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Area forecast discussion 
National Weather Service Blacksburg Virginia
112 am EDT Wednesday Sep 2 2015

Synopsis...
a ridge of high pressure will remain over the region through the
end of the week. Upper level disturbances tracking through the
ridging will result in widely scattered mainly diurnal showers
and storms into Friday. A weak backdoor cold front will drop
south through the area by the end of week bringing somewhat drier
and cooler air for the weekend.

&&

Near term /through today/...
as of 1150 PM EDT Tuesday...

Showers have faded and skies will stay clear to partly cloudy
south to more clouds in the northern County Warning Area closer to the upper
shortwave. Will maintain fog late tonight. No other significant
changes to the forecast were made.

Previous early evening discussion...

Just a few showers out there early this evening in association
higher Theta-E gradient over the Piedmont and WV/far SW Virginia. Best
deep moisture convergence resides in the Piedmont. We have weak
impulses stretched across WV into Virginia...aiding in weak lift.
Overall...not impressed with threat past 9pm...so I expect any
showers to fade before midnight. Will be dealing with some high
clouds through the night...especially the northern County Warning Area...with scattered
low/middle clouds.

Made only minor adjustments to previous forecast...with most
occurring with the sky cover. Will have to see if the overnight
sky cover can influence fog formation. Will look at that later
this evening.

Previous forecast discussion issued middle afternoon...



Starting September off on the warm side with afternoon
temperatures ranging from the low 80s west to the low 90s over
the Piedmont. Slow moving showers have develop with afternoon
heating...but have held near the Interstate 64 corridor for the
most part...with only a few sprinkles elsewhere across the
mountains. We can expect these showers to fade toward sunset as
heating ends...and expect clouds to fade as well...making for
partly cloudy skies entering the night.

With dewpoints expected to hold in the 60s overnight...light winds
and only scattered clouds...patchy fog appears likely once again
across the area...at least during the early half of the night. Middle
and high clouds will begin to increase shortly before dawn as an
upper level disturbance approaches from the Ohio Valley...which
may cause some of the fog to thin before sunrise. Otherwise...
expect a muggy warm night...with lows holding in the low to middle
60s areawide.

Any remaining fog will burn off quickly after 10 am Wednesday with
another day of strong heating. The upper level disturbance will
drift across the northern half of the area during early afternoon...
and expect greater coverage of shower activity across the
mountains. Forecast model soundings indicate that there will be
enough instability present for a few of the showers to intensify
into thunderstorms during middle afternoon. As the disturbance will
be slow moving...linger across our area through sunset. Afternoon
high temperatures will reach the upper 70s/low 80s across the
mountains...and climb as high as the low 90s across the Southside.

&&

Short term /tonight through Friday/...
as of 345 PM EDT Tuesday...

The GFS moves the Ohio short wave slower than the NAM/sref. Given
the lack of a kicker for this vorticity maximum...the slower GFS seems more
reasonable...but even on this slower track the short wave will still
be southeast of the forecast area by Thursday afternoon. Will extend
the probability of precipitation in the forecast for Wednesday
night. Stayed closer to mav guidance through the period.

Backdoor front moves south through the New England and middle Atlantic
coast. Models were showing some convergence in eastern Virginia but
still questionable how much lift this feature may provide for the
County Warning Area. Best probability of precipitation will be in
the northeast counties of the forecast area...east of the Blue
Ridge. As this front moves down the coast and high pressure builds
south over the Appalachians...winds will turn from north to east.
This easterly component of the wind at the surface through low
levels may also expand the cloud cover and probability of
precipitation along the Blue Ridge.

&&

Long term /Friday night through Tuesday/...
as of 100 PM EDT Tuesday...

Lower 500 mb heights remained trapped under the ridge over the
southeast United States Friday night through Sunday. Sunday night
into Tuesday...the center of the upper ridge...drifts south into the
middle Atlantic region. Heights will be close to 594dm according to the
00z GFS. At the surface Friday night any remnants of the backdoor
front wash out and by Saturday The Wedge will be fully established
and this feature will remain in place through Sunday. High pressure
will cover the Middle Atlantic States through Tuesday. Unseasonably
warm temperatures...with 850 mb temperatures approaching +20...move
into the region with the higher heights.

A drier air mass begins to approach the region as The Wedge builds
in on Saturday. Sunday and Monday...European model (ecmwf) was showing much drier air
above 850mb. Below 850 mb the drier air remains east and south of
the forecast area through Monday.

&&

Aviation /06z Wednesday through Sunday/...
as of 100 am EDT Wednesday...

Other than for debris middle/high clouds...fog the main concern
overnight with potential again given less clouds of seeing the
valleys such as klwb/kbcb drop into LIFR/IFR for a period prior to
daybreak. Also some fog possible around klyh-kdan where brief MVFR
to IFR possible while coverage looks quite spotty at kblf where
limited to just a very brief inclusion of MVFR conditions around
dawn.

For Wednesday...should see any low clouds/fog fade with the onset
of heating allowing for VFR conditions to quickly return by middle
morning if not sooner. Then expect the upper wave over the Ohio
Valley to slowly shift southeast into Virginia. This will allow
more instability with steeper lapse rates and lift. Showers and
storms will be more scattered Wednesday afternoon with best
forcing across the foothills/Piedmont to the New River valley. As
of now will keep the going thunderstorms in the vicinity at kroa/klyh and kbcb after
19z/3pm...but think coverage will be less further west and
southeast...so only going with a vcsh at klwb-kblf while keeping
mention out of kdan.

Overall expect VFR conditions Wednesday with ceilings of 4-6kft with
towering cumulus and cumulonimbus in the afternoon.



Extended aviation discussion...

The forecast area will remain in a weakly forced synoptic
environment through Thursday...with chances for diurnal
convection mainly from the Blue Ridge west...and some MVFR/IFR
fog/stratus development at night...especially at kbcb and klwb.

For Friday into Sunday will see high pressure in control though
the position of the high over the middle Atlantic may slide more MVFR
ceilings off the Atlantic into our eastern taf locations.

&&

Equipment...
as of 400 PM EDT Tuesday...

The dew point reading at kjfz...Richlands Virginia AWOS...is incorrect.
At this time we do not know when this sensor will be repaired.

&&

Rnk watches/warnings/advisories...
Virginia...none.
NC...none.
WV...none.

&&

$$
Synopsis...jh/nf
near term...nf/wp
short term...air mass
long term...air mass
aviation...jh/nf/wp
equipment...

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