Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 212024

Area Forecast Discussion
National Weather Service Blacksburg VA
324 PM EST Sat Jan 21 2017

A weak warm front south of the area will continue to be the
focus for impulses to travel along overnight into Sunday,
resulting in added rounds of rainfall tonight through Sunday
afternoon. The front should lift north into the region Sunday
night as a stronger area of low pressure approaches from the
west. This low pressure system will slide slowly east across
across the area Monday before exiting Monday night, allowing
for drier weather under high pressure for Tuesday.

As of 305 PM EST Saturday...

Active near-term forecast period begins tonight continuing into a
good part of the short-term period due to multiple rounds of steady
rainfall to affect the Blacksburg forecast area.

Late-afternoon surface analysis depicts a 1003 mb low near Atlanta,
enhanced to some extent by ongoing strong convective line marching
across southern Georgia, with eastward-trailing warm front from the
low center to a position from Greenville/Spartanburg SC to just
north of Florence SC to Wilmington NC. Lingering in-situ wedge lies
north of it across our area, keeping most of our forecast area in
the cool, cloudy side. Aloft, diffluent mid-level regime with
increasing 500 mb height falls approaching far southwest Virginia.
Composite radar mosaic reveals band of steady rain along the
northern periphery of warm front/mesolow advancing to the northeast,
its northern extent from Knoxville TN to near Charlotte NC.

For Tonight: First band of steady light to at times moderate rain to
progress primarily across the Grayson Highlands, southside Virginia
and northwestern North Carolina between roughly 21z and 04z per high-
res HRRR and RAP. Highest PoPs are focused across that general area,
with a lower/gradient in PoPs north to northwest essentially west of
the Blue Ridge. Appears that the northern fringe of negative
Showalter indices snakes its way into those aforementioned southern
areas. Included slight chance Thunder mention accordingly, but I did
not want to hit that harder given the best potential for convection
remains well south of our forecast area into central/western NC on
southward. Rainfall amounts for tonight range from a few hundreths
in the northwestern part of the CWA to around four-tenths along the
North Carolina Piedmont into southside Virginia.

In the wake of the rain, indication from most guidance is that the
wedge/CAD rebuilds back in with light east flow. Thus for the after
midnight period, I`ve again introduced patchy fog for most of our
forecast area as we`ll be waiting for the second, more
significant/widespread round of rain approaching from the southwest.
Forecast lows in the mid 40s to low 50s under persistent overcast
and areas of rain showers.

For Sunday: Upper trough closes off in the lower Mississippi Valley
early Sunday morning. Aforementioned warm front then lifts north
across much of our forecast area. This focuses a period of moderate
rainfall with rainfall rates enhanced somewhat by (1) convection -
elevated negative Showalter Indices advect into our central portion
of our CWA per NAM/GFS and (2) potential for southeasterly upslope
enhancement in the foothills and Piedmont areas in Virginia and
North Carolina. SPC maintains a Marginal/5% severe and Slight/15%
severe Sunday, primarily across our Piedmont NC counties into the
southside of Virginia due to strong vertical wind profiles. However
only very marginal if any surface-based CAPE with much of the
already modest instability being elevated. Potential is there for
thunder but given the limited amount of instability owing to early-
day CAD, not thinking we`ll see any severe. A conditionally greater
risk for stronger cells remains well to the south from central NC to
FL and as reflected in SPC`s Day-2 Convective Outlook.

Rainfall stands to be the heaviest during the morning into early
afternoon hours. Interesting that to some varying extent amongst the
12z models there is something of a lull in the rain during the
afternoon. So while PoPs start high-Likely to Categorical, I`ve
tried to show a reduction in PoPs to reflect a break in rain, though
they are still high-Chance (40-50%) and only stand to increase again
from SW - > NE Sunday night. All in all, anticipate a wet Sunday
with at least some rise on area rivers and streams. See the Hydro
section which speaks more to the hydrologic aspect given a few days
of at least steady rains (spanning into the short-term period).

Looking for highs Sunday to range from the mid 50s to around 60.

As of 245 PM EST Saturday...

Next round of heavier rainfall including possible embedded stronger
storms across the far south will be ongoing Sunday evening as very
strong lift/diffluence arcs north ahead of deep low pressure over
the western Carolinas. This will likely bring the heaviest period of
rainfall given more convective nature under 1.25 inch PWATS
just ahead of the advancing warm front and subsequent possible
triple point just off to the southwest. However just how much of
the south and/or east can work into the warm sector remains in
question, with models showing the low level wedge in the wake of
earlier rainfall holding near the VA/NC border before weakening
by early Monday. Since shear is quite impressive and strength
of warm advection stronger than usual, per such a stacked system
given the upper low passing near the Blue Ridge, opting to
include more thunder mention espcly southeast into Sunday night.
Also cant rule out at least some elevated rumbles elsewhere
overnight espcly Sunday evening southern Blue Ridge where seeing
a brief but strong upslope component. Otherwise pops returning
to categorical into early Monday across the region with lows in
the 40s but perhaps rising some early on.

Upper dry slot may act to cut off showers across the east Monday
morning as the center of the upper low drifts into western VA
and eventually to the east/northeast of the area by Monday
night. This likely to shift bands of heavier rainfall over the
west during Monday as the deformation zone develops aided by
wrap around moisture behind the exiting upper low. However
uncertainty remains espcly by Monday with the track of this
system including the degree of more concentrated heavy rain,
and where precip may be more convective resulting in higher
rates. Models have again shifted east with QPF totals given more
convective nature over the southeast and more in the way of
stratiform rain closer to the low at this point. This on top of
previous rainfall would likely bring the highest threat for
flooding pending exactly where added bands set up, so keeping
mention in the HWO and hydro section of the AFD for now. Highs
Monday remain tricky with potential for values to zoom to around
60 Piedmont while staying in the 40s northwest, with low/mid
50s in between.

Axis of deeper moisture should continue to fan out to the west
before shrinking along the higher western mountains Monday night as
the low and associated lift depart. However increasing upslope
northwest flow in the wake of this system likely to keep showers
going western third overnight, while some drying and cutoff in pops
takes shape east of the Blue Ridge. Temps aloft also appear too warm
for much snow so leaving out mention except highest elevations late
Monday night. Main concern overnight will again be with strong winds
as the low heads offshore and the pressure gradient between high
pressure to the south increases. Latest input of strong subsidence,
brief cold advection and a lowering inversion into local scheme data
off the GFS, still suggests potential wind advisory criteria
mainly NC mountains. Therefore have again included a mention
in the HWO there overnight. Otherwise running with diminishing
pops overnight with only chance pops lingering north/west late
and little elsewhere. Lows mostly 30s to lower 40s.

As of 215 PM EST Saturday...

Strong upper trough will be lifting out on Tuesday making way for a
brief period of shortwave ridging that will slide east and across
the area later Tuesday into Wednesday night. May still be some
upslope driven rain and high elevation snow showers Tuesday morning
before much drier air slides in from the southwest under a nose of
surface high pressure that will extend north from the Gulf coast.
This should bring clearing skies Tuesday afternoon with sunny skies
on Wednesday before clouds increase again later Wednesday night
ahead of the next upstream cold front. Winds should remain quite
strong into midday Tuesday with potential advisory level speeds
espcly NC mountains into far southwest VA. Will be the mildest
period for quite some time with highs 40s mountains to upper 50s
east Tuesday, followed by 50s to lower 60s Wednesday under warmth
aloft and westerly flow.

Return to more winterlike conditions will ensue by weeks end with a
cold frontal passage by Thursday ahead of a broad longwave trough
that will drop south into the region and persist into next weekend.
However latest guidance not nearly as strong or deep with this
feature making for more Canadian type air vs. much colder thickness
seen yesterday that was supportive of Arctic air. However will see
persistent cold advection develop by Thursday with weak passing
shortwaves gradually helping to bring in colder air aloft by
Saturday. A band of showers possible along/behind the front Thursday
into Thursday night although iffy given only sheared southern
energy. Therefore will only include a low pop shower mention
including possible western snow showers overnight. Otherwise will
evolve to mostly upslope driven clouds and periodic light snow
showers far west, to mainly clear/sunny east Friday into Saturday,
under a rather dry and more westerly flow driven environment. Highs
mostly 30s to around 40 mountains to mid 40s east into the weekend.

As of 1255 PM EST Saturday...Active, changeable aviation
conditions anticipated through the TAF period with several
periods of poor flight conditions.

General improvement in aviation flight categories anticipated
through 00z as fog which plagued the TAFs this morning has
finally begun to weaken with visibilities in the 2-3 SM range at
ROA and LYH. Unfortunately flight categories are expected to
degrade further with approaching rain from the southwest.
Anticipate ceilings to deteriorate back to MVFR to IFR with
visibilities 4-7 SM in rain, being steadiest/lowest at DAN. In
fact there may be little in the way of rain at northwestern
airports such as BLF, ROA and LWB.

Appears rain should shift eastward into eastern Virginia by
04-06z. Expect a break in rain chances, with a developing in-
situ wedge in behind the rain again focusing a period of IFR to
VLIFR conditions due to stratus and fog starting around that

Second wave of steadier rainfall then looks to build from the
southwest during the morning hours Sunday, with coverage of rain
covering a larger portion of our forecast airspace along with
a few rumbles of thunder. Due to low forecast confidence in
specific airports being affected by thunder, did not mention in
the TAF at this point, but the best shot would be at LYH, DAN
and ROA. Flight categories IFR to VLIFR through 18z Sunday, due
to low ceilings, with visibilities 3-6 SM in rain briefly heavy
at times.

Winds generally light and variable (trending southeast) through
most of the TAF period.

Extended Aviation Discussion...

A slow-evolving but potent storm system affecting a good part
of the central and southeast CONUS will lead to a potentially
long period of sub-VFR conditions through Monday with stratus,
fog, and periods of rain, in addition to thunderstorms. Once the
storm passes on Tuesday, conditions will improve with respect to
cigs and vsbys, but it will become breezy with northwest winds
gusting to 30 kts behind the departing storm on Tuesday.


As of 310 PM EST Saturday...

There will be several rounds of rainfall affecting the Blacksburg
hydrologic service area beginning tonight and lasting through
Monday. The greatest amount of rain over a 24-hour period looks to
be on Sunday with 1 to 2 inches of rain across most of the forecast
area, with lighter rainfall amounts expected tonight across the
southern tier of counties/Dan and Yadkin River basins and again
Monday along and west of the Appalachian Divide into southeast West
Virginia, the Mountain Empire of Virginia and into parts of the New
River Valley.

Using a model blend, heavily weighting WPC in that blend, forecast
rainfall amounts tonight through Sunday range from 1.5" for most of
southeast West Virginia and the Alleghany Highlands, to well over 2
inches along the east-facing slope of the Blue Ridge.

Current expectation is for gradual main-stem river rises with
faster/greater response on smaller rivers/creeks, along with some
potential for ponding on roads and in areas of poor drainage.
Rainfall rates do not appear excessive at this point. The only river
forecast point projected to go just into Minor flood stage is the
Dan River at South Boston. That may be a worst-case, and it would
take a greater amount of rain over a shorter period (more convective
rain rates) to increase the risk for minor flooding across the
forecast area. Given that confidence is low in flooding, a flood
watch has been issued at this time and would rather wait to see what
tonight`s rainfall brings before that is considered.



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