Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 211132

Area Forecast Discussion
National Weather Service Blacksburg VA
732 AM EDT Wed Jun 21 2017

A front will stall from the Mid Atlantic states to the Mid
Mississippi Valley today and tonight as Tropical Storm Cindy moves
from the Gulf of Mexico to the Texas/Louisiana coast. By Friday a
cold front will extend from the Great Lakes through the Central
Plains will Cindy tracking between the front and high pressure off
the Southeast Coast. The front will push through the Mid Atlantic
region Sunday and Monday.


Scattered light showers east of a Farmville to Reidsville line this
morning. Area with a probability of precipitation will gradually
shift southeast throughout the day. Expect a majority of the showers
to be southeast of Charlotte, Halifax and Caswell Counties by

Large swath of high clouds cover much of the Gulf Coast region and
Southeast Unites States into the Tennessee Valley and Mid Atlantic
region. Upper ridge remains off the Southeast Coast and with little
change in the overall synoptic pattern expect the cloud cover to
persist. Clouds will hold back temperature rise today and will limit
cooling tonight.

Cold front from New York state to central Illinois will stall from
the Ohio Valley into the Maryland panhandle by late today. Front
becomes stationary far enough north that southwest Virginia,
southeast West Virginia and northern North Carolina will be between
the moisture and chance of precipitation along the front and the
deep, tropical moisture surging through the Tennesse Valley late


As of 415 AM EDT Wednesday...

An interesting and complex forecast this period as T.S. Cindy
moves initially northwest toward the TX/LA border, then north
along the TX/LA border, then turns northeast late Fri, then ENE
into Saturday, essentially following the I-40/I-81 corridor
into southwest VA by Saturday afternoon.

As we look at the current time, there is an ongoing interaction
between the frontal boundary that moved through our area Monday
and then stalled just to our southeast and the outer circulation
of Cindy, with a deformation zone evident in the vicinity of
these combined features over the SC/NC area. As we progress into
Thursday, the core of Cindy and surface circulation from Cindy
will separate from the baroclinic zone as upper-level ridging
off the southeast coast splits apart the baroclinic
zone/deformation zone and forces a weak upper-level short wave
to track northeast away from the core of Cindy. All models are
in good agreement on this scenario. So, our dry weather of
yesterday and today will quickly come to an end Thursday as the
deep tropical moisture, which originally tapped into Cindy and a
Gulf moisture feed, translates northeast into eastern
TN/southwest VA Thursday. Instability is fairly minimal at this
point, so mainly looking for rain showers, arriving in
southwest VA, southeast WV, and northwest NC during the morning,
ramping up quickly to high chance/likely pops by evening,
especially west of the Blue Ridge. Any pockets of instability
will allow for isolated thunder, but nothing severe is expected
Thursday as overall the air mass is fairly stable and dynamics
are weak. The main concern will be for locally heavy rainfall.
Given that recent rainfall has not been widespread, but rather
heavy in localized pockets plus the fact that we have had a
couple of days to dry out before the Thursday rain arrives, we
do not anticipate any significant hydro problems yet Thursday.

Friday, the situation becomes more interesting, especially
during the afternoon. Upper heights begin to fall across the
region as the broad persistent trough over the Great Lakes
deepens once again. The core remnants of Cindy, as noted above,
will essentially be tracking east to east-northeast along the
I-40 corridor through western and central TN Friday, where there
are indications of substantial rainfall in those areas. However,
our CWA will remain in advance of this tropical core. Tropical
moisture will already be in place across our CWA, having spread
into the area from the pre-Cindy feature noted above, although
not quite sure it qualifies as a PRE (predecessor rain event).
Any pockets of heating, which seem quite feasible, will result
in ample instability combined with the deep moisture in place to
result in some fairly robust thunderstorms producing torrential
downpours, heavy wet precipitation loading, hence localized damaging
wind gusts, and likely frequent lightning. SPC has consequently
placed the area in a marginal risk, which agrees with my
assessment of the situation as well. As SPC notes, the severe
threat is highly conditional on expected heating, but a scenario
similar to what we saw last Thursday would not be out of the
question. It is not unusual for the day preceding the main
system (in this case Cindy) to end up being the more active
weather day of the two. Of course another concern with any
thunderstorms Friday would be localized flooding or flash
flooding as with PWATS approaching 2 inches and tropical
moisture to boot, thunderstorms will be very efficient rainfall
producers and this could really end up being the major concern.
At this point, it looks like it would be localized enough to
preclude the need for a Flood or Flash Flood Watch, but such
headlines are certainly possible with later forecast packages.

Saturday, the core remnants of Cindy are progged by all models
to track right over the CWA, enter southwest VA early in the
day, exiting the coast near Virginia Beach by evening.
Widespread cloud cover and a saturated atmosphere will limit
the thunderstorm threat Saturday, but persistent heavy rainfall
in areas that receive excessive rain Friday will certainly be
primed for flooding and/or flash flooding. However, the
thunderstorm threat seems less of an issue due an increased
stable, saturated atmosphere.

Look for well above normal minimum temperatures thanks to the
tropical atmosphere in place this period, with slightly below
normal maximum temperatures owing to the extensive cloud cover,
tropical air mass, and precipitation.


As of 430 AM EDT Wednesday...

The remnants of Cindy will quickly move out of the picture late
Saturday and no longer be a factor in our forecast. Over the
past several days, the models had been advertising a transition
to a much drier pattern across the region evolving into next
week as Cindy left the region, but that is not quite as clear
this morning. Broad upper troughing will remain in place across
the Great Lakes, keeping a somewhat unstable, albeit, weak west-
northwest flow aloft across our region. A baroclinic zone will
unfortunately linger across the southern part of our CWA or
perhaps just to the south. The upper trough is not deep enough
to shunt the baroclinic zone and deeper moisture far enough
south of us for it not to be a potential factor in our forecast.
With these features in place as listed and the potential for
upper-level disturbances to impact northern areas, we cannot
advertise a dry forecast, unfortunately. As the weak progresses
and the upper trough deepens through midweek, cooler, drier air
attempts to spread south and this should push the baroclinic
zone further south away from the CWA with a better potential for
minimal convection at that point. For now, have use the
Superblend pops in the extended as there is really no clear
consensus on any dry or not dry weather among the models. Most
suggest southern sections, at least, will likely see diurnal
convection. Temperatures look to average a tad below normal,
especially in northern areas through the period with lows in
the 50s and highs in the 70s, trending more toward lows in the
60s and highs in the lower 80s south.


As of 725 AM EDT Wednesday...

Extensive shield of high clouds spreading from the lower
Mississippi Valley to the Carolinas and Mid Atlantic region
this morning. Patchy fog in the New River Valley through
northwest North Carolina will dissipate by 14Z/10AM. High
confidence that KBCB will improve by 13Z/9AM.

MVFR over southern Virginia and northern North Carolina this
morning will gradually lift and improve back up to VFR by mid
morning. This timing follows the trends in the HRRR and GLAMP
guidance. Isolated light showers may impact KDAN but will no
reduction in visibility. Deep moisture and chance of
precipitation approaches from the southwest at the 12Z/8AM end
of the TAF forecast period, but expect ceilings to lower but
remain VFR through 12Z.

Extended Aviation Discussion...

Ceilings lower to MVFR Thursday evening into Friday with first
surge of showers advancing SW to NE. Potential for low-level
wind shear over the mountains on Thursday night and Friday.
More sub-VFR conditions Friday evening into Saturday with
periods of moderate to heavy rain. Confidence is medium on
arrival time of the rain. A front will cross through the area on
Saturday and Sunday with a continued chance of precipitation.




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