Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 231940
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
340 PM EDT Fri Jun 23 2017

.SYNOPSIS...
A cold front, energized by the remnants of what was Tropical
Storm Cindy, will progress across the forecast area tonight.
Heavy showers and embedded thunderstorms are anticipated late
this evening, a few of which may produce gusty winds and heavy
downpours. The cold front will slowly push southeastward into
eastern Virginia and North Carolina on Saturday. A cooler and
much drier air mass then filters in for the rest of the weekend.

&&

.NEAR TERM /THROUGH SATURDAY/...
As of 339 PM EDT Friday...

While currently on the quiet side, heading into a more active
evening with several potential issues to contend with, owing to the
approaching cold front which has merged with Cindy`s remnants.

Through the rest of the afternoon.... we continue to carefully
monitor the potential for isolated thunderstorms. Visible
satellite reveals some cloud breaks taking place from the
central/southern Blue Ridge eastward. LAPS-based CAPE values are
on the order of 1000-1500 J/kg, and we do have strong low-level
winds already in place (40-50 kts are common per area VWPs).
The strong low-level winds contributing to deep layer shear
values near 40 kts per SPC mesoanalysis. If any isolated storms
do develop, as reflected in recent runs of the HRRR and the 3-km
NAM, they may produce localized gusty winds as they move
northeastward off the Blue Ridge. Otherwise, looking at
generally quiet conditions into the early evening.

For tonight... weather turns increasingly more active, as we
will watch ongoing storms across the Ohio and Tennessee Valleys
progress eastward across the forecast area. Present indication
from a consensus of high-resolution guidance is to bring an
evolving line of heavy showers and embedded thunderstorms into
our western WV/VA/NC doorstep by around 01z, progressing
eastward to the Blue Ridge foothills toward midnight. While it
is a diurnally unfavorable period of time for strong convection,
wind shear values both through the lowest 0-3km and deep layer
owing to Cindy`s wind field only increase. Though it`s difficult
to fully trust higher-resolution guidance later in the model
cycle, through midnight several members do depict some
concerning radar structures as storms move into far western
counties. I suspect that we`ll have a high-shear/low-CAPE
environment in place tonight, even well after dark. In that
environment, even heavy showers could offer the potential to
produce gusty winds which may topple dead or shallow-rooted
trees. Based on 0-1 km shear values progged to be in the 40 to
50 kt neighborhood, this also poses a concern for isolated
embedded spin-ups in stronger convective elements in the line.
Given those reasons, to raise awareness of the potential threat
I`ve added enhanced wording for gusty winds in the zone forecast
through midnight at least, roughly from the Blue Ridge
foothills westward into southeast WV. After midnight, heavier
showers and storms seem to lose structure as they drift into the
Piedmont areas, so I`ve kept the gusty wind wording to central
and western counties away from the VA/NC Piedmont areas.

Another concern in this humid, tropical air mass will be heavy
rainfall potential. Even showers should be efficient rainfall
producers. Potential for hourly rain rates to be enhanced by
convective elements as well. I`ve maintained heavy rainfall wording
in the zones for the tonight period. Fortunately, rainfall we did
receive last night was not substantial. And the fast-moving nature
to tonight`s rain/thunderstorms will probably keep flooding more
localized. Guidance QPF values continue to generally paint some
consensus in depicting the highest forecast values northwest of our
forecast area, and official forecast ranged from a half to one inch
west of I-81, a quarter to third of an inch to the Blue Ridge
escarpment, tapering to a couple tenths or less east. For those
reasons, confidence was too low on an issuance of a flash flood
watch with this forecast package. For more on hydro, see the
Hydrology section below.

Cold front will have advanced east of the Blue Ridge toward Saturday
morning. However, the 12z NAM, ECMWF and GFS suggest that the front
may make more slow southeastward progress into the NC Piedmont and
foothills areas. While much drier, more refreshing air filters into
the northwestern two-thirds of the forecast area on Saturday, I`ve
kept lower Chance level PoPs through the day for showers and
thunderstorms for the southeastern third.

With the front roughly bisecting the forecast area, lows tonight
range from the lower to middle 60s west of the Blue Ridge but will
be a muggy lower to mid 70s from the Roanoke Valley eastward. Highs
tomorrow should top out in the mid 70s to lower to middle 80s.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/...
As of 339 PM EDT Friday...

By 00z Sunday (8 pm Saturday Night), most of the medium range models
forecast the position of the cold front across the far southeast
portion of our forecast area. This boundary interacting with
lingering instability will result in keeping a slight chance for
storms from South Boston to Yanceyville early Saturday evening.

Dry conditions are expected for the remainder of the short term
period as a deep northwest-west flow prevails across our region
through Monday.

In the upper levels, a broad upper trof will gradually deepen across
the Great Lakes into the northern U.S., which will allow dew points
to drop through the period resulting in cooler temperatures Monday
morning compared to Sunday morning. Leaned toward the cooler
guidance values or lows Monday morning. Highs will also be cooler
Monday as 1000-850mb thicknesses continue to fall. For late June,
humidity levels will be quite comfortable.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
As of 339 PM EDT Friday...

The long range models are in good agreement with the overall upper
air pattern during this period with an anomalously deep upper trof
over the eastern U.S. early in this period, with 500mb heights 2-3
standard deviations below normal,transitioning to a zonal flow by
mid week, with ridging building by the end of the week.

The 12Z GFS is still fastest with the eastward progression of the
upper trof Tuesday night into Wednesday, but this has little impact
on the tranquil weather expected during this time. A weak frontal
boundary moving through early Tuesday morning may generate some
spotty showers in the mountains, but forecast RAOBS indicate there
will be very little moisture available. Otherwise, PWAT values will
remain below average until Thursday night/Friday so expect a
prolonged period of dry weather. By late in the week...a deep
southwest flow developing on the west side of an upper ridge and
ahead of an approaching upper trof in the upper mid west will
increase moisture and provide the opportunity for weak upper level
disturbances to pass over our area. This will result in increase
chances for scattered thunderstorms during this period.

As for temperatures, forecast 850mb temps from the GEFS are 2-3
standard deviations below average, which will lead to some cool
mornings Tuesday and Wednesday. Adjusted low temperatures down a
couple of degrees below guidance during this period. Temperatures
will moderate late in the week.

&&

.AVIATION /20Z FRIDAY THROUGH WEDNESDAY/...
As of 148 PM EDT Friday...

Overall active aviation forecast period in the first 12 hrs.

VFR to VFR/MVFR conditions should prevail through 00z. May be
an isolated to widely scattered thunderstorm but confidence is
not high to include in the TAFs at this time. Areas of
mechanical turbulence along the spine of the Blue Ridge are
likely this afternoon with a southwesterly low- level jet of
around 50 kts persists. Surface winds should also be around 6-12
kts, with gusts to 22 kts at BLF.

Deterioration to poor flight conditions expected from 00-09z is
the overall messsge. A cold front interacting with Cindy`s
remnants produces a substantial line of moderate to heavy
showers and embedded thunderstorms. See TAF for specific
airports on timing; as of now, this was indicated with
prevailing VCSH given that we are > 6 hrs out before likely
impact. Frequent use of amendments may be likely to narrow down
a more specific period of greater risk. While a couple stronger
cells are possible and their associated lightning risk they
pose, even heavier showers may produce gusty, erratic winds at
Roanoke, Bluefield, Lewisburg and Blacksburg. Gusty wind
potential is more conditional and more lower- confidence for
Danville and Lynchburg. Expect widespread MVFR conditions with
temporary IFR/LIFR visibility in heavy rain. Low-level wind
shear possible ahead of with the frontal passage. Outside of
thunderstorms, winds begin southwest 6-12 kts before veering to
west.

For Saturday, front should make slow but steady progress
southward, with steadily improvement to VFR conditions along
with a west/west-northwest wind shift 6-8 kts.

Extended Aviation Discussion...

VFR conditions to then prevail Saturday night through Sunday night.
Potential for radiation fog and related visibility restrictions
Sunday night.

A cold front passing on Monday may produce VFR/possible MVFR
conditions and breezy northwest winds behind it. VFR then lasts
through Wednesday.

&&

.HYDROLOGY...
As of 735 AM EDT Friday...

Rainfall forecast/models/ensemble river forecasts suggests greater
threat for flooding will be west and north of our forecast area.
Average rainfall amounts of 1 to 2 inches expected from the NC
mountains to Mountain empire of SW VA to southeast WV with 2 to 3
inches further west into the KY/TN/OH corridor.

Not seeing enough signals to warrant a watch, but given efficient
rainfall rates, any convective elements or training will increase
the threat. Timing of heavy rain will be this evening into
Saturday morning.

In summary, based on models looking wet, but still not high enough
confidence in a flood/flash flood watch.

&&

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

&&

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


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