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FXUS61 KRNK 240116

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

A cold front, energized by the remnants of what was Tropical
Storm Cindy, will progress east across the forecast area 
tonight. Showers and embedded thunderstorms are anticipated into
the overnight, 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, followed by a 
cooler and much drier air mass for the rest of the weekend. 

As of 855 PM EDT Friday...

Leading edge of weakening convection to the west slowly heading
east toward the western counties attm in association with the 
Cindy remnant racing through NE Kentucky and into West Va. Once 
the low passes should see shra/tsra get a bit better push off to
the southeast with perhaps low topped bands impacting the 
mountains into the early morning hours. However latest regional 
radar loops show a weaker area of showers within the line 
heading toward southwest VA likely due to strong lift just north
and a passing storm cluster to the west. This may tend to fill 
in and intensify given the passage of the main core of 850 mb 
winds across the western sections in the next few hours and low 
level instability off evening soundings but iffy. 

Other concern is with the heavy rain potential mainly northwest
sections as the persistent band of heavy rain now northwest of 
RLX drops southeast once the remnant low passes. Some solutions 
including the HRRR do bring between 1 and 2 inches to portions 
of Greenbrier county between 03-09Z while others weaken the 
heavier rainfall once better support exits. Given uncertainty 
and isolated nature to possible heavy rain, wont hoist a watch 
and handle with short term products if needed at this point. 
May also see some of the showers push east of the mountains 
after midnight but in a much weaker state. Thus main update was 
with timing of higher pops northwest and to bump up QPF mainly 
Greenbrier/Summers into Bath county. Otherwise expecting a 
breezy and very muggy overnight with higher gusts possible in 
showers despite lack of thunder. Lows 60s west to low/mid 70s 

Appears exodus of showers will occur a bit faster later tonight
with decent drying developing by early Sat. Since this supported
by most solutions, opted to lower pops by daybreak Sat and
remove most chances during the morning for now.

Previous discussion 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.


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.


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.


As of 650 PM EDT Friday...

Overall active aviation forecast period in the first 12 hrs. 

VFR conditions should prevail into early this evening across the
mountains and out east through about midnight. Areas of 
mechanical turbulence along the spine of the Blue Ridge are 
likely to continue into this evening as the low level jet ramps
up to around 50 kts ahead of convection to the west. Surface 
winds should also remain gusty at times with KBLF perhaps
gusting to 30 kts at times. 

Deterioration to sub-VFR flight conditions will occur from west
to east after sunset with most of the showers either passing 
east or dissipating during the early morning hours of Saturday. 
This ahead of a cold front along with the remnants of Cindy that
are likely to produce at least a broken band of moderate to 
heavy showers and embedded thunderstorms. Models have continued 
to slow down the eastward progress of this line while showing a 
weaker scenario with best energy passing by to the northwest. 
Thus have only included a prevailing period for showers at 
KBLF/KLWB with continued VCSH/VCTS and TEMPO elsewhere although 
thunder chances look rather small at this point. Appears some
low clouds may develop across the mountains as the showers exit
with potential IFR/LIFR mainly around KBLF late. Strong wind
gusts to 35 kts may also accompany any of the heavier showers.

For Saturday, the front should make slow but steady progress
southward, with steadily improvement to VFR conditions along
with a westerly wind at 8-15 kts with potential gusts to around
25 kts during Saturday afternoon.

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.


As of 911 PM EDT Friday...

Rainfall forecast/models/ensemble river forecasts suggests greater 
threat for flooding will remain northwest of our forecast area
into tonight. Average rainfall amounts of .50 to 2 inches 
expected from the NC mountains to Mountain empire of SW VA to 
southeast WV with 2 to 4 inches further west into the KY/TN/OH 

Not seeing enough signals to warrant a watch, but given efficient 
rainfall rates, any convective elements or training will increase 
the threat especially across the Greenbrier Valley of West
Virginia into the overnight where the heaviest rainfall is
expected. Timing of heavy rain will be in a 3 to 6 hour window 
from late this evening into early Saturday morning with drier 
air quickly returning Saturday.




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