Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Blacksburg Virginia
817 PM EDT Sat Apr 19 2014

Synopsis...
slow moving low pressure along the southeast coast will drift
east and offshore tonight. Strong high pressure will build into
the area from off the middle Atlantic coast Sunday providing a return
of drier air into early next week. Another cold front arrives
later Tuesday with the next chance of showers and storms.

&&

Near term /through Sunday/...
as of 815 PM EDT Saturday...

Latest radar trends supporting earlier GFS model depiction of
expanding area of precipitation northeast to as far north as the
NC/Virginia state line early this evening...before being shunted off to
the east during the early morning hours as upper diffluence and
associated upward vertical motion fields through moist middle layer
moves away from the Blacksburg forecast area.

As a result...just issued quick update to boost threat of
precipitation across the foothills and Piedmont of north-central
NC for much of the rest of this evening. However...continued
entrainment of drier air in lower levels should ensure that
overall precipitation amounts along and south of the Virginia state
line remain light...generally less than one tenth of an inch.

Rest of forecast for tonight still appears on track...so no
additional adjustments were needed.

As of 215 PM EDT Saturday...
deformation type axis around the upper low now over southeast Georgia remains
persistent at this time with a lingering band of rain mostly over northwest NC
where feed of low level easterly fetch remains best. However
farther north...drying continues under a more predominant NE flow
with quite a dropoff in moisture crossing the Virginia/NC border. Upper
low will again be slow to move overnight with this feature being
blocked east to off the southeast coast by Sunday morning. Models
continue to struggle with the leftover deformation band overnight
in just how long axis of deeper moisture will remain and whether
or not it jogs back north as another wave pivots in from the west.
Latest GFS again remains wettest in keeping precipitation going from northwest NC
into Southside Virginia...while the ec/hrrr less with only spotty
residual -ra...and most other solutions dry. Given current trends
will continue to lean toward the GFS/ensemble blend and keep
higher probability of precipitation far southern sections into this evening which may be
overdone given current dry advection. Otherwise mixture of high/middle
clouds likely with deeper moisture perhaps pushing back north
across the east overnight with the next impulse aloft. Kept low
temperatures similar to MOS except colder northwest valleys where radiational
cooling should be best.

Upper low finally shifts far enough offshore during Sunday to allow
a drier easterly flow to develop as high pressure along the middle
Atlantic coast strengthens under gradual shortwave ridging aloft.
This should allow skies to clear by afternoon per model relative humidity cross
sections with highs bouncing back into the 60s to around 70 given
heating of dry air and slight 850 mb warming.

&&

Short term /Sunday night through Tuesday night/...
as of 200 PM EDT Saturday...

Moderating temperatures are expected through at least Monday
night as an upper level ridge of high pressure builds over the
Appalachians. Low level southerly warm air advection and plenty of
sunshine will contribute to this. However...one last cool night is
expected Sunday night. Some patchy frost will be possible for a
couple of hours around sunrise for parts of the New
River...Greenbrier and southern Shenandoah valleys.

By late in the day Monday...cloud cover will start to overspread
the area from west to east in advance of an approaching cold
front. This trend will continue into Monday night with mostly
cloudy to cloudy skies across the entire area by sunrise Tuesday.
With the clouds will come increasing chance of showers late Monday
night through the day Tuesday as the front approaches and then
crosses the region. Model convective parameters still offer the
potential for thunderstorms during the day on Tuesday. Despite
more cloud cover and the precipitation...the strength of the warm
air advection will help temperatures on Tuesday be a few degrees
warmer than those of Monday. A mix of middle 60s to lower 70s looks
promising across the area.

Tuesday night...coverage of showers will be on the decrease as
the front exits the region. By midnight...all that is expected to
remain is a small region of isolated to scattered showers across
the preferred upslope areas of southeast West Virginia thanks to
gusty northwest winds in the wake of the cold front.

&&

Long term /Wednesday through Saturday/...
as of 1245 PM EDT Saturday...

On Wednesday...we still expect winds to be on the gusty side
during the morning hours in the wake of the Tuesday/Tuesday night
cold front. While there will still be some upslope clouds across
southeast WV in the morning...the remainder of the region will be
mostly sunny to sunny. Clouds will be on the decrease across southeast WV
during the day as winds weaken as drier high pressure builds
toward the region.

Surface high pressure will move across the area Wednesday night
and become situated off the East Coast by Friday.
Concurrently...an upper level ridge will build over the
Appalachians while strengthening southwest flow allows for good
warm air advection. Expect Wednesday night through Thursday to be
a period of no precipitation and moderating temperatures.

A cold front will approach the area Thursday night and cross
the region Friday into Friday night. Anticipate temperatures to
remain mild in advance of the front Thursday night into
Friday...but with increasing cloud cover and chances of showers.
Model guidance currently offers the potential for some convective
weather with the passage of the front. Have included a slight
chance of thunderstorms accordingly on Friday.

Cooler air arrives Friday night into Saturday...while lingering
rain showers decrease in coverage...ending last across southeast
West Virginia where upslope conditions will maintain the
precipitation the longest.

&&

Aviation /00z Sunday through Thursday/...
as of 730 PM EDT Saturday...

Drier low level NE flow has continued to maintain VFR status at
all taf sites with mostly a middle to high broken/overcast canopy in place.
Expect this overall trend to prevail tonight with widespread MVFR
to IFR conditions remaining well south of the Virginia/NC state line.

Last in a series of weak impulses to our south will push
northeast through central NC overnight...which may allow for a
temporary northward push of precipitation this evening into the
early morning hours...remaining mainly along and south of the
NC/Virginia state line. Proximity of rain shield and deeper low level
moisture may help lower ceiling at kdan back down to near (but not
likely below) 3k feet before midnight...but as this wave moves off
to the east of the area later tonight...any threat of lower
ceilings at kdan should erode back to prevailing middle/high level
canopy before daybreak Sunday.

Extended aviation discussion... the upper low slowly exits to the
east Sunday and high pressure returns maintaining VFR conditions
into early next week. May still have some middle/High Deck ceilings
across the east Sunday morning before slow clearing takes place
from west to east by/during the afternoon.

The next cold front approaches Monday night with more showers and
possible thunderstorms...with localized MVFR flight restrictions
on Tuesday. The front exits Tuesday night...resulting in a return
to VFR weather Wednesday...except for the potential of some MVFR
ceilings over the far western upslope sites in southeast West Virginia. High
pressure builds in for Thursday...with widespread VFR likely.

&&

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

&&

$$
Synopsis...jh/rab
near term...jh/wert
short term...ds
long term...ds
aviation...jh/rab/rcs/wert