Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 250852

Area Forecast Discussion
National Weather Service Blacksburg VA
452 AM EDT Thu Apr 25 2019

A stalled front across the region will lift slowly northeast as
a warm front today, passing north of the area tonight. A cold
front will cross the Mid Atlantic region on Friday followed by
high pressure on Saturday. Another fast moving low pressure
system will bring another cold front across the area later


As of 245 AM EDT Thursday...

Stalled front remains across southern sections early this
morning while the 850 mb boundary is farther north near the Ohio
River where ongoing bands of showers persist. Next stronger
ripple of upper energy passing to the west later today will
again attempt to lift the front back north allowing much of the
region to return to the warm sector on a backing southwest
component aloft. However even with the front across the area,
focus remains limited under weak ridging aloft, so appears any
showers through midday quite isolated at best. Instability
increases this afternoon while faint upper energy slides across
during peak heating. This could pop isolated showers mainly
across the west and along the front near the I-64 corridor late
where will include chance pops for now. Otherwise looking at
intervals of sunshine and clouds with more cloudy periods
north/west near the front. Will be an increasingly warm and more
humid afternoon under low level southwest flow that will again
send highs into the 80s south/east with 70s elsewhere, and
perhaps a bit cooler north pending exactly where the boundary
remains until mid/late afternoon.

Main southern stream wave will lift by to the west overnight and
merge with a northern stream trough sliding through the upper
Midwest. This will propel a cold front east to just west of the
Appalachians by daybreak, preceded by a band or two of showers
within the moistening/deepening south to southwest flow aloft.
However with the boundary layer just off the surface progged to
remain dry until late, slowed down higher pops a bit with any
likelys not until after midnight, reaching the Blue Ridge toward
dawn. Otherwise cloudy with increasing chance pops from west to
east during the evening, although this may be generous given
the residual front to the north, and best corridor of lift to
the west until very late. Thus wont include any thunder mention
late given more shallow nature to showers at this point. Otherwise
a mild night under increasing southerly breezes as the jet
aloft starts to ramp up, that should keep lows in the 50s if not
above 60 in spots.


As of 400 AM EDT Thursday...

Early Friday the lingering warm front of the past three days
will be north of the region near the WV/VA/MD border. An
increasingly meridional upper trough will be phasing across the
mid part of the U.S. helping to push a cold front eastward into
the OH and TN Valley. General timing of this new front has it
arriving in the western part of the CWA around daybreak and
reaching the Blue Ridge around 18Z. Instability is not
particularly impressive with CAPES generally 500 J/kg or less.
However low-level shear of 40-50kts at times during the day is
impressive. Latest HIRES CAM models show a narrow line of
convection developing late morning/early afternoon near the Blue
Ridge and then translating eastward rather quickly into the
LWX/AKQ CWAs and intensifying. Best threat for severe and
locally heavy rainfall will remain east of our area. However,
the exact location of a squall line/QLCS development still
remains under question and since it could develop a bit sooner,
SPC has outlooked about 13 of our eastern most counties for a
marginal risk of severe Friday. Will mention the threat via the
HWO/eHWO. Marginal hail and wind gusts are the main threat.
Rainfall amounts are expected to remain mostly under 1 inch as
the system moves through the area rather quickly and also the
poor morning timing for the western areas.

Temperatures will be cooler Friday primarily because of
precipitation and cloud cover. It will be late afternoon before
any cool advection begins to take place, so we should still
manage to muster highs near 80 across the Piedmont with 60s in
the western mountains. The more sun and later in the day the
front arrives/convection develops in the east, the greater the
threat for any severe thunderstorms.

The weekend looks rather chilly with rather strong cold
advection into the region on the heels of a period of gusty
northwest winds. A notable increase in pressure rises and cold
advection behind the Friday front has resulted in a marked
increase in winds Friday night into Saturday. Our local wind
guidance tool suggests that gusts into Wind Advisory criteria,
perhaps even a few spots in the mountains along the Blue Ridge
could reach High Wind Warning criteria. Will wait until the
rain/severe threat passes, but may well need to entertain a Wind
Advisory for Friday Night/Saturday with later forecast packages.

While the flow aloft becomes more zonal into Sunday, a fast
moving short wave skirts through the Ohio Valley into NY/PA on
Sunday. This will primarily be another wind maker for our region
with the dynamics/moisture/convergence needed for showers
remaining mostly to our north. Cannot totally rule out a few
showers near the I-64 corridor or across the western mountains
from orographic lift, but rainfall amounts will be miniscule.
The main concern will again be the wind with the GFS MEX MOS
showing potential winds of 20-25kts sustained at ROA once again.

The weekend looks rather chilly as 850mb temperatures plummet
from pre-frontal levels around +16C to around -2C by Saturday
morning and hover around 0C through the weekend, being
reinforced by the secondary trough on Sunday. High temperatures
Saturday and Sunday will be in the 50s west to near 70 far
southeast, with low temperatures in the 30s and 40s. Some
freezing temperatures possible in the mountains, but likely too
much wind for any frost in the Piedmont at this point.

/Confidence in Forecast Parameters/
Temperatures - Moderate,
Precipitation Probabilities - Moderate to High,
Winds - Moderate,
Thunderstorm Threat - Low.


As of 430 AM EDT Thursday...

An warm and increasingly unsettled period expected as a frontal
boundary/baroclinic zone lingers to our north through much of
the period in response to a strong subtropical ridge amplifying
across the Southeast U.S. There is always the potential that
some of the progressive/highly kinematic short waves tracking
across the northern U.S. could push the baroclinic zone further
south causing it to linger more in our area than across OH/PA.
Will need to watch. At any rate, it will be warm with 850mb
temperatures hovering around +15C or so. Diurnal/Pulse-like
convection appears to be the most favored mode through the
period with the better dynamics/instability remaining to our
north dependent on the exact location of the boundary. In
summary, an unsettled and warm period with an almost daily
chance for thunderstorms.

/Confidence in Forecast Parameters/
Temperatures - Moderate to High,
Precipitation Probabilities - Moderate,
Winds - Moderate to High,
Thunderstorm Threat - Low to Moderate.


As of 105 AM EDT Thursday...

Mainly VFR anticipated overnight with mostly mid/high clouds
around and perhaps some strato-cu at KLWB near the stalled
front. This boundary may provide the focus for isolated showers
along the I-64 corridor, but probability/confidence too low to
include in any of the TAFs. Moisture pooling along the front
will allow for some high based cu or alto-cu with thin layer of
cirrus above. Cirrus is expected to thicken on thursday with
sct-bkn high based cu and/or layer of alto-cu. Lower cloud
elements, promoting MVFR, may form along and north of stalled
front, but this would mainly be confined to areas north of I-64
KLWB-KCHO and points north.

Throughout Thursday surface to 500 MB flow backs to the
southwest and the front ends up moving back north as a warm
front. Winds are expected to be light and variable until front
makes a more bonafide push to the north Thursday afternoon.
When this occurs winds will primarily be out of the SW at 6-12


An area of low pressure is forecast to move northeast through
the Ohio Valley Thursday night and into New England Friday. The
associated surface cold front will cross the Mid-Atlantic
Region Friday. Expect a period of MVFR to IFR flight
restrictions in widespread showers and possibly thunderstorms -
especially Thursday night into Friday - before a return to VFR
weather on the backside of the departing system as high pressure
pushes back into the region on Saturday. Another weak cold front
will cross the area later Sunday with perhaps a period of
mountain MVFR in showers before high pressure returns widespread
VFR for Monday.




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