Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 301801

Area Forecast Discussion
National Weather Service Blacksburg VA
201 PM EDT Thu Mar 30 2017

High pressure will continue to wedge south along the eastern
slopes of the Appalachians today before passing offshore this
evening. Low pressure will pass to the northwest of the area
tonight into Friday with a trailing cold front crossing the
region by Friday night. High pressure follows this system with
mainly dry weather expected over the weekend.


As of 1000 AM EDT Thursday...
Made a slight adjustment to the sky grids to show the clear area
over our far Western territory as seen on current METSAT. Also
adjusted QPF up slightly to acknowledge area of drizzle from
Blue Ridge eastward into Henry Co. Otherwise, everything else
looks on track.

As of 300 AM EDT Thursday...

Wedge continues to evolve across the region early this morning with
satellite pics showing low clouds filling in along/east of the Blue
Ridge with better cool advection entering from the north per MSAS.
Latest short term models indicate a better surge in low level
moisture advection through sunrise per deepening southeast flow with
low clouds spreading over all except the far west. Also depicting
some spotty light rain or drizzle developing by daybreak eastern
slopes with stratus lowering, resulting in fog across the ridges
this morning. Although this likely overdone, will leave in going low
pops for light rain central sections early on, with coverage
gradually shifting southwest to along the southern Blue Ridge into
this afternoon as warm advection starts to increase above the low
level cool pool. Otherwise looking at a much cloudier day in the
wedge with perhaps a lot more sun far west and some breaks
east/southeast as a bit drier air sinks in. High temps very trick
with readings likely to not rise much from 12z values along portions
of the Blue Ridge while possibly surging west of I-77 where will be
on the periphery of the cool air. Mos spread remains large pending
solutions depiction of the depth of the wedge and subsequent cloud
cover. Think best to continue on the cooler side similar to the
latest consensus and supported by the new Euro highs which is
mostly in the 50s.

High shifts offshore this evening while an area of strong upper
level low pressure slides by well to the northwest by early Friday.
This will shift a warm east toward the mountains late with an
increasing southerly jet aloft coming in line with a rapid increase
in upper diffluence from west to east espcly after midnight. Will
initially be wedged within a stable environment to start with the
CAD getting eroded from aloft western counties and out along the
Blue Ridge by daybreak. Models again a bit faster while showing lots
of convergence/lift over much of the region late as upslope precip
becomes enhanced via approach of lead energy aloft before merging
into the precip shield ahead of the front aloft to the west. This
should spell periods of moderate to perhaps heavy rain overnight
with likely more elevated/embedded convection that could run out
east of the mountains by dawn. Some potential espcly if the initial
wedge is weaker over the far southwest to see a broken band of
deeper more surface based convection spill east right along the axis
of strong shortwave energy aloft. Thus beefed up thunder chances
south/west given shear and will mention in the HWO per latest SWODY1
slight/marginal outlook. Winds ahead of this feature may also be an
issue along the western ridges as forecast soundings show a 45-50 kt
jet just above the inversion later this evening. However with the
flow veering more southerly think should stay below advisory
criteria for the most part so wont headline for winds at this point.

Appears QPF of an inch or more possible southwest sections including
parts of the Blue Ridge through late tonight but should not be
enough to cause flooding issues given rather dry conditions to start
at this point. Otherwise running with a rapid increase in pops to
high end categorical all sections overnight with lows 40s to mid 50s
before rising some late.


As of 330 AM EDT Thursday...

Looking at a rather complicated but also an active forecast for
Friday. 00z model guidance has trended about 3 to 6 hours faster
than prior model cycles with regard to early-day rainfall. Though
specific spatial details and intensity are still somewhat fuzzy,
given a potential earlier eastward shift of early-morning precip, a
second/redevelopment of showers and storms appears possible Friday

Will attempt to describe these prospects below...

Friday morning: This precipitation, likely to be a steady band of
moderate rain, stems from overnight, weakened convective activity in
the Deep South/Tennessee Valley. As upper low begins to deamplify,
an axis of steady rain is indicated on a negatively-tilted trough
axis. Strongly diffluent upper level flow should also accompany this
band of rain. Though surface-based instability will be lacking given
the time of day, elevated instability alone should be enough to at
least include a chance of thunder with its northeastward advance.
Effective-layer vertical wind shear magnitudes may be as high as 60
kts with at least some backed, southeasterly 2-m AGL winds shifting
to low-level southwesterly. Wouldn`t think prospects for severe are
all that great with this initial band given the marginal instability.

