Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 171911

Area Forecast Discussion
National Weather Service Blacksburg VA
311 PM EDT Sat Mar 17 2018

A warm front will shift into the area from the southwest this
evening, followed by a low pressure system moving across from
the northwest tonight. This system will move south into the
South Carolina and Georgia by Sunday morning. Another complex
storm system works in from the west next week, with wintry
weather possible.

As of 230 PM EDT Saturday...

Area currently remains in an overall lull between the initial batch of
light precipitation that has exited the region and ahead of a stronger
shortwave aloft that should pass across late this evening. Models
continue to develop a cluster of deeper convection with this feature as
aided by weak surface reflection and convergence on the nose of
stronger 850 mb warm advection to the southwest. However recovery
of instability remains slow per most heating west of the mountains,
but appears deeper shear likely to overcome this as it works
into the southwest sections by 00z. This along with steep lapses
and an increasing unidirectional jet aloft could result in
initial clusters of strong/severe storms far west by early
evening. Then appears these multicellular storms could merge
into a small MCS complex capable of damaging winds as it slides
southeast into northern NC later this evening per latest HRRR.
This reflected in the latest forecast significant severe
parameters and the SWODY1 including a small swath of slight risk
across southern sections.

Therefore will continue high pops for deeper convection southwest third
espcly from 22-23z through midnight with mostly showers north of
Route 460 where parameters show more elevated nature to any instability.
Once this feature exits will still have the lagging 850 mb
boundary that will sag in from the north and perhaps result in
another swath of showers that may run from around LWB southeast
across central/eastern sections overnight before exiting early
Sunday. This supports another round of at least high chance pops
that should diminish around daybreak on Sunday. Lows mainly in
the 30s to low 40s but could get colder far north where precip
may end as a few snow showers late.

Deep moisture fades from drying aloft early Sunday as upper support
exits and shortwave ridging starts to take shape. This in conjunction
with weak high pressure under light north/northwest flow should allow
for clearing from north to south by afternoon with comfortable highs in
the 50s to lower 60s similar to the latest ECMWF mos.

As of 300 PM EDT Saturday...

The transition of the upper pattern from an eastern ridge to a broad
ridge is now looking to be a rather complex evolution as the ridge
flattens and several spokes of southern stream short wave energy
gradually carve out a successively deeper trof. Expectation is for
generally wet weather through midweek.

The first part of Sunday night looks to be dry, but a developing
wedge will combine with low pressure emerging along the coast to
bring southeasterly flow and robust isentropic lift over the region,
which leads to increasing rain chances overnight.

The situation remains relatively unchanged for Monday with
increasing dynamic support and increasing coverage of rain as a wave
moves through the Mississippi valley and approaches the Appalachians
from the west. The wave will move through the region Monday night
with a good dose of widespread precipitation. There also looks to be
potential for convective development upstream outside of the wedge
that would slide into the region and greatly enhance precipitation,
though any specific details are murky at this point. Also of note is
the steady cooling of the airmass from north to south which allows
for a gradual transition/mix of ptype far north Monday night but any
slushy accumulations look minimal through early Tuesday.

After perhaps a brief respite early on Tuesday, the next wave will
approach late in the day and renew precipitation across the region
with good forcing over the wedge and potential convection
developing/moving in from outside of the wedge. Complex situation
continues to unfold Tuesday night as additional short wave energy
fuels coastal low redevelopment with good isentropic, large scale
synoptic ascent, and mesoscale deformation over the region as the
cooling wedge deepens. Very early in the game but accumulating wet
snow looks to be a possibility across a good part of the region by
daybreak Wednesday.

With the developing upper trof expect temperatures to be trending
downward to well below normal through midweek.

As of 300 PM EDT Saturday...

Additional short wave energy continues to fuel the train of coastal
low formation through Wednesday which will keep a good chance of
some wet snow across the region. Diurnal effects in late March may
limit any accumulations to along/west of the Blue Ridge but far too
early for specifics. As the closed upper low moves across the region
synoptic forcing lingers along with the decreasing possibility for
wet snow through early Thursday. Northwest flow will also be on the
increase and will support upslope snow showers at the higher
elevations west of the Blue Ridge into Thursday night. The upper
trof axis finally pushes to our east and allows for a break of fair
weather on Friday before a warm front starts to push precipitation
back in our direction from the west late Friday night.

Temperatures will start the period well below normal, then moderate
a bit but remain on the cool side through the end of the week.

As of 1245 PM EDT Saturday...

Initial band of light showers fading upon moving across northern
portions of the area early this afternoon with VFR currently
returning to all except the KBLF-KLWB corridor where westerly
flow keeping persistent sub-VFR cigs in place.

Otherwise, should see a continued break in coverage for most of
the afternoon before more convective nature showers and storms
start to develop off to the west and propagate toward the region
by this evening. This may actually help scour the lower cloud
bases out of the west before clusters of showers and storms
arrive, perhaps as late as 00z/8PM around KBLF. Latest short
term models then have a complex of storms sliding southeast,
perhaps clipping KBCB/KROA by 02z/10PM and KDAN/KLYH by
midnight. Thus given increasing confidence have included some
TSRA mention either in prevailing or TEMPO groups for all except
KLWB/KLWB where may stay just north of the small cluster of
storms this evening.

Once this complex exits after midnight likely to see more
showers redevelop along the 850 mb cold front that will lag and
take until possibly daybreak Sunday to clear KDAN. This in
combination with lingering low level moisture in the wake of the
initial wave should keep sub-VFR around most spots overnight
with ocnl IFR within showers.

Any remaining sub-VFR conditions early Sunday should quickly
improve to VFR across most of the region by midday as drier air
works in from the north/northwest.

Confidence is medium to high on all parameters this afternoon
but low on vsbys and cigs after 00z/8PM Sunday.

Extended Aviation Discussion...

A more organized low pressure system will track into the area
from the central and southern Plains for Sunday night and
Monday. This is expected to bring sub-VFR ceilings and
visibilities and widespread rain by Monday afternoon. MVFR to
IFR conditions are then expected in widespread rainfall Monday
night into Tuesday. As the low deepens near the coast, wrap
around precipitation including a prolonged period of snow and
snow showers with gusty north to northwest winds will likely
keep sub- VFR in place for much of Tuesday night through
Wednesday night. Drier air under weak high pressure should
finally return VFR on Thursday.


As of 940 AM EDT Friday...

The Mount Jefferson NWR remains off the air and will likely be
sometime next week before it is operational again.




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