Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 180209

Area Forecast Discussion
National Weather Service Blacksburg VA
909 PM EST Fri Feb 17 2017

High pressure both at the surface and aloft remains in control
through tonight. Increasing clouds and limited chances for rain
then approach from the southwest later Saturday into Saturday
night. Dry and unseasonably mild temperatures are expected late
in the weekend into early next week, possibly approaching daily
record highs.


As of 906 PM EST Friday...

The forecast is pretty much on track with clear skies this
evening. Temperatures are a little slower to fall with some
lingering mixing of the winds. Still should see differences
between elevations overnight with valleys/rural areas dropping
into the mid 30s while ridges/urban areas like Roanoke fall to
the lower 40s. Should see some cloud cover arrive from the
southwest late.

Previous discussion from Friday afternoon...

An outstanding Friday afternoon across the central Appalachians
and Piedmont region today, with mild temperatures in the 50s to
60s under subsidence both at the surface and aloft, along with
a moderate southerly breeze. Dewpoints are still on the low
side, leading to afternoon relative humidities in the 20 to 30
percent range. To the southwest, we`re watching an upper level
low and a low- level Gulf of Mexico moisture return that will
primarily influence the latter half of the forecast period -
Saturday and more so Saturday night.

For Tonight: Mid-level ridge will continue to build over the
forecast area, with clear skies and lightening southerly winds the
rule. Conditions should be near ideal for strong radiational cooling
when you add those factors in with what is a dry air mass, even in
the midst of steadily warming 850 mb temperatures (near +10C by
morning). While the 12z MET MOS appears to be too cold, I`ve blended
in the milder MAV MOS guidance into a consensus blend to better show
the likely spatial variation (i.e. milder temps along the ridges and
locally cooler temps in the lower river valleys). That gives lows in
the upper 30s in the lower elevations/valleys to the mid 40s along
the ridges.

For Saturday: We start the day off clear, but will see
increasing/thickening cloud cover as we progress through the morning
and early afternoon. This is moisture is advected northward along
the eastern periphery of the aforementioned deamplifying upper low.
Today`s models have begun to trend back on rainfall amounts, and the
NAM is the wetter compared to the 12z GFS and ECMWF output through
Saturday afternoon. I`ve kept only slight to low Chance-level PoPs
in the grids through the afternoon, as BUFKIT soundings reveal
substantial dry plume of air at low-levels that would need to
saturate up. Feel that the best chance for showers would be late in
the day and mainly confined to the Mountain Empire region and
perhaps the Grayson Highlands/NC mountains. Despite more cloud cover
than today, particularly the latter half of the day, with milder 850
mb temperatures I`ve shown highs a couple degrees warmer than
today...60s to lower 70s. These are at or near record highs, and
as the potential for additional records are possible this
weekend, see the Climate section for these records at our long
term climate sites. Lighter southwesterly winds expected on
Saturday than today.


As of 308 PM EST Friday...

Saturday night, the axis of a shortwave trough is expected to shift
eastward through the region. Precipitation is still expected in the
form of rain showers across the western extent of the area in
association with steeper lapse rates near the axis. Lower in the
atmosphere, the associated 850mb and surface front will already be
east of the area, and general flow will be northwest. This will
yield subsidence across eastern parts of the area, and greatly limit
the extent to which precipitation arrives across the Piedmont. This
same northwest flow will help to maintain lingering isolated showers
across western parts of the area into Sunday, with decreasing
coverage during the day.

Sunday night into Monday night, surface high pressure will build
into the area from the west, and then shift to closer to the coast
by late Monday night. Higher in the atmosphere, an upper level ridge
will be building into the area, with its axis over the region by
late Monday night. This pattern is expected to yield dry conditions
across the region, with a general trend toward milder temperatures.
The GFS offers a solution that bring a little bit moisture into the
area on the return southerly flow by late Monday night, with light
rain or drizzle near the crest of the Blue Ridge. Currently only
entertaining increased cloud cover during this time period without
mention of any precipitation potential.


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 631 PM EST Friday...

VFR conditions will prevail tonight into Saturday. After 12z
Saturday, there will be a general lowering/thickening of
predominantly mid clouds.

Gusty Southwest winds around 4-10 kts with gusts to 20 kts
will diminish this evening to 5-8 kts. With a low-level jet of
35 kts approaches the west tonight into Saturday morning, There
is the potential for instances of low- level wind shear. KLWB
is the airport with the best opportunity between 09z and 13z.
Winds will be from the south at 4-9kt Saturday.

Extended Aviation Discussion...

Next potential for sub-VFR conditions is late Saturday into
Sunday as a disturbance moves into the area from the Deep South.
Warmer temperatures will accompany this system, so any
precipitation should be in the form of rain showers. Lower
ceilings associated with these showers may linger into Sunday
before clearing and return to VFR Sunday night through Tuesday.
MVFR rain showers are possible Tuesday night into Wednesday.


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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