Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Blacksburg Virginia
242 am EDT sun Aug 18 2019
high pressure will remain over the region today through Monday
resulting in a continuation of the hot and humid weather into
next week. A cold front approaches by the middle of the week
bringing increasing chances of showers and storms Wednesday into
Near term /through tonight/...
as of 240 am EDT Sunday...
Weak surface ridge has slipped into the area early this morning
resulting in less clouds with some fog around again early on.
This scenario to continue over the next 24 hours with a weak
high just to the southwest separated from eastern subsidence via
a weak residual boundary along the Blue Ridge. Models again
suggest that despite high heights per mid level ridging and weak
focus that we will see added afternoon convection again today.
Appears best chances this afternoon across the west where will
see weak convergence between light easterly flow to the west of
coastal low pressure, and southwest flow mountains around the
high to the west. Best ribbon of Theta-E also progged along/west
of the Blue Ridge where differential heating/orographics, and
eventually outflow will lead to isolated to scattered convection
as the light southwest flow helps unzip coverage. This supports
a bit more expanse of chance pops although still likely looking
at more widely scattered nature per short term solutions as well
as the href. Lack of organization should preclude the overall
severe threat but pulse nature likely given dry air aloft...high
lapse rates and strong instability from heating. Expect only
isolated to little coverage heading in the Piedmont where weak
subsidence to the west of the coastal low should win out.
Otrw partly to mostly sunny after early fog and hot/humid again
with 85h temps of better than +20 deg/c helping push highs into
the low/mid 90s east and 85-90 west. Heat indices still below
criteria but around or just above 100 degrees in spots out east
by mid/late afternoon.
Lingering isolated convection may hang on a bit longer into the
evening given likely more afternoon coverage and possible
mergers into bands. Thus keeping low pops espcly west into mid
evening. Things again wind down overnight with clearing skies
and fog development in the valleys, including where earlier
downpours occurred. Lows mostly 60s to around 70 Piedmont.
Short term /Monday through Wednesday night/...
as of 230 am EDT Sunday...
Little change in the overall synoptic pattern as building heights keep
northern stream energy well off to our north, and a Bermuda high
remains anchored over the Atlantic. This will keep hot and humid Summer
weather in place across the Appalachians and central mid Atlantic
region through the first part of next week. Transient convectively
induced short waves will move across the region and, combined with low
level instability and orographic effects, will keep showers and
thunderstorms in the forecast. Given the lack of dynamic support the
overall threat for severe storms is low, but will have to be mindful of
outflow boundary interactions and clustering of cells pulsing up some
strong storms from time to time during the afternoon/evenings. Also
anticipate some heavy downpours and potential localized Hydro issues
will remain a concern in the moist environment.
By Wednesday a weak front will be approaching from the northwest. The
front is rather lethargic since there is not a big push behind it, and
expect actual passage of the boundary will not occur until later in the
week. However there will be enough influence from the front to increase
coverage and chances for showers/storms for midweek.
Temperatures will start the week hot with upper 80s/lower 90s west of
the Blue Ridge, 90s to near 100 degrees east. This is well above normal
and heat index values will be in the low 100s east of the ridge which
is just below heat advisory criteria. So a good idea to stay well
hydrated and limit outdoor activity Monday afternoon. A slight
moderation in readings will have highs mid 80s/around 90 west, low/mid
90s east by Wednesday. Lows will be in the 60s west with some 50s in
the valleys, upper 60s/lower 70s east.
Long term /Thursday through Saturday/...
as of 230 PM EDT Saturday...
The deterministic long range models are in fair agreement that an
upper level trof digs over the eastern U.S. Thursday/Friday which
will drive a cold front through our area. There still is
uncertainty about the magnitude of the upper level trof, as shown
by the gefs temperature/dewpoint plumes with a large spread in
values from Thursday night to Saturday. Thus, forecast confidence
in temps during this time is lower than average.
Confidence continues to remain high that scattered to numerous
thunderstorms will be possible along and ahead of this front
Thursday. Again, beyond Thursday, confidence decreases about
the location of storms with the European model (ecmwf) and GFS keeping precip
chances Friday and Saturday in the far southern portion of our
area where these models expect the sfc front to be located. The
Canadian model, however, with its much less amplified upper
level trof forecast, is more robust with precip expanding north
on Saturday. Bottom line is that Friday appears the most likely
day for dry weather in our region during this period.
Temperatures to cool to values closer to normal or even below
normal Thursday night/Friday before modifying as the upper trof
lifts and sfc high shifts off shore on Saturday.
Aviation /06z Sunday through Thursday/...
as of 100 am EDT Sunday...
Continued VFR to start late tonight before patchy dense fog
becomes an issue at a few sites overnight especially those that
had rain at or in vicinity of the terminals today.
Again appears that best fog coverage will be in the deeper
valleys such as klwb and kbcb although coverage remains
uncertain similar to last night. May also see more fog around
kdan before perhaps clouds with low along the coast slip back
across the area toward daybreak. Klyh a bit more uncertain but
at least a brief period of MVFR looks likely there late so kept
MVFR mention. Otrw lowered kdan a bit to include a couple hours
of IFR, with tempo IFR at kbcb. Klwb should drop mainly due to
flow near river bringing dense fog at the Airport by 09z/5am.
Confidence in this is lower than usual due to dry ground
recently and calm winds.
After fog and low clouds lift early Sunday morning, afternoon
widely scattered thunderstorms can be expected mainly over the
mountains. However, coverage is such to not have in the tafs for
now. Otrw looking at another round of scattered to perhaps
broken VFR cumulus fields, including some cumulonimbus buildups along the
southern Blue Ridge.
Expect any convection to again fade shortly after sunset Sunday
evening, leading to another round of patch fog including periods
of sub-VFR by early Monday morning.
Extended aviation discussion...
Moisture increases Monday and Tuesday ahead of a an upstream
cold front. This should bring an increasing chance of
showers/storms espcly by Tuesday afternoon with more periods of
sub- VFR conditions in addition to the overnight/morning fog. An
approaching cold front looks to bring a more widespread threat
for showers and storms including sub- VFR Wednesday into
as of 310 PM EDT Saturday...
Record daily maximum temps: