Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 291127
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
727 AM EDT Wed Mar 29 2017

.SYNOPSIS...
High pressure will build into our region from the north, then
slide east Thursday. The next storm system arrives from the west
triggering showers and thunderstorms on Friday. Mainly dry
weather is expected over the weekend in the wake of this system.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
As of 300 AM EDT Wednesday

At 06Z (2 AM EDT)...a deep northwest flow was firmly established
across the forecast area in the wake of a cold front that extended
from eastern NC southwest into SC and west into Texas.
Upslope stratus has spread south from the mountains well into the
New River Valley.

Models are in very good agreement that a wedge of high pressure
builds into our area today from the north which allows sfc winds to
veer to the east by late afternoon/early evening. Where the models
struggle today is with the extent of cloud cover which will have an
impact on the temperature forecast. Leaned toward the more
pessimistic guidance for clouds based on current location of low
clouds compared with short range models. This solution allows for
clearing late morning into early afternoon before weak low level
convergence and easterly upslope flow generate clouds along and near
the Blue Ridge by late afternoon. With most guidance running too
warm on high temps over the past few days...leaned toward the
slightly cooler guidance with highs in the lower-mid 70s in the
south to 60s in the mountains.

Forecast soundings show a fairly deep column of saturated air (800mb
and lower) developing late tonight as the wedge of high pressure
strengthens. Confidence continues to grow that light precipitation
and widespread low clouds will develop as southerly winds above the
wedge transport higher PWAT values and the western portion of the
forecast area falls under the nose of a surging theta-e ridge. With
widespread cloud cover expected...leaned toward the warmer guidance
numbers for lows.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH FRIDAY NIGHT/...
As of 400 AM EDT Wednesday...

A continuation of dreary, cloudy conditions driven by easterly flow
associated with a cold air damming signature will continue into much
of Thursday. Interestingly both the 00z NAM, GFS and 21z SREF Mean
all point to modest/light QPF values along the southern periphery of
the Blue Ridge (from Roanoke southwestward to the NC mountains and
into a portion of the foothills). This may be driven by shallow
upslope flow. However, large scale regime points to strong mid-level
height rises/implied subsidence above shallow stable layer so this
may in fact be something more along the lines of drizzle. Challenge
will be temperatures and both 00z MOS output are only slightly
warmer than their prior runs. Offered highs from the New River
Valley eastward in the 50s due to the overcast, but west of the Blue
Ridge (eastern TN Valley counties in VA northeast into southeast WV)
may be some 5-10 degrees warmer due to southerly flow.

Region will still be dominated by cold air damming, however its
influence should begin to weaken after midnight owing to
increasingly stronger forcing/precipitation moving in from the west.
PoPs gradually increase as the nighttime progresses, highest by
early Friday morning. NAM-based wind fields are quite strong above
stable layer (nearing 50 kts). That is a setup for potential
mountain waves and associated breezy to at times gusty
southeast/south winds at higher elevations in our southwestern CWA
(Smyth and Tazewell Counties southward into the Grayson Highlands/NC
high country). Will include mention of gusty conditions in the HWO.
Accompanying the southerly flow will be an increase in PWAT values
to around 1" by morning. Should really only see temperatures fall a
few degrees at most to lows in the upper 40s to lower 50s.

That sets the stage for Friday, which stands to be the most active
day in the short-term period. Vertically stacked low should be near
the western Ohio Valley by 12z Friday, with an associated mid-level
trough extending southeastward into GA and coastal SC. That trough
will likely be the focus of what should be a band of rain and
embedded thunderstorms, the thunderstorms driven primarily by
elevated instability/mid-level lapse rates around 6.5-7 C/km. How
strong any thunderstorms develop can become would be contingent on
surface-based instability, and that should really be nil given the
weakening CAD which may only lead to isothermal low-level profiles
at best. Effective-layer shear is likely to be rather strong at
values around 50 kts though, and the SHERB parameter supports a
potential high-shear/low-CAPE setup. Certainly is something that
will need to be watched in the coming days. Only kept thunder
mention at slight chance at this point with elevated CAPE values
only 250 J/kg off the 00z NAM; also note SPC`s SWODY3 which reflect
general thunder for most of our CWA except for far eastern counties
(5% severe/"Marginal"). Categorical PoPs are indicated for Friday as
we should see at least a period of steady rain, primarily focused in
the late morning to mid/late afternoon. Highs should be in the low
to mid 60s for many on Friday. Rainfall amounts of around a half to
three-quarters of an inch, highest along and in the lee of the Blue
Ridge into the VA/NC piedmont.

In the wake of the trough, the parent upper low begins to deamplify
as it makes its way into West Virginia Friday evening. Generally
diminishing PoPs in central sections, although does stay high chance
to Likely in southeast West Virginia counties due to the
deamplifying upper low track and northwest upslope flow.
Precipitation character should also trend more showery in these
areas. Should see lows in the mid 40s to low 50s Friday evening.

For Saturday...upper low shifts off the coast with upslope rain
showers in eastern West Virginia coming to an end early in the day.
Mostly clear skies then predominate Saturday afternoon and evening
for all areas. Highs in the mid 50s/low 60s across southeast WV, the
lower 60s for the New River Valley/Alleghany Highlands to the Blue
Ridge, and downslope-driven low 70s east of the Blue Ridge. Cold
advection aloft (850 mb temps falling to +4 to +6C) along with clear
skies due to building surface ridging supports lows Saturday night
into the upper 30s to mid 40s.
&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
As of 400 AM EDT Wednesday...

Conditions remain dry into Monday morning, then we wait on the
next system tracking across Texas to the Tennessee Valley
Tuesday. Precipitation pattern may be similar to Thursday-
Friday`s event with light rain over a weak wedge followed by
short waves from an upper level low/trough to our west.

Temperatures trend above normal again for Sunday. Temperatures cool
down Monday with rain falling into a wedge. Tuesday will be wet with
slightly above normal temperatures as wedge erodes.

&&

.AVIATION /11Z WEDNESDAY THROUGH SUNDAY/...
As of 725 AM EDT Wednesday...

MVFR to IFR ceilings continue to blanket many of the mountain
airports this morning, including KLWB, KBLF, and KBCB. These
clouds will be slow to erode, but by midday to early afternoon
VFR conditions should prevail. A few MVFR cigs near KDAN and
KLYH are still possible through the early afternoon, but
confidence is low and ceilings were not included in the TAF for
this period.


High pressure will continue to build in from the north through
Thursday developing a low level wedge of cooler air east of the
mountains. This will result in the re-development of IFR to MVFR
ceilings for all TAF sites after 06Z (2 AM EDT) tonight and
continuing into Thursday.

Extended Aviation Discussion...

The next storm system moves in the Mississippi Valley Thursday
night into Friday. The highest probability of rain will be on
Friday along with sub-VFR cigs/vsbys with the showers and storms.
Once the low passes, northwest flow sub-VFR ceilings will
remain into Saturday across the mountains with gusty northwest
winds possible.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...None.
NC...None.
WV...None.

&&

$$
SYNOPSIS...PH
NEAR TERM...JH
SHORT TERM...AL
LONG TERM...AL/RCS
AVIATION...AMS/PH


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