Area Forecast Discussion
Issued by NWS Blacksburg, VA-- Remove Highlighting --
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FXUS61 KRNK 291506
Area Forecast Discussion
National Weather Service Blacksburg VA
1006 AM EST Tue Nov 29 2016
-- Changed Discussion --A weak front will remain just west of the area this afternoon into
early Wednesday allowing a series of upper disturbances to track
along the boundary. This will result in periodic rounds of
rainfall this afternoon and again later tonight into Wednesday. A
stronger cold front will pass through the region Wednesday night
bringing drier and cooler weather for the end of the week.
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.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --As of 950 AM EST Tuesday...
Axis of heavier showers now crossing the Blue Ridge along the
residual outflow from overnight and along a warm front lifting to
the northeast. Decent rainfall amounts along the swath of precip
with a good third to one half inch currently. Latest HRRR shifts
this band, in advance of the stalled front along the western
slopes, quickly east into the afternoon with most significant
rainfall exiting the southeast by mid afternoon if not sooner.
Cant totally rule out a rumble of thunder over the far south and
east as guidance shows some weak instability under a swath of good
upper diffluence within the very mild air that has advected north
from overnight. However appears most convection likely to be even
farther to the southeast where deeper instability will reside so
leaving out mention.
Otherwise bumped up pops to higher likely/categorical along the
axis of rainfall as it progresses east into the piedmont through
early afternoon. This should bring about a lull in all but the far
southern sections where plan to leave in some low pops for late in
the day once the main band slides east. Southwest winds remain
quite strong aloft but currently too high to mix very strong
speeds to the surface despite lack of much of an inversion where
temps have warmed. Highs again appear mild, mostly 60s to even
around 70 southeast, but tricky farther north where still stuck in
the cool pool north of the warm front. Thus lowered highs back
closer to 60 far north/west where may see a brief surge once the
rain tapers per latest Met mos.
Previous discussion as of 400 AM EST Tuesday...
First round of much needed rainfall moving through the region
early this morning. This rainfall is associated with a
dampening/shearing out short wave emanating from a deep upper low
centered across the Midwest. Rainfall amounts so far have all been
less than an inch ranging from around 2/3rd or a letter better
across northwest NC into Grayson county VA to only a few hundreths
across the Piedmont. Far from a drought buster, but certainly
welcome, especially with the best rainfall in the most severe
drought Blue Ridge mountain forest areas. The last piece of energy
from this short wave will traverse the region through the morning
hours with additional light rainfall amounts, mostly from this
point forward across the northern and eastern portions of the
forecast area. Additional rainfall during the afternoon/evening
will be limited, but cannot completely drop pops below mentionable
values as broad southwest flow aloft prevails and additional
upstream disturbances are on the way. As the upper low continues
to move slowly east through Wednesday, a more significant and
final disturbance is progged to round the base of the trough and
bring another round of rainfall beginning late tonight in the west
spreading further east Wednesday. With a better tap into the Gulf,
this one promises to bring more beneficial rainfall to the area,
but will leave further discussion on that to the next section.
The GFS is fairly aggressive on instability late tonight/early
Wednesday as the upper-level feature approaches and elevated
thunder cannot be ruled out, but given that this is the only model
indicating such and the potential for a cool, stable boundary
layer, will not include thunder through this period at this time.
The strong surface winds associated with the 50kt+ LLJ will shift
east of the CWA during the morning hours. A few gusts toward 40kts
still not out of the question through the morning hours along the
higher peaks of the Blue Ridge across Bedford/Amherst, but not
enough coverage to warrant a new Wind Advisory. A less impressive
LLJ will approach the region from the west late tonight in
association with the next short wave, but speeds look to be well
below advisory criteria east of the divide and thus will not issue
any further wind headlines at this point.
850mb temps surge well into the double-digit range today of +12C
to +13C well north into the northeast U.S. A non-diurnal
temperature trend was already under way this morning with balmy
late November temperatures in the 50s and which have been rising
overnight. By afternoon this will equate to temperatures well into
the 60s except 50s higher mountain elevations, some 20 degrees
above normal. Look for a very mild night tonight/Wednesday
morning as strong southerly winds continue to transport warm/Gulf
air this direction. Low temperatures Wednesday morning will be
some 30 degrees above normal in the mid 50s to near 60!
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.SHORT TERM /WEDNESDAY THROUGH FRIDAY NIGHT/...
As of 200 AM EST Tuesday...
On Wednesday, a shortwave trough will head northeast across the
region on the east side of an upper low/trough centered across the
north central part of the nation. At sunrise, rain is expected across
primarily the western and southwestern portion of the area. As the
day progresses, coverage will translate eastward in concert with a
shift eastward of the upper trough. By Wednesday evening, the
shortwave trough will be exiting the eastern portion of the area
with some trailing precipitation across the eastern quarter of the
The upper systems associated 850mb trough axis will cross the area
Wednesday evening, and northwest flow will commence. Although, the
latest guidance suggests not the best cross barrier flow with an
orientation more west-northwest. Look for scattered rain showers
across the western slopes of southeast West Virginia through the
overnight hours. Enough cold air may arrive by sunrise Thursday for
a mix with or change over to snow showers.
