Scientific Forecaster Discussion
fxus61 krnk 271706
Area forecast discussion
National Weather Service Blacksburg Virginia
106 PM EDT Tue Jun 27 2017
an upper-level disturbance moving across the area today will produce
partly to mostly cloudy skies and isolated light showers. The
passage of the upper disturbance will provide a reinforcing shot of
unseasonably cool, dry and pleasant temperatures tonight through
Wednesday. Temperatures and humidity levels then begin to rise with
continued dry weather on Thursday. Conditions then turn more
unsettled Friday and into the upcoming weekend.
Near term /through tonight/...
as of 1255 PM EDT Tuesday...
The early afternoon update will reflect yet another decrease in
the coverage of showers this afternoon. No location now has more
than a chance of an isolated shower. Starting to see a few
returns on kfcx radar, but these are very weak and could simply
be virga at this point. Have adjusted hourly temperature, dew
point, wind and cloud cover to reflect the latest observations
and expected trends through the afternoon. Forecast high
temperatures have not been adjusted by more and plus or minus
one or two degrees a just a few locations.
As of 955 am EDT Tuesday...
Have continued the trend of decreasing precipitation chances
today. Our latest forecast update removes any potential the
remainder of the morning hours, with the first isolated showers
developing between noon and 100 PM EDT. Have also adjusted
hourly temperature, dew point, wind, and sky cover forecast
grids to reflect the latest observations and expected trends
into the early afternoon.
As of 733 am EDT Tuesday...overall no significant changes made
to the going forecast. Did adjust sky cover downward slightly
through the next couple of hours. A review of incoming 06z
guidance and cam models has shown a trend toward less coverage
of daytime showers; though i've reduced pops somewhat,
maintained isolated showers in the forecast for most locations
but preferentially within a few miles on either side of the Blue
Ridge. As 1000-500 mb thermal trough builds over the area,
expect destructive sunshine promoting cumulus development, a few
which may pop some showers but dry sub-cloud air mass is a
significant hindrance toward measurable rainfall. No changes to
temps at this point.
Previous near-term discussion issued at 345 am follows...
Skies early this morning are mostly clear to partly cloudy, with
scattered mid-level cloudiness affecting areas along and west of I-
81. These clouds lie ahead of a cold front in association with a
reinforcing shot of cooler, drier cp air across the upper Midwest.
Broad 500 mb longwave troughing/cyclonic flow in place across much
of the eastern two-thirds of Continental U.S. This morning. Two shortwave
disturbances were evident in morning water vapor imagery: the
stronger shortwave over northeastern Ohio with the weaker feature over
For today...axis of broad longwave trough progresses across the
forecast area through the first half of the day. Shortwave trough
now over in should progress eastward across WV into the Virginia Piedmont
by mid-afternoon. Daytime heating may be slowed to some extent by
increased mid-level cloudiness, but expect some cumulus to develop
by mid/late morning into the afternoon with progged weak
instability. That should be enough to generate isolated to scattered
showers along the cold front. Based on BUFKIT forecast soundings off
the NAM, rap and GFS which all depict a dry and well-mixed sub-cloud
air mass, it's questionable if rain will even reach the ground.
Could see several cu today generate a good deal of virga, but did
keep quantitative precipitation forecast less than 0.03" today. Skies should be partly to mostly
cloudy through early afternoon, with greater clearing associated
with building surface ridge behind the frontal zone. Should be
somewhat breezy as well with northwest gusts as high as 25 mph
especially during the afternoon when the boundary layer mixes out.
Highs today remain unseasonably cool but refreshing given late-June,
which range from the mid 60s to lower 70s along and west of the Blue
Ridge to the mid and upper 70s in the Virginia/NC Piedmont and foothills
For tonight...cold, dry advection regime as broad high pressure area
now over the upper Midwest builds southeastward, becoming centered
over the forecast area by early Wednesday morning. Any cloudiness
erodes quickly and many areas should go clear by sunset. Questions
for tonight pertain to how quickly do northwesterly winds ease and
on fog development. It seems likely per 00z nwp suite that there
should be enough of a gradient to permit slower radiational cooling,
but especially around/after midnight, winds should drop off and
temperatures stand to plummet pretty quickly given the dry air mass
and 850 mb temps around +8 to +10c. I'm not as confident on the
potential for radiation fog development for the overnight as we'll
be lowering dewpoints for a good part of the night; areas that do
get rainfall do stand a better chance, though. All in all, a
comfortable night with lows some 7-10 degrees colder than late-June
climo values, running from the mid 40s to the lower/mid 50s.
Short term /Wednesday through Thursday night/...
as of 345 am EDT Tuesday...
High pressure centered over West Virginia Wednesday morning will
slide east Wednesday afternoon and push into the Atlantic Ocean
Thursday. A northwest to west flow aloft behind the exiting
trough will advective cooler air into our region Wednesday. High
temperatures Wednesday afternoon will range from the lower 70s in
the mountains to the lower 80s in the Piedmont.
The transition to higher humidity levels and warmer temperatures
begins Wednesday night into Thursday as higher low level Theta-E air
lifts north into our area as a deepening west to southwest flow
arrives. Low temperatures Wednesday night will vary from the mid 50s
in the mountains to the lower 60s in the southeast.
A few showers or thunderstorms may be possible Thursday afternoon
across southwest portions of the forecast area with the nose of a
low level Theta-E ridge, increasing pwats and differiental mountain
heating. Do not cancel your outdoor plans because of storms will be
quite limited. Highs Thursday will be a couple of degrees warmer
than Wednesday with readings from the mid 70s in the mountains to
the mid 80s in the Piedmont. Under clear to partly cloudy conditions
Thursday night, low temperatures will generally be from the the
upper 50s in the mountains to the mid 60s in the Piedmont.
Long term /Friday through Monday/...
as of 345 am EDT Tuesday...
A return to weather conditions more typical for Summer are expected
during the long term period, as the long range models show good
agreement transitioning from an upper level zonal flow early in the
period to an upper trof forming somewhere around the Great Lakes and
extending south into the eastern U.S. By late in the weekend into
This will result in increasing temperatures and humidity, as well as
an increase in storm chances. An upper level disturbance moving out
of the western Gulf of Mexico into eastern Virginia on Friday/Friday night
will help lead a surge in higher precipitable water air our area. This higher
moisture combined with a series of upper level disturbances...with
the usual differences in timing shown in the long range
models...will help generate scattered showers and storms each day
during the long term period, especially during the peak heating in
the afternoon and early evening hours. With increasing CAPES, but
low shear through the period,a few pulse severe storms and locally
heavy rain are possible.
Warmest temperatures are expected Sunday and Monday as 850mb
forecast temps approach +20c. This may generate heat index values in
the low to mid 90s in the east.
Aviation /18z Tuesday through Sunday/...
as of 725 am EDT Tuesday...
Mainly VFR through the 12z taf forecast period. Cold front
continues to slowly progress east out of central WV, and ahead
of it lies a plume of scattered-overcast mid-level clouds. A few showers
are possible as cumulus clouds develop this afternoon, though
dry sub-cloud layer likely leads to any isolated showers
producing no restrictions to visibilities. Indicated vcsh for
the late am/early PM period, with threat of isolated showers
generally ending at or before 20z. Will see west to northwest
winds 6-9 kts with gusts to near 20 kts as the mixed layer
deepens this afternoon. Confidence in this period is moderate to
high on ceilings, visibilities and winds.
Continued VFR currently anticipated for the evening. There is
the chance for patchy radiation fog and associated IFR to LIFR
visibility restrictions but confidence is not high on occurrence
and its areal coverage would depend to a large extent on if any
Airport can experience a wetting rainfall this afternoon.
Building high pressure ridge promotes lightening surface winds
and clearing skies. Confidence in the Tuesday evening period is
low to medium on visibilities, high for ceilings and winds.
Extended aviation discussion...
VFR conditions then anticipated until Friday, with potential
for overnight IFR to LIFR radiation fog Wednesday and Thursday
Friday into Saturday, the potential for sub-VFR conditions
increase as showers and storms become more numerous. Saturday
appears to feature a greater coverage of showers and storms
associated with a cold frontal passage. Likewise, an increase
in the boundary layer moisture will also lend itself to better
chances of late night/early morning patchy fog.