Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000
FXUS61 KRNK 170903
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
403 AM EST Sat Feb 17 2018

.SYNOPSIS...
High pressure over the mid-Atlantic today will shift off the New
England coast this evening. Low pressure tracks from the Gulf
Coast states to the Carolinas during this time. This system then
exits east late tonight with high pressure building in Sunday.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 320 AM EST Saturday...

Temperatures still running a little milder out there, and 00z model
analysis versus upper air soundings showed models initialized a
degree to 3 degrees colder at 8h from RNK to BNA, however, colder
air aloft is arriving behind the front. Kind of a nuisance event
coming up today and timing of measurable precip as it moves into a
cooler airmass will lead to what ptype occurs. Models vary on the
thermal structure, but overall no changes are planned to where the
current advisories are.

Given that temps are trending a little warmer in the low levels,
with a shallow colder layer above the surface, the another narrow
warm nose above this cold layer and the fact the winds are not that
strong out of the east, appears icing will not be as widespread, but
still could see a tenth to two-tenths of an inch along the Blue
Ridge especially from Roanoke to Meadows of Dan, with patchy icing
elsewhere along the Blue ridge and isolated portions of the
Alleghanys.

Coupled with warmer and wet ground temperatures, icing will mainly
be confined to trees and power lines, and with wind not much of an
issue, not expecting too much impact on power, but isolated outages
are not out of the question, mainly Roanoke/Floyd/Franklin/Patrick.

Further north, colder air will allow for a mix of snow/sleet to
stick around longer, but again appears accumulating snow/sleet will
be confined to higher terrain, with less in the valleys and sleet
will cut into totals as well. Cut back somewhat snowfall but have 2
to 4 inches at the higher terrain of Greenbrier/Bath and
Amherst/Rockbridge through tonight, with some of the higher amounts
also occuring as snow showers kick in behind the departing surface
low off the coast with high pressure building into the mid-MS Valley.

Given the speed of this event, overall impact is less. Still those
traveling from the New River Valley and Blue Ridge north of
Hillsville northward to WV into northern VA should be prepared for
slick spots at times.

Timing of precip shows rain/snow arriving into the NC mountains
around 8-9am, spreading northeast thru midday. Most should see
precip today but best coverage will be along and northwest of the
Blue Ridge, and north of highway 460. Temperatures today will not
move too much, may fall initially at precip onset, then steady out
or rise a degree or two. So at dawn, temperatures will range from
around 30 in the Alleghanys to Greenbrier higher ridges, to lower to
mid 30s elsewhere along/west of the Blue Ridge and north of the
Roanoke River in VA, to upper 30s to lower 40s southside VA into the
NC foothills/piedmont. By 4pm, temperatures will be a little cooler
along the Blue Ridge especially north of the NC/VA border with lower
to mid 30s. Lower 30s also lingering into the Alleghanys and
portions of the Shenandoah to New River Valleys, with mid to upper
30s elsewhere, but possibly seeing lower 40s across far southwest VA
west of Tazewell and Marion.

Overnight, temps slide down into the upper 20s to lower 30s, with
clearing taking place east of the mountains, and snow showers
tapering off by dawn, though will possibly see some freezing
drizzle along the western slopes per model forecast soundings
showing little if any moisture in the snow crystal growth temp
zone.

&&

.SHORT TERM /SUNDAY THROUGH TUESDAY/...
As of 356 AM EST Saturday...

Sunday begins with high pressure building across the northern half
of the Blacksburg forecast area. This high pressure area will shift
low-level thermal gradient at least briefly southward. After early-
morning stratus in the Greenbrier mtns into the far western mountain
counties burns off, a pretty nice day appears on tap with mostly
sunny skies expected to push temperatures from the mid-40s to
mid/upper 50s.

Aforementioned thermal gradient begins to return back northward as a
warm front Sunday night (00z NAM/ECMWF camp) or early Monday (00z
GFS). This warm front is on the leading edge of low-mid level height
rises ahead of what will turn out to be a highly-anomalous
subtropical ridge and surge of low-level Gulf moisture. Regardless
of which solution(s) are chosen, greatest larger-scale lift passes
us by to the northwest - more in the Ohio Valley. Sided toward the
NAM/ECMWF idea for timing the front across, which begins to approach
our far southwestern counties toward midnight. Will keep PoPs to no
better than low-end Chance (30%-ish or less) west and south of
Roanoke before midnight, and dry to the north/eastern tier of
counties. Best chance for rain is really into the early morning/pre-
dawn hours where PoPs increase to high Chance/lower Likely, lowest
chances furthest east more removed from the best frontal forcing.
What this will do is set up somewhat of a complicated temperature
pattern where clouds and warm advection earlier in the eastern TN
Valley/NC mountains will lead to early lows, whereas enough
opportunity radiational cooling happens through a large part of the
overnight across the Piedmont northwest into the
Shenandoah/Highlands areas, before rising closer to early morning.
Used a non-diurnal temperature curve to better capture these
details, with lows mid-upper 30s mainly from I-64 to Lynchburg, and
the lower to mid 40s further south and west.