Thursday through Friday night, the upper low/trough near the Great
Lakes region will make gradual progress eastward towards and over
the Canadian Maritimes. This will keep the region under a prevailing
west to northwest flow, all while an west-east oriented surface
ridge axis noses into the western parts of the area. We expected
flurries/sprinkles to conclude the precipitation Thursday morning
across the western parts of Greenbrier County, WV. The vast majority
of the area will be dry Thursday into Friday night. A disturbance
within the northwest flow is expected to pass through NY/PA on
Friday. While most of the energy and moisture with this system will
stay north of the area, enough may make it south enough to yield
scattered upslope flurries/sprinkles across western Greenbrier
County Friday into Friday night.
Temperatures will trend colder during this portion of the forecast.
Readings on Wednesday will be 15 to 20 degrees above normal. By
Friday, temperatures closer to normal for this time of year are
.LONG TERM /SATURDAY THROUGH MONDAY/...
As of 330 PM EST Monday...
Saturday brings possibly a transition day toward wetter
conditions late weekend, but there are unresolved differences
among the models in this regard. Highs near climo for early
December. Still considerable divergence persisting in the model
solutions for the latter part of the extended. 00Z/28 Euro came in
quite wet for weekend with a deep trough located over Texas and
the Gulf opening up with strong moisture return along with
favorable dynamics. Latest 12z/28 operational EC run maintains
similar solution with this fairly bullish scenario for additional
precipitation. This in distinct contrast to last few GFS/CMC runs
which keep western trough much further west, back across AZ or
northern Mexico with a flat zonal flow across the east and little
to no northward moisture return through Sunday. Latest PMDEPD
discussion is favoring the run- to- run consistency of the GFS and
maintaining high pressure over our area through Sunday and
suppressing significant precipitation well to the southwest. Have
raised PoPs into chance range starting early Sunday but confidence
is not particularly high.
.AVIATION /15Z TUESDAY THROUGH SATURDAY/...
As of 645 AM EST Tuesday...
Poor aviation conditions expected through the TAF valid period. A
broad upper low pressure area centered across the western Great
Lakes will drift slowly east through Thursday. An initial short
wave and associated cold front were moving through the region this
morning with a couple of waves of light rainfall. The later area
of light rain will traverse the forecast area during the morning
hours, exiting to the east during the early to mid afternoon.
Little additional measurable precipitation is expected during the
afternoon. The next wave of precipitation, associated with the
main disturbance rounding the base of the deep upper trough and
originating from the southeast states, will approach the area late
tonight and early Wednesday. There are some indications that there
may be a period of MVFR or better cigs west of the Blue Ridge for
a brief period this afternoon toward 00Z before conditions sock
back in for good for the rest of the night. However, this
improvement is not expected east of the Blue Ridge where south-
southeast winds will hold the low-level moisture in place.
Otherwise, mostly MVFR-IFR cigs through midday, becoming mostly
IFR-LIFR after 00Z in DZ and FG. The next round of rain will
arrive around 09Z in the west, closer to 12Z in the east.
Visibilities will generally be MVFR to low end VFR through mid
day, VFR during the afternoon, becoming mostly MVFR to locally IFR
overnight in DZ, FG, and -RA.
The strong low-level jet that traversed the area overnight has
generally pushed northeast of the area and is weakening. Winds
have dropped below advisory criteria. Will carry LLWS at KLWB for
a couple more hours, but not include anywhere else. Winds across
the Piedmont have largely decoupled showing east 3-5kts.
Otherwise, mainly seeing SSE-SSW winds 5-7kts with low end gusts
yet in a few places. Winds through much of the TAF valid period
will be SSE-SSW 4-8kts with low end gusts by afternoon many
locations, decreasing after 00Z and trending more SSE.
Medium confidence in ceilings and visibilities through the TAF
Medium confidence in wind speed through the TAF valid period.
Medium to high confidence in wind direction through the TAF valid
Extended aviation discussion...
Improving conditions expected after the upper low and final short
wave move east of the area Wednesday night. Conditions should
become largely VFR east of the Blue Ridge into the weekend.
Upslope clouds and potentially a weak clipper system could bring a
low chance of rain or snow showers into the Alleghanys Thursday
and Friday along with lingering MVFR cigs before conditions
improve there as well.
As of 430 AM EST Tuesday...
Beneficial rainfall of 1/2 to isolated near 1 inch amounts
observed across the southwest part of the forecast area, which has
been the driest area with the most significant drought and forest
fire conditions in recent weeks. Thus, this rainfall is very good
news. Now with winds also in a diminishing trend, fire threat has
been greatly reduced. Additional rainfall expected
tonight/Wednesday should further alleviate any fire weather
concerns in the short term.
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