Friday late morning and aftn: In the wake of the early-day
rain/embedded thunder, a number of higher-resolution models
including the 3-km NAM and ARW depict a window of potential
redevelopment, primarily from the Blue Ridge/North Carolina
foothills eastward. Shear profiles aren`t quite as pronounced as in
the morning given more veered/southwesterly surface winds, and the
coverage and strength of convection are contingent on afternoon
cloud breaks/insolation. Forcing coming in with the upper low and
cold front, and at least some colder temperatures aloft would
support thunderstorm redevelopment. Instability is still rather
modest at values around 500 J/kg or less, though could be greater if
more peeks of sun were expected. It`s really during this period that
would likely need to be more closely monitored for stronger
thunderstorm development. SPC reflects this in their Day 2
Convective outlook, carrying a Marginal Risk/5% severe probability
in the Piedmont area. In the grids, I`ve depicted this as a lowering
from Categorical morning PoPs to high chance/low likely, ramping to
likely-level PoP for the afternoon.

Temperatures Friday are contingent upon any cloud breaks; I did
raise highs up into the mid 60s for most areas except low 60s for
western Greenbrier County. Rainfall amounts Friday are roughly a
third to half-inch with potential for greater in isolated spots due
to convection.

Friday Night: Upper low moves across West Virginia and off the coast
late. Any instability-driven showers/storms dissipating and/or
having already shifted eastward. Northwesterly component to low-
level flow supports a period of upslope cloudiness and rain showers
in the Alleghanies and southeast West Virginia. Clouds should begin
to scatter out elsewhere, with lows in the mid 40s to around 50.

Saturday/Saturday Night: Ridging builds in from the northwest with
cold thermal advection occurring through the daytime hours. Mainly
sunny skies with a northwesterly breeze. Highs top out in the mid
50s to low 60s from the Blue Ridge west, but are some 10 degrees
warmer east of the Blue Ridge aided by downslope warming.

Sunday/Sunday Night: Ridge builds to our east during the day, with
southwesterly warm advection occuring in Smyth and Tazewell
counties. Mostly sunny skies should again predominate boosting highs
into the 60s to low 70s. However, later Sunday into Sunday night
models indicate that we again return into a cold air damming regime.
Increasing cloudiness with easterly onshore flow in the evening
would be the result. Essentially, Sunday night may be a carbon copy
of tonight. Started to trend lows a bit warmer using some of the
warmer raw guidance, which supports lows in the upper 40s to low


As of 315 AM EDT Thursday...

Lift will then steadily increase along with stronger overrunning
late Monday into early Tuesday as the main wave passes to the west
and a warm front pushes north into southern sections Tuesday
morning. This will bring another round of at least light/moderate
rain espcly Monday afternoon into early Tuesday before the warm
front passes north per latest wetter ECMWF. Convective threat again
focused along the retreating warm front and perhaps within the warm
sector Tuesday pending timing and subsequent heating ahead of the
trailing cold front Tuesday afternoon. System exits Tuesday night
into Wednesday with shortwave ridging returning, allowing for a
period of drier weather for middle of next week. High temps to stay
above normal, mainly 60s mountains to low 70 east, except cooler
within the wedge Monday when some 50s could occur. Lows to start out
on the cool side, mostly 40s, before rebounding under clouds/warm
advection to mainly 50s into midweek.


As of 645 AM EDT Thursday...

A wedge setup has the region mired in sub-VFR conditions. The
exceptions being LYH and DAN where somewhat drier air is present
to help mix out the layer. Overnight, expect all locations to
fall to at least MVFR as warm frontal passage will bring
increased moisture and showers to the region. Chances for an
isolated thunderstorm exist particularly in the early part of
the day Friday as an associated cold front approaches the
region. Conditions look to improve later on Friday with the
passage of the cold front, however MVFR to IFR conditions could
persist over the western mountains. Winds look to be relatively
weak until the front passes through the area, with a lone
exception at BLF where southerly flow tends to produce gusty

Extended Aviation Discussion...

High pressure should bring improving conditions during Saturday
although appears sub-VFR cigs along with a few residual showers
may linger at KBLF/KLWB into Saturday afternoon. Otherwise will
see VFR return later Saturday through Sunday under diminishing
winds as high pressure slides across. Next similar low pressure
system to that seen at the end of this week will approach by
Monday with sub-VFR conditions likely returning Sunday night
into Monday as rain again transitions to showers and storms to
start the week.




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