Biggest question mark for Monday is timing the warm front
northeastward and the degree of post-frontal cloud clearing and its
effect on mixing/temperatures. The warm front probably will still be
making slow progress northeast across the northern half of the CWA
(roughly along/north of Route 460) early Monday. Light showers most
likely to continue until early afternoon, with leftover cloud cover
likely to restrict mixing significantly in northeastern counties and
the Piedmont. 850 mb temperatures are on the order of +10 to +12C
range by mid-afternoon. BUFKIT soundings off the NAM and GFS at
Lynchburg each show poor potential for mixing, which leads to highs
in the mid 50s in the Piedmont closer to what the 00z MAV suggests.
Better chance at temperatures reaching or exceeding 60 is from
Roanoke southwest. All told, a rather low-confidence temperature
forecast. If better mixing occurs than currently indicated, these
temps probably are much too cold, perhaps by as much as 5-10 degrees
closer to the 00z MET idea. In terms of rainfall with the warm
front, amounts are generally a quarter inch or less and not
sufficient enough to yield any hydrologic issues.

Anomalous 591-dm 500 mb ridge becomes established off the Carolina
coast Monday night, continuing into Tuesday. For Monday night,
there`s a fair amount of RH that`s indicated in the GFS guidance
which along with a light south-southwest breeze should keep
temperatures from falling very far. Despite PWATs on the rise,
ridging aloft should keep forecast area on the dry side. Overall a
mild evening with temperatures in the 50s, with dewpoints also in
the upper 40s to low 50s.

Tuesday should be the warmest of the period, with 850 mb
temperatures between +12 and +14C. Much like Monday, Tuesday high
temperatures again will be heavily determined by how much
sunshine/breaks in clouds is realized. Since it`s unlikely we see
clouds clear any more than partly cloudy, will keep temperatures for
now in the upper 60s to lower 70s, with potential for even milder.
Even those lower values are still 20-30 degrees warmer than climo.
It will also feel like an early-mid spring day as dewpoints rise
into the upper 50s-near 60.

&&

.LONG TERM /TUESDAY NIGHT THROUGH FRIDAY/...
As of 356 AM EST Saturday...

Synoptic pattern features strong subtropical ridge of high pressure
from the central/eastern caribbean up toward Bermuda.  As a result,
well above average mid-level heights are forecast across the eastern
U.S. with values in the 582-588 DM range. Relative to climatology,
this is 2 to 2.5 standard deviations above normal.

This set-up favors much above normal temperatures for the Southeast
States with the baroclinic zone forced to reside on the west side of
the Appalachians from East Texas to the Ohio Valley and into the
eastern Great Lakes. Repetitive rainfall is likely within this
corridor during the week with our forecast area far enough east of
the boundary to escape anything significant. Toward the end of the
week, models hint that a surface front will impinge on the forecast
area, but overall weather conditions for mid week feature above
normal warmth and only a chance for showers pending any instability
within the warm subtropical airmass.

Models suggest potential for a backdoor front to impinge from the
north-northeast Thursday, so somewhat cooler/wetter conditions are
favored for the end of the week.

&&

.AVIATION /09Z SATURDAY THROUGH WEDNESDAY/...
As of 1150 PM EST Friday...

VFR conditions will start the TAF period at most locations as
drier air moves in from the north. IFR to MVFR clouds on the
extreme western slopes of the central Appalachians will
dissipate overnight.

A wedge will begin to set up overnight, trapping the colder air
at the sfc. An upper-level disturbance will pass over the
region tomorrow bringing a shot of mostly cold rain into the
region by the afternoon. Evaporative cooling into the drier
layer at the ground will cause precipitation to become a mixed
bag of rain, snow, sleet and freezing rain, mainly at higher
elevations caught under the wedge. Ceilings and visibilities
will drop to MVFR/IFR in precipitation. Medium to high
confidence on the timing of the precipitation spreading from
southwest to northeast across southwest Virginia, northwest
North Carolina, and southeast West Virginia between 14Z/9AM and
18Z/1PM. Little improvement is expected before the 06Z end of
the TAF forecast period.

Extended Aviation Discussion...

VFR returns again on Sunday under weak high pressure. A warm
front lifts back north into the area on Monday with another
round of sub-VFR ceilings and visibilities with rain,
especially mountains. High pressure strengthens offshore
resulting in a return to overall VFR Tuesday ahead of the next
cold front well west of the Appalachians. Scattered MVFR showers
are expected Wednesday into Thursday.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...Winter Weather Advisory from 10 AM this morning to midnight
     EST tonight for VAZ011-014-017>020-022>024-032>035.
NC...None.
WV...Winter Weather Advisory from 10 AM this morning to midnight
     EST tonight for WVZ044-507-508.

&&

$$
SYNOPSIS...KK/WP
NEAR TERM...WP
SHORT TERM...AL
LONG TERM...AL/PM
AVIATION...AMS/JR



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