Area Forecast Discussion
Issued by NWS Blacksburg, VA

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897
FXUS61 KRNK 131805
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
205 PM EDT Sun Aug 13 2017

.SYNOPSIS...
A weak frontal boundary will remain stalled just to our south
for the next several days. A broad area of low pressure aloft,
located across the Great Lakes, will continue to send a series
of upper-level disturbances toward the region from the west.
These disturbances will interact with the front helping to
enhance periods of showers and thunderstorms at times through
the week. A new frontal system will move into the region from
the west Friday.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 100 PM EDT Sunday...

No major changes from the earlier forecast update. The
beginnings of some isolated showers are now being noting on
regional radars along the spine of the Northern Mountains of
North Carolina. Still anticipating a gradual development
northeastward along this higher terrain as the afternoon
progresses. Likewise, parts of Southside Virginia and north
central North Carolina are also expected to see some isolated
coverage by the late afternoon. Based upon the latest
observations and expected trends through the afternoon, have
increased forecast high temperatures on average by two to three
degrees for a number of spots across the region.

As of 1000 AM EDT Sunday...

Have not made a lot of changes to the general trend of the
forecast for today. However, have made some minor adjustments as
to how we trend to the over expected pattern of the day. For
example, have reduced the rate by which isolated showers return
to the southwestern sections of the area, and then spread
northward, by a few hours based upon the latest trends of the
HRRR guidance. Although the final coverage of precipitation by
the late afternoon is not to be all that much different than our
earlier prediction.

Have also made minor adjustments to the hourly temperature, dew
point, wind/gusts, and sky cover based upon the latest
observations and expected trends through the early afternoon.

As of 415 AM EDT Sunday...

What was appearing to be relatively dry day compared to the past
few and the upcoming several, does not appear so dry now. Model
trends continue to stall the frontal boundary, which moved
through the area slowly late Friday and yesterday, further
north. This will leave the associated moisture further north as
well. Lingering showers and isolated thunderstorms early this
morning across the far southeastern portions of the CWA,
associated with an upper-level disturbance, are moving east and
will move out of the region before daybreak. By afternoon,
moisture will return northward through especially the southwest
and western parts of the CWA as surface high pressure drifts
from the eastern Great Lakes toward New England. This will help
to set up easterly flow and enhance moisture and lift along the
Blue Ridge going into Monday. Would look for rain showers, and
possibly even a thunderstorm in the far southwestern areas,
during the afternoon/evening hours. Moisture will continue to
closely creep northward overnight as the east-southeast flow
further enhances lift along the Blue Ridge. Rain chances will
increase from the south overnight. There may also be areas of
drizzle and fog, especially along and near the Blue Ridge from
northwest NC into southwest VA overnight. So, this may leave a
period this morning where much of the CWA, especially the
northern sections, will have a rain free period, but still lots
of clouds. Throughout this section of the forecast, the best
rain chances will be located east and south of the Blue Ridge,
with the lowest chances toward the I-64 corridor.

850mb temps drop back into the +12C north to +14C south range
today. This combined with the developing northeast to east flow,
hence weak wedge, should result in similar or cooler daytime
max times than what was observed Saturday. The air mass will be
slightly drier with lower dewpoints northern areas today. A few
glimpses of sun may help warm northern areas a bit more than
southern areas where clouds will be more solid. MET MOS seemed
to have the best handle on the cooler temperatures as it was
generally a full category lower on max temps than the GFS or
ECMWF MOS. Thus, given the synoptic situation in place, leaned
heavily toward the MET MOS. This will yield highs near 80 across
the Piedmont, with mostly 70s elsewhere. Lows remain muggy at
night with increasing humidity again, mainly in the 60s, but
near 70 across the eastern and southeastern parts of the
Piedmont.

&&

.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
As of 245 AM EDT Sunday...

High pressure centered over the northeastern states will wedge south
into the Carolinas Monday. The parent high will move off the New
England coast Monday night with a bubble high remaining over
southern Virginia and western North Carolina. A series of mid level
short waves, being ejected out of an upper level trough over the
Midwest, will move over the wedge keeping the chance for showers
along and east of the Blue Ridge Monday. The highest probability for
showers through the day will be along the NC/VA border.
Northeasterly flow may keep the morning dry for areas north of hwy
460. As the parent high shifts towards the New England coast, low
level flow will become more east to southeast, increasing the
chances for showers to the north Monday afternoon and evening.
Downsloping southeast flow should help deter rain from falling
across extreme southwestern Virgina and southeastern West
Virginia. However, the wedge will partially erode across the
Mountain Empire to the Bluefield area. This area may see some
late day thunderstorms that will fade during the evening. All in
all, Monday will be a damp cool day with highs generally in the
70s. Areas outside of the wedge may touch 80F. Rain chances and
areal coverage decrease Monday night. Areas not seeing rain
overnight will see fog developing and hanging around into
Tuesday morning.

The wedge is expected to erode Tuesday as the upper level trough
approaches from the west. Areas across south-central Virginia may
not see the wedge erode until mid to late afternoon. Forecast
soundings still showing a moist environment when the wedge breaks.
So, as the sun peeks through the clouds, afternoon heating will lead
to showers and thunderstorms Tuesday afternoon and evening.
Afternoon temperatures should warm towards normal levels with near
80F west of the Blue Ridge to the low to mid 80s east. Fog is again
possible Tuesday night, especially in mountain valleys.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
As of 1130 AM EDT Sunday...

Looking into the second half of the week, believe most of the
shower and thunderstorm activity will be hit-and-miss variety,
driven by daytime heating, as the weather forecast models do not
indicate any highly organized disturbances moving our way.
Models are hinting at a cool front approaching from the
northwest by week`s end, which may bring greater coverage of
rainfall, however given the weak push from high pressure behind
the front, it remains uncertain how far south this front will
progress.

For Wednesday, held temperatures a degree or two below the guidance
consensus as generally southeasterly windflow continues to draw
cooler Atlantic moisture into the central Appalachians. Temperatures
enter a modest warming trend towards the end of the workweek as a
weak warm front lifts across the region, allowing surface winds
to shift more southwesterly, tapping more humid air from the
Gulf of Mexico. Temperatures may be a degree or two cooler next
weekend, depending on the amount of cloud cover associated with
the cold front approaching the mid Atlantic.

&&

.AVIATION /18Z SUNDAY THROUGH FRIDAY/...
As of 130 PM EDT Sunday...

Isolated afternoon convection is still anticipated across the
southwestern and southern sections of the area this afternoon.
Ceilings for the most part will remain VFR with some patchy MVFR
ceilings in the same regions where