momentum transfer and the low
level wind field amplifies late in the TAF period.
Extended Aviation Discussion...
Showers and isolated thunderstorms will be east of KLYH and
KDAN by 00Z/7PM. Drier weather and increasing northwest winds
follow behind the front on Saturday night through Sunday
morning. High pressure will cover the region behind the front
for late Sunday into Monday with lighter winds.
A warm front is expected to stay near us Monday night into
Tuesday keeping a threat of showers around with sub-VFR