Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 181443

Area Forecast Discussion
National Weather Service Blacksburg VA
943 AM EST Sat Feb 18 2017

High pressure across the region this morning will gradually
give way to increasing moisture ahead of a weakening upper level
area of low pressure this afternoon into tonight. This system
exits the area early Sunday followed by high pressure that
should remain over the region into early next week.


As of 942 AM EST Saturday...Variable mid-level cloud cover (I
suppose I`d call it more intervals of mid-clouds) continue to
periodically pinwheel across the Central Appalachians/Piedmont
Region this morning, associated with moisture advecting
northward acround a mid-level low centered over the Deep South.
As we remain on the northeastern periphery of greater/deeper
moisture feed implied on morning visible imagery to our
southwest, I opted to adjust sky cover a bit for a more
optimistic cloud cover forecast for the first part of the day,
changes most notable particularly in the Piedmont/Southside VA
area. Expect at least filtered sun through the morning/early
afternoon across the forecast area.

Expect a greater areal increase/lowering in cloud shield
beginning around early/mid afternoon. As far as rain chances go,
with the predominantly northward motion of the main rain band
over AL/TN, I`m becoming less and less convinced at the
prospect of seeing much of any precipitation before sunset. In
fact, a consensus from recent higher-resolution convection-
allowing guidance doesn`t bring in light precip around/after
01z. So I started to trend back precipitation chances especially
southern areas. Best chance of seeing some light showers would
be late this afternoon in western Smyth and Tazewell counties.

No other changes made to highs at this point.

Previous near-term discussion issued at 215 AM follows...

Upper ridging in place to start today will gradually get undercut by an
opening upper low to the south that should fan quite a bit of high/mid
clouds into the region early on, followed by thickening canopy this
afternoon. However latest forecast soundings show quite a bit of
residual dry air stuck beneath the increasing moisture aloft, which
looks to delay moistening of the low levels until late in the
day or this evening. Latest guidance also keeping any measurable
precip mainly confined to far western/southwestern sections
toward late afternoon when diffluence aloft in advance of the
upper wave is progged to be best. This supports mainly low chance
pops generally along and west of the I-77 corridor with mainly
increasing clouds elsewhere per latest HRRR output. Despite
cloud cover appears just enough mixing under very warm 850 mb
temps to push highs into the 60s mountains, with some low 70s
possible east pending just how fast clouds thicken. This could
produce a shot at record highs in spots mainly west per the
climate section below.

Remnant upper low will cross the region tonight with models
differing on just how much precip works east given best
shortwave energy passing around the system to our south, while
deeper moisture tends to split to the southwest. Forcing looks
rather weak until perhaps the trailing deformation axis enhances
a little along the northwest edge of the passing core of the
upper system where lapses will be better overnight. This
suggested mostly by the wetter NAM with most other solutions
quite dry per latest ECMWF, and the GFS which keeps most of this
trailing shower coverage over the far west. However with the
low level flow already turning more northwest, NAM appears
overdone, while the potential to get much precip to shift east
of the Blue Ridge also quite iffy within increasing downslope
flow late. Therefore kept pops chance over the west with less
coverage heading east overnight. Even possible for spots over
the far southeast to stay dry as best lift passes south early
on, and northwest trajectory helps clip off deeper moisture
heading east late. Lows mostly in the 40s under developing weak
cool advection on the back of the passing low, although may stay
around 50 or warmer east if rainfall is less and the cloud
canopy remains in place through daybreak.


As of 245 AM EST Saturday...

A weak upper level trough will track over the south-central
Appalachains Sunday morning, generating a low off the southeast
coast. This low will become the dominate feature, pulling moisture
away from the deformation zone moving over the area Sunday morning.
Both the 00Z NAM/GFS keep a fairly large area of light showers over
the mountains Sunday morning. However, the 00Z ECM is mostly dry
expect near dawn with a few rain showers in Western Greenbrier. We
believe the NAM/GFS has a better handle with the deformation zone,
even though rainfall will remain light. Through the afternoon, both
the trough and surface low move off the southeast coast with high
pressure building from the west. Before this high moves over the
region during the evening, a pressure gradient will bring breezy and
gusty conditions to area along and west of the Blue Ridge. Nothing
near advisory level. Some cooler air will filter into the region
Sunday afternoon, but not enough to drop temperatures any where
close to normal. Highs Sunday will range from the low to mid 60s
west to the upper 60s to lower 70s east.

