Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Blacksburg Virginia
348 PM EDT Wednesday Apr 23 2014

Synopsis...
cool high pressure over the Ohio Valley will build southeast
passing overhead tonight and Thursday. Another cold front will
cross the area from the west on Friday.
&&

Near term /through Thursday/...
as of 210 PM EDT Wednesday...

Mainly a temperature forecast over the next 24 hours as the upper
trough departs overnight and shortwave ridging builds overhead on
Thursday. This will allow the cool surface high to slide southeast across
the region later tonight and then to the coast on Thursday. This
will keep very dry air in place but allow for diminishing winds
tonight and the onset of weak SW flow Thursday afternoon as the
ridge departs. May start to see some high clouds return from the
west later Thursday otherwise clear overnight and mainly sunny
Thursday.

Main concern with low temperatures tonight given potential for good
radiational cooling after midnight as dewpoints remain low and
winds go light under clear skies. This would support values going
below freezing mountains...with some 20s valleys...and well into
the 30s out east where the growing season has started. However
similar to temperatures a few nights back...dewpoint depressions look
quite large given dry air which wont support much frost outside
The Bottoms/low spots...while coldest values will be brief near
dawn pending how far winds decouple. Thus after coordination with
surrounding sites...have opted out of a frost advisory for now
while increasing the patchy coverage east late with lows mainly
middle/upper 30s. Should quickly recover on Thursday per 850 mb warming
and weak SW flow aided by heating of dry air which will push most
into the 60s if not low 70s in spots.
&&

Short term /Thursday night through Saturday/...
as of 330 PM EDT Wednesday...

Short wave coming out of the middle Mississippi Valley will cross
through the Ohio Valley Thursday night...reaching the far western
County Warning Area around 12z/8am Friday morning. This feature
weakens as it crosses the area. The corresponding surface cold
front is expected to pass through during the day Friday.

Will speed up arrival time of precipitation Friday morning. With
this timing...appears the best opportunity for thunderstorms will
be east of the Blue Ridge...ahead of the front in the afternoon.

Very little change in airmass behind the front on Friday night. Have
maintained a low chance of precipitation in the far northern County
Warning Area as a secondary front comes in from the north on
Saturday.
&&

Long term /Saturday night through Wednesday/...
as of 330 PM EDT Wednesday...

A more pronounced push of cold air will come in behind a front on
Saturday night. The front will stall south of the forecast area on
Sunday. High pressure builds in north of the boundary and will
keep the winds from the east on Sunday. Have kept Sunday maximum
temperatures below guidance with the expected east component of
the winds and increasing cloud cover.

The upper pattern becomes more amplified through this time frame.
Will have an upper ridge over the eastern United States Saturday
night through Monday. Then a closed low tracks through the central
United States Tuesday and Wednesday.

Surface low pressure develops in the Central Plains by Monday with
broad flow off the Gulf of Mexico. Moisture will lift over the
stalled boundary in the Tennessee Valley on Monday. Models were
offering different solutions for the track of the surface low
Tuesday and Wednesday with the European model (ecmwf) much farther north and west
than the GFS.

Wpc was leaning toward a track of the surface low through the Ohio
Valley with a strong cold front coming through the region on
Wednesday. With overall southwest flow and the front remaining to
the west until Wednesday...have trended toward warmer temperatures
for Monday through Wednesday.
&&

Aviation /20z Wednesday through Monday/...
as of 1255 PM EDT Wednesday...

Widespread VFR conditions under mainly clear skies will prevail
during the next 24 hours into Thursday afternoon. Gusty northwest winds
will continue this afternoon...with sustained 12-17 kts including
gusts to 20-25 kts. Winds will diminish and become light tonight
with return of light south to southwesterly flow Thursday as high
pressure passes overhead and off the middle Atlantic coast.

Extended aviation discussion...

A cold front will cross the area early Friday...with a brief
period of MVFR restrictions possible as showers/isolated storms
move east with the front through the area. Strong high pressure
builds in Friday night into Saturday with VFR expected under gusty
northwest winds. Winds diminish Saturday night followed by high
pressure passing across Sunday with fair dry weather. Moisture
will start to return north Sunday night as a boundary lifts NE
ahead of low pressure across the central U.S. This will likely
bring sub-VFR conditions later Sunday night and espcly on Monday.
&&

Fire weather...
as of 330 PM EDT Wednesday...

Red flag warning remains in effect from the Virginia Blue Ridge
northward into north central Virginia for continued gusty northwest
winds and low humidity. Fire Danger Statement elsewhere for increased
fire danger where winds wont be quite as strong and fuels a bit
wetter.

Otherwise winds will range from 15 to 20 miles per hour sustained...with gusts of
25 to 30 miles per hour through early this evening. Humidity minimums near
20 percent will also be common resulting in continued marginal red
flag criteria until around sunset. In addition...it has been 7
days since measurable rainfall has occurred in Roanoke valley and
the immediate Blue Ridge counties...including the New River and
Shenandoah valleys. This has allowed leaves and brush to become
dry and very receptive to fire...10 hour fuel moisture dropping to
around 7 percent or lower.

Concern is for rapid fire spread if a wildfire occurs.

Winds should finally diminish overnight as high pressure builds in
with humidity levels recovering to 70-80 percent or higher after
midnight. Winds will be much lighter on Thursday and gradually
turn more SW under 10 miles per hour. However relative humidity will again
drop with heating during the afternoon with levels likely dropping
to below 25-30 percent across most of the region. A cold front may
bring some wetting rain in the form of scattered showers and
isolated storms during Friday.

&&

Rnk watches/warnings/advisories...
Virginia...red flag warning until 8 PM EDT this evening for vaz007-
009>020-022>024-032>035-047.
NC...none.
WV...none.

&&

$$
Synopsis...jh/PM
near term...jh
short term...air mass
long term...air mass
aviation...jh/PM
fire weather...jh/PM