Scientific Forecaster Discussion
fxus61 krnk 221225
Area forecast discussion
National Weather Service Blacksburg Virginia
725 am EST Mon Jan 22 2018
weak high pressure at the surface and aloft early today will
give way to a strong upper-level low pressure area and
associated cold front moving east from the central U.S. The cold
front will bring widespread rainshowers and isolated
thunderstorms to the area late today and tonight, followed by
colder temperatures, gusty northwest winds, and mountain snow
showers by Wednesday. Temperatures will moderate to above normal
again later in the week before the next frontal system arrives
during the weekend.
Near term /through tonight/...
as of 415 am EST Monday...
Weather conditions will deteriorate fairly rapidly today from
our nice weekend weather as a deep upper low pressure area and
associated cold front move toward the area from the west. The
system is a full-scale cyclone with a significant snow storm on
the north end and severe thunderstorms on the south end.
Fortunately, neither one of these inclement conditions is
expected in our region to any degree, but there are nonetheless
some concerns that will need to be addressed as the weather
system moves through our area later tonight and Tuesday.
Through the afternoon, clouds, moisture, and winds will be on
the increase across the County Warning Area as the weather system approaches.
While there may be a few sprinkles or light showers out ahead of
the front during the afternoon, strong forcing should support a
qlcs-type convective system, which will enter southwest Virginia just
before sunset and translate eastward to the Piedmont by
midnight. With a 65-70kt low level jet accompanying this activity, will
need to monitor for convection that may be able to mix down to
the surface with these gusty winds. While the area is outlooked
for thunder, currently severe probabilities are not indicated in
our area. It is interesting to note that each successive model
run advertises slightly stronger forcing and instability as this
system moves into the area, so it will need to be watched.
Brooks-Craven severe parameter has been creeping up into the
5000-10000 range and at least weak instability has been
indicated for several model runs. However, there are
indications that a slight "in- situ" wedge may be in place, so
depending on the degree of cloud cover and light precipitation
that occurs in advance, will largely determine the intensity of
the main convection as it arrives later today. Brief heavy
rainfall is expected with the stronger convective cells as well,
but given the overall dryness and lack of meaningful
precipitation in the past couple of months (very cold and dry!),
Do not foresee any Hydro issues as this system will be moving
through very quickly.
All of last weeks snow cover is gone, even though the ground is
still quite cold from 2+ weeks of single digit and teen
temperatures. With gusty southwest winds and accompanying warm
advection with 850mb temperatures around +10c, look for most
areas east of the Blue Ridge to be well into the 60s today, with
50s further west for highs. Temperatures will remain above
normal overnight as the true colder air will not arrive with
this system until later Tuesday into Tuesday night. Thus, for
today temperatures will average about 20 degrees above normal
and will still be about 15 degrees above normal Tuesday morning.
So at this point, despite some minor concerns with thunderstorms
and gusty winds, nothing meets headline criteria. Any concerns
during the forecast period will be addressed via the severe weather potential statement.
Short term /Tuesday through Thursday night/...
as of 345 am EST Monday...
The cold front will be pushing through the region east of the Blue
Ridge across the Piedmont Tuesday morning before exiting the region and
taking the showers and thunderstorms with it. Will be watching
the front until it is clear of the area for any embedded
convective elements capable of bringing strong winds to the
Behind the front temperatures will be falling and upslope snow showers
will develop west of the Blue Ridge aided by short wave energy
streaming around the large upper trof over the eastern US. While the
snow is not expected to be particularly heavy, it will be a prolonged
period of show showers west of the ridge through Wednesday night that
will result in a couple of inches of accumulation at the higher
elevations through the 2 day period. The low level wind field will
amplify considerably behind the front and BUFKIT soundings indicate
efficient downward momentum Transfer. This will create a prolonged
period of strong, gusty winds especially west of the Blue Ridge through
Long term /Friday through Sunday/...
as of 155 PM EST Sunday...
Dry weather expected into Friday as high pressure moves across
the area then sets up over New England, while upper trough digs
across the central and northern plains. Will start to see
moisture work northeast on southwest flow late Friday into
Better threat of rain showers moves in late Saturday into Sunday
ahead of slow moving front, as the front becomes parallel to the
upper flow. This looks like a good chance for seeing an inch or
more of rain. Colder air will be delayed until Sunday night but
we could see some accumulating snow showers in the mountains
per strong forcing aloft with trough.
Temperatures will average above normal this period, falling back
toward normal or below beyond Sunday.
Aviation /12z Monday through Friday/...
as of 700 am EST Monday...
Broken-overcast VFR ceilings will prevail through much of the day as
high-level moisture increases from the west in advance of a
deep upper low and associated cold front currently located in
the Midwest. Ceilings will mostly be between 080 and 100. Late
in the day, a linear organized area, potentially a squall line
or qlcs will approach from Ohio/Tennessee entering the western portion of
the County Warning Area near or just before 00z, then moving across the County Warning Area
through around 08z in the east. This activity will be associated
with IFR-LIFR ceilings and IFR visibilities, which will continue
overnight. Another concern will be strong gusty winds associated
with the convective activity. Wind gusts of 30-40kts at the
surface are not out of the question as a strong 60-70kt low level jet
traverses the area. Wind direction will be mostly south-southeast-south-southwest
through the day, becoming west-southwest with the line of showers. Isolated
thunder is possible, but not likely, so it has been left out of
Notam...the ASOS at kdan remains inop tonight due to a power
supply failure and likely wont return to service until sometime
later Monday or perhaps after that. Therefore the observation
remains missing at this time. Also since lower ceilings and/or
patchy fog may impact the site, taf amendments will not occur
until data is once again available from the ASOS unit. This
includes an addition of amend not schedule in the terminal forecast at
kdan at this point.
Medium confidence in ceilings through the taf valid period.
Medium confidence in visibilities through the taf valid period.
Medium to high confidence in wind speed and direction through
the taf valid period.
Wind will continue to be the main problem behind the cold front
later on Tuesday into Wednesday. Strong gusty northwest winds
are expected behind the front Tuesday into Wednesday. VFR
ceilings should return to the Piedmont by Tuesday afternoon, but
MVFR ceilings with low clouds and potentially -shsn can be
expected across the westernmost sites, and possibly as far east
at times as kbcb. High pressure and VFR conditions along with
diminishing winds should return to the entire area by Wednesday.
Dry weather including good flying conditions can be expected
Thursday and Friday.
as of 325 PM EST Friday...
National oceanic and atmospheric administration weather radio, wxl60 (roanoke transmitter) which
broadcasts at 162.475 mhz remains off the air. The phone company
reported that the circuit line had been cut and will take until
Monday January 22nd to fix it.