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878
FXUS61 KRNK 272327
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
727 PM EDT Thu Jul 27 2017

.SYNOPSIS...
Low pressure was over the Ohio Valley with a cold front extending
into the the Southern Plains. This low will move southeast tonight
through Friday night, which will bring a cold front across the Mid
Atlantic region on Friday and Friday night. High pressure follow
behind the front covering from the Great Lakes to Virginia and North
Carolina by Tuesday.

&&

.NEAR TERM /THROUGH FRIDAY/...
As of 625 PM EDT Thursday...Sent an early update to increase
QPF and to add heavy rain wording through midnight mainly for
locations along and southwest of a line from Bluefield to
Blacksburg to Hillsville and North Wilkesboro. Regional radar
shows bands of embedded heavy rain producing thunderstorms over
northern and eastern Kentucky that are progressing east-
southeast. The area I`ve included for heavy rain wording looks
to be on the eastern extent of favorably high moisture flux
convergence from developing 30-kt southwesterly low-level jet
advecting in PWAT values nearing 2.0" per SPC mesoanalysis.
WPC`s mesoscale precip discussion issued around 2 pm does a nice
job describing the larger-scale meteorological elements
associated with heavy rain in this area as well. HRRR and the
18z 3-km NAM progress this ongoing area across the Mtn
Empire/Grayson Highlands and into the NC High Country through
midnight. As profiles have generally stabilized from earlier
showers and thunderstorms, think convective threat is over
outside of some intra-cloud lightning flashes. In the next
couple of hours, will also need to monitor ongoing convection
over the northern NC Piedmont for localized heavier rainfall
issues. Overall message for tonight is such that the already-
limited severe threat has ended, but in turn is turning toward
localized hydro concerns.

As for the after-midnight period, it`s a little unclear on the
spatial extent of rainfall per 18z guidance suite. The NAM and
the CAM guidance generally keeps us dry with a lot of cloud
cover around; while the GFS hints at re-developing at least
showers roughly along/north of I-64 associated with broad
diffluent mid-level flow ahead of potent shortwave now over the
upper Midwest. Started to reduce PoPs for areas south of Rt 460
after midnight, but keeping them relatively high across the
northern third of counties. Further adjustments may be needed
later. All in all, expect a cloudy and rather muggy evening with
lows in the mid/upper 60s to mid 70s.

Previous discussion issued at 205 PM follows...

A weakening line of convection will fall apart over the New
River and Roanoke Valleys this afternoon. Following this line of
convection is an area of light rain with embed cloud-to-cloud
lightning. This area of rain will stay over a county for two to
three hours before moving east. There is another line of
convection coming across eastern Kentucky with another line
behind that one. These two lines will move over the Mountain
Empire and North Carolina Highlands later this afternoon.

Convection east of the Blue Ridge is likely however, severe
potential is limited. Morning soundings at RNK/GSO shows plenty
of moisture in the low levels (below 6kft), but abundant dry air
above. Both soundings are also showing a very strong cap
between 6500-8000 ft. It was this capping inversion that kept
the first line of convection weak coming over the mountains this
morning. The difference this afternoon is that diurnal heating,
especially across Southside has allow instabilities to creep up
to moderate levels. A few storms may find a weakness in the cap
and break through to become severe. These lines of convection
and areas of rain will help saturate the column and erode
capping inversion by this evening.

An ill-defined surface low is expected to track along a cold
front tonight into Friday. Models are developing two areas of
convection with this front. One area of convection is expected
to occur across Tennessee early Friday morning and track
southeast into Alabama and Georgia. This line of convection is
expected to fade through the morning, but will send an outflow
boundary across the southern Appalachian and into North Carolina
in the afternoon. The second area of convection will occur
across the southern Ohio Valley tonight then track east into the
Mid Atlantic region Friday. This area of convection will have
the stronger dynamics as a closed low drops out of the Great
Lakes.

Being in between these two convective systems, our chances are
very good to see showers through the day Friday. Convection
coming across the area today will take out some of the
instabilities but will saturate the environment. PWATs will
increase to nearly 2 inches Friday. With our recent dry spell,
we are not expecting any widespread flood concerns today.
However, depending on how much rain falls today and does rain
continue overnight, may prime the area for Flash Flooding
tomorrow. For now, we will hold steady and only highlight in the
Hazardous Weather Outlook (HWO).

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/...
As of 125 PM EDT Thursday...

Closed low moves southeast from the Ohio Valley into Virginia
by Saturday. Upper low moves offshore and trough axis will be
along the coast on Sunday. This brings a vorticity maximum over
the region on Saturday. This synoptic lift combined with good
lapse rates due to colder temperatures aloft will result in
scattered afternoon showers. More questionable is the presence
of enough instability and lift for thunderstorms. BUFKIT
forecast soundings showed little, if any CAPE, Saturday
afternoon.

Models, especially the 12Z NAM, suggest 850MB temperatures by
Sunday morning as low as +8. NAM. SREF and ECMWF were not that
cool. Clouds and wind will limit cooling potential on Saturday
night. will be leaning toward blend on MOS/MAV guidance for lows
Saturday night.

Deeper moisture wraps around the low pressure system with the
dry slot over southwest Virginia Friday night and moisture back
in the area on Saturday and Sunday. A surface low off the
southeast Virginia coast may enhance the north to northeast
surface winds across the region on Saturday night and Sunday.
Will be increasing wind speeds at higher elevations this time
frame.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
As of 1245 PM EDT Thursday...

Upper low opens up and lifts northeast back into the northern
jet stream Sunday night through Tuesday with 500MB heights
rising Tuesday and Wednesday over the Mid Atlantic region. Still
under long wave troughing over the eastern United States
through the end of the week with models showing some signs of
through amplifying by Friday. 850MB temperatures gradually warm
Monday through Wednesday back close to +18 to +20 for Wednesday
and Thursday.

May take until Monday for low pressure system to move far
enough away that wrap-around clouds exit foothills and piedmont
of Virginia and North Carolina. Will maintain dry forecast
Sunday night through Thursday. Once surface high moves offshore
on Friday a slight chance of precipitation will return to the
mountains.

&&

.AVIATION /23Z THURSDAY THROUGH TUESDAY/...
As of 726 PM EDT Thursday...

Light, unrestricted isolated/scattered rain showers producing
generally VFR conditions through about 03z. Next wave of
moisture/heavier showers now over eastern KY then advances
southeast to areas mainly along and southwest of I-77,
potentially bringing MVFR-visibility showers to Bluefield and
the southern Blue Ridge and Mtn Empire, perhaps as far
northeast as Blacksburg. Threat mainly from 03-05z. Wouldn`t
discount rumble or two of thunder and embedded intracloud
lightning but likely to be sporadic. Ceilings should begin to
fall to a BKN/OVC VFR/MVFR range after 06z. Kept VCSH going
after 06z across the TAFs, though this may be overdone and
potential for adjustment possible in 06z TAF issuance. Winds
mainly light southwest, variable to calm at times. Confidence in
aviation ceiling/visibility and timing elements medium this
period, though is medium to high on winds.

Low-confidence TAF forecast for Friday. Appears that VFR to
possibly MVFR ceilings may linger with early-day showers a
possibility especially further northwest. Potential will exist
for re-development of showers and thunderstorms as early as
noon. Timing, strength and extent of thunderstorms highly
contingent on sufficient cloud breaks. Any thunderstorm may be
capable of torrential downpours and associated IFR to LIFR
visibility and lightning. A few stronger storms may be
possible, again contingent on cloud breaks. Current TAF calls
for extensive, though VFR cloudiness with VCTS indicated for
the afternoon hours (starting 16-18z). Winds turn
south/southwest 4-8 kts.

Aviation Extended Discussion...

Saturday starts to dry out some but upper low will enhance
northwest flow over the area keeping sub-VFR cigs stuck in
BLF/LWB similar to winter scenarios with isolated showers
around.

Expect widespread VFR to return Sunday/Monday under high
pressure following the front.

&&

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

&&

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