Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 142008

Area Forecast Discussion
National Weather Service Blacksburg VA
308 PM EST Thu Dec 14 2017

Variably cloudy skies will continue through tonight. A
weak disturbance passing through the region Friday may produce
light wintry precipitation during the morning hours. Though
amounts are not significant, it may impact the Friday morning
commute. High pressure then returns Friday afternoon through
Sunday. After Friday, the next chance for precipitation is
Sunday night into early next week.


As of 306 PM EST Thursday...Variably cloudy afternoon across
the central Appalachians, Piedmont and mid-Atlantic region this
afternoon. Still some shallow moisture being drawn southeastward
on lower-tropospheric northwesterly flow, despite surface
ridging building into the region. Broad cyclonic mid- level flow
exists across the eastern two-thirds of CONUS. In that flow, a
shortwave trough/vort max now over the central Plains will serve
as a focal point for sensible weather into the morning hours

Though sky conditions are variable currently, greatest further west
one goes, a west-east increase in cloud cover is expected per bufkit
soundings during the late-evening/around midnight, especially for
the northern/central half of the forecast area. Should see good
radiational cooling in most areas through that period of time with
most locations in the 20s to low 30s around midnight.

Beyond midnight, forecast becomes a bit less certain given some
discrepancies in 12z NWP solutions on extent of QPF. Though it`s
really a difference between no QPF and limited QPF, the potential
for significant impact is possible for locales in/around the Blue
Ridge, as much of what falls may be in the form of freezing drizzle.
Aformentioned mid-level trough acts on low-level frontal zone, and
generates limited to no QPF as it moves eastward, inducing weak
cyclogenesis off the NC coast later Friday. The NAM and the higher-
resolution CAM solutions generate QPF values less than 0.05", but
closer inspection of the sounding indicates the moisture layer is
shallow and confined between -2 and -8C, suggesting supercooled
droplets with a dry ice crystal layer. Using the top- down
methodology, this leads to either freezing drizzle or flurries
within about 30 miles either side of the Blue Ridge for the pre-
dawn hours into mid-morning Friday. On the other hand, the
GFS/ECMWF depict hardly any QPF as the wave aloft makes its way
into the mid- Atlantic region. From an impact perspective,
freezing drizzle produces the same level of impact as light
freezing rain: potential for accidents and glazed trees. Given
that confidence in the NAM solution is low, will not issue any
winter advisories at this time but will allude to the potential
for difficult travel during the morning commute from freezing
drizzle in the HWO.

Temperatures should slowly warm into the mid/upper 30s with
decreased clouds in the afternoon. The upper Piedmont of NC should
see highs in the lower 40s. As clouds clear, westerly winds increase
with some occasional breezes to 25 mph.


As of 301 PM EST Thursday...

Will see increasing subsidence as well as drier air punch into the
region Friday night into Saturday behind the passing complex of
shortwave energy from later Friday. This along with high pressure
should make for quiet weather to start the weekend with little more
than a flurry far northwest mountains Friday night. However will
again see northwest winds ramp up behind the departing system with
the next shot of modest cold advection. Expect strongest speeds
Friday night as the 850 mb jet increases to around 40 kts and the
inversion lowers but for now soundings indicate speeds below
advisory levels. Should see winds diminish on Saturday as surface
high pressure builds across allowing for a switch to warm advection
in the afternoon. Otherwise partly cloudy Friday night with lows
in the 20s to mainly sunny on Saturday with highs in the 40s.

Heights slowly build Saturday night with southeast ridging taking
shape Sunday into Sunday night. However residual upper low heading
northeast out of Texas looks to ride up along the west side of
ridging across the Tennessee Valley Saturday night/Sunday with some
of this lift spilling east toward the region by Sunday night. Models
remain inconsistent in just how much deeper moisture will make it
into the area given westerly flow and ridging aloft as some
solutions dry most precip up crossing the ridges at this point. Thus
will continue the trend of bringing low shower chances into the west
later Sunday and chance pops most sections Sunday night. Should be
warm enough for all rain given late day arrival with temps in the
40s Sunday, and only falling into the 30s to around 40 Sunday


As of 301 PM EST Thursday...

Appears mild period to maintain itself for much of next week as
flatter upper flow around ridging across the southeast states
remains in place for most of the period. Will see some digging
northern stream shortwave energy make for a brief return to passing
eastern troffiness around midweek but overall weaker now with each
run. Other than for much warmer temperatures, main concern will be
with shower chances that could linger into Monday, and then perhaps
expand again Tuesday with a residual shearing upper system from
Mexico that will attempt to undercut the ridge. Guidance spread
remains quite large in northward extent of deeper moisture into
westerly flow aloft as well as timing through Tuesday night. Appears
initial system much weaker so only running with low pops into Monday
night with somewhat higher chances Tuesday pending added support
coming out of the southwest. Should finally see a cold front cross
from the northwest Wednesday allowing for drier but only slightly
cooler weather before temperatures rebound again Thursday as high
pressure slides offshore. Highs to rebound to well above normal
levels for early next week with mostly 50s to near 60 east before
cooling some to 40s/near 50 mountains to 50-55 east Thursday.


As of 1256 PM EST Thursday...

General mix of VFR/MVFR ceilings across the TAFs, though MVFR
upslope stratus mainly for Lewisburg and Bluefield. Ceiling
trends should continue through midnight. Northwest winds
initially 6-12 kts gusts to 20 kts abate through early evening
to light and variable as boundary layer winds ease. Until then,
potential for mountain wave clouds/turbulence in and around the
Blue Ridge.

Aviation forecast then trends less certain looking into the
overnight and midday Friday period. Another mid-level
disturbance traverses the region around 07z. Recent 12z GFS and
NAM solutions are at odds in terms of precipitation as this
upper-level system ripples through our area. The NAM is the
wettest, and examination of forecast soundings support potential
for light snow/flurries or freezing drizzle between 07-15z
along the Blue Ridge and adjacent terminals. Have sided toward
the worst-case NAM with -FZDZ indicated in most TAFs except
Bluefield and Danville. Freezing drizzle would produce slick
runways and greater mixed-ice accumulation on wings. Ceilings
trend MVFR most TAFs except at Danville where VFR conditions
should prevail. Light and variable winds, tending light
east/northeast through the rest of the TAF period.

High confidence in aviation elements through midnight. Medium
confidence on ceilings and winds after midnight, Medium to low
confidence on weather type and visbys after midnight.

Extended Discussion...

Should see conditions improving toward VFR late Friday afternoon
into the evening as aforementioned upper disturbance/weak
surface low moves off the Atlantic coast. VFR conditions should
prevail until Sunday night. A better shot exists for sub- VFR
conditions from a system system forecast to emanate from the
mid-Mississippi Valley Monday into Tuesday.




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