A northern stream surface ridge will merge with the ridge over the
area Sunday night. These two surface features place a shallow weak
wedge east of the Blue Ridge. Some clouds will accompany the wedge
Monday afternoon into Tuesday morning, but no precipitation is
expected to fall until Tuesday night as the next system enter the
region. This wedge will place uniform temperatures over the area
with highs in the mid to upper 60s Monday. Increasing cloudiness is
expected Tuesday as rain shield approaches the area. These clouds
and a light easterly flow will help keep Tuesday cooler with highs
in the low to mid 60s. Normal highs for this time of year are mid
40s across the mountains to near 50F east.


As of 308 PM EST Friday...

During this portion of the forecast, the upper ridge over the region
will shift east in response to a shortwave trough heading eastward
through the Great Lakes region. Southerly flow in advance of this
trough, and in the wake of the ridge, will tap Gulf of Mexico
moisture, and advect it northward into our region by Tuesday night.
Look for increasing chances of showers during this time period with
a cold frontal passage late Tuesday night into early Wednesday.
Showers will linger during the day Wednesday.

Wednesday night into Thursday, solutions between the GFS and ECWMF
are consistent with a quick return to a southerly wind flow pattern
across the region. What differs is the degree to which moisture
impacts the area with some light rain across the area Wednesday
night into Thursday.

Thursday night into Friday, the guidance is in fairly decent
agreement of amplifying or closing off an upper trough/low across
the Upper Mississippi River Valley. Very strong southerly low level
flow is progged to develop in advance of this system`s associated
cold front that will be near the Mississippi River by Friday
afternoon. This flow will tap the Gulf of Mexico, once again sending
moisture into our region in the form of scattered rain showers and
cloud cover. While there are indications from the GFS that surface
based instability may be sufficient for some thunderstorm activity on
Friday, will leave it out of the forecast at this time given it
is alone on this solution for our area, and it is the Day 7
time period.

Temperatures during this portion of the forecast will cool slightly
on Tuesday, but still average some ten degrees above normal.
Temperatures will start to rebound again Wednesday through Friday
and be in the neighborhood of fifteen to twenty degrees above normal.


As of 535 AM EST Saturday...

Overall VFR to prevail today under increasing mid and high
clouds, with possible MVFR arriving at KBLF late in the day as a
weak upper level disturbance shifts northeast toward the region.

Cloud bases thicken and lower overnight with deteriorating
conditions espcly western locations this evening where a
scattering of showers will occur. Expecting MVFR or lower over
the mountain locations tonight into early Sunday with locations
from KROA east likely remaining VFR for the most part. Vsby
restrictions in showers remains low confidence at this point
with latest models varying from mostly VFR under spotty coverage
to sub-VFR mountains, and even some light showers out east
tonight. Appears best shot at seeing MVFR or worse vsbys will be
along the KBLF-KLWB corridor with periods of MVFR possible at
KBCB and only brief VFR vsby reductions at KROA/KLYH.

Improving flying conditions are expected Sunday as drier
northwest flow kicks in behind the departing wave from Saturday
night. However upslope flow may keep lower cigs around KBLF/KLWB
until the afternoon, with a return to mainly VFR under gusty
northwest winds elsewhere by midday if not sooner.

Extended Aviation Discussion...

High pressure should bring VFR Sunday night through Tuesday.
MVFR rain showers are possible Tuesday night into Wednesday with
the next weak cool front crossing the region midweek.


As of 302 PM EST Friday...

Record High Temperatures and Year of Occurrence

Feb 18
Blacksburg, VA..65 in 2011
Bluefield, WV...64 in 1997
Danville, VA....77 in 1976
Lynchburg, VA...75 in 2011
Roanoke, VA.....72 in 2011

Feb 19
Blacksburg, VA..65 in 1994
Bluefield, WV...66 in 1994
Danville, VA....73 in 1981
Lynchburg, VA...76 in 1939
Roanoke, VA.....77 in 1939

Feb 20
Blacksburg, VA..68 in 1984
Bluefield, WV...67 in 1986
Danville, VA....74 in 1971
Lynchburg, VA...76 in 1930
Roanoke, VA.....75 in 1939




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