Scientific Forecaster Discussion
fxus61 krnk 201111
Area forecast discussion
National Weather Service Blacksburg Virginia
711 am EDT Tue Aug 20 2019
high pressure will remain over the region today into tonight
resulting in a continuation of the hot and humid weather. A
cold front approaches by the middle of the week bringing
increasing chances of showers and storms Wednesday into
Thursday. This front should finally work southeast through the
area by the weekend.
Near term /through tonight/...
as of 245 am EDT Tuesday...
Still mostly a persistence forecast again today with any patchy
fog early giving way to afternoon convection within the
hot/moist airmass across the area. This beneath high pressure
and weak upper troffiness that will provide just enough cooling
aloft and perhaps weak shortwave energy to help spark at least
widely to scattered storms into this evening. Better focus to
start development early this afternoon appears along/west of the
Blue Ridge where models suggest slightly better low level
convergence, per developing south/southeast flow, aided by an
axis of higher Theta-E and differential heating ahead of a weak
shortwave to the northwest. Guidance then remains a bit
uncertain with organization and just how far to the east any
clusters/multicells might evolve given weak flow aloft. However
does appear with outflow around and merging cells that even the
east will see some isolated coverage by mid to late afternoon.
Therefore covering most with chance pops including spots of
likelys west/northwest where models seem to have some consensus.
Latest forecast sounding profiles also suggest more of a pulse
nature severe/heavy rain threat across the region per dry air
aloft above high low level moisture within weak steering and
steep lapses. This may again support isolated severe cells as
indicated by the latest day1 Storm Prediction Center outlook. Otrw partly to mostly
sunny with highs again similar to the hotter nbm, in the upper
80s to mid 90s, including eastern heat indices of around 100.
Isolated convection likely to linger espcly southeast into the
evening ahead of a weak low sliding by to the southeast. Then
expecting mainly partly cloudy elsewhere before patchy fog
development takes place again overnight. Models also indicate
that more low clouds may head into western sections in the wake
of the passing low and deeper advection of moisture from the
southwest. In addition, will need to watch the remnants of the
projected upstream mesoscale convective system that may make a run, or at least some
development off residual outflow late tonight across the far
northwest as winds aloft increase. Otrw trimmed back pops for
the most part after early this evening with lows mostly 60s
mountains to 70-75 east.
Short term /Wednesday through Friday night/...
as of 300 am EDT Tuesday...
The upper level pattern will be gaining some amplitude through the end
of the workweek, though the appalachains and central mid Atlantic
region will only be on the fringes of the trof moving through the
northeastern US. This will ultimately push the slowly approaching
frontal boundary into the region but ensemble guidance continues to
show a slower timing and only slowly sags the front just south of the
area by Friday night. The net result will be increasing chances for
showers/thunderstorms along with increasing coverage through Friday.
Guidance shows a series of short waves crossing the area and this will
introduce the potential for upstream mesoscale convective system development to slide into the
region along the baroclinic zone. While guidance is ambiguous as to
specifics it appears that the greatest threat will reside off to our
west, where there is better instability, and to our north where dynamic
support is more robust. Will monitor the convective trends closely over
the next few days to see if the threat becomes better defined.
Precipitable water anomalies will be on the increase as the baroclinic
zone settles over the region so the potential for locally heavy
rainfall and potential Hydro issues will remain a concern through
Expect temperatures to be trending cooler, moreso on Friday as
clouds/precip and the frontal boundary keep highs in the low/mid 80s
east of the Blue Ridge, mid 70s/around 80 to the west.
Long term /Saturday through Monday/...
as of 345 PM EDT Monday...
A pattern change will slowly evolve into the weekend as an upper
trough centered across the Great Lakes continues to deepen and
help push a west-east oriented cold front slowly south through
the area. Models disagree on the speed with which the front will
drift south of the area enough to shut off precipitation
changes. At this point, that does not appear with strong enough
potential to remove pops from any forecast period within this
time frame. In fact, the weekend could be quite wet and
unsettled as the front slowly drifts southward through the area. This
front will eventually stall across the southeast by the weekend.
Given time of year and the overall lack of good upper-level
support to push the front south, have kept high chance to even
periods of likely pops across the southern two thirds of the
region into the weekend. It appears from the viewpoint of most
models that drying will be limited to northern areas, generally
near and north of the I-64 corridor.
Temperatures are expected to be a bit closer to normal as 850mb
temperatures drop back into the mid and upper teens and clouds
and precipitation remain abundant. Northern areas may eventually
end up being warmer during the daytime than southern areas by
the weekend, as clouds and precipitation will be more abundant
south/east of the Blue Ridge. Nighttime low temperatures will
likely remain at or above normal during the period, especially
across southern areas.
Aviation /12z Tuesday through Saturday/...
as of 705 am EDT Tuesday...
Mainly VFR anticipated with the exception of patchy fog
early this morning and isolated to scattered showers/thunderstorms
this afternoon/evening. Scattered thunderstorms this afternoon
will promote local MVFR/IFR, but only widespread enough to
include a tempo for lower conditions at klwb where expecting
better coverage. Will maintain a thunderstorms in the vicinity mention elsewhere this
afternoon with best chances likely over the mountains and
perhaps southeast near kdan. Mean wind aloft is forecast under
15 kts, so storm cells that develop should be slow moving.
Any residual convection again looks to fade with loss of
heating this evening although isolated storms could linger into
late evening or shortly past midnight per lingering instability
and outflow. Otrw looking at mostly VFR again tonight with
patchy fog/stratus likely the main issue overnight. However some
solutions do show lower cigs associated with stratus heading up
from the southwest riding up along the Blue Ridge late. Thus
sub- VFR cigs also possible over western locations late although
quite uncertain at this point.
Extended aviation discussion...
The airmass will remain unchanged through mid-week with daily
threat for showers and thunderstorms. A front will approach from
the northwest later in the week, resulting in an uptick in
shower/storm activity with potential for organized deep
convection across the Ohio Valley which may move downstream and
into the mid-Atlantic region as an mesoscale convective system or series of mcss.
Improving conditions with more in the way of VFR will be
possible for the weekend, pending just how far south of the
area the front makes it.
as of 250 am EDT Tuesday...
Record daily maximum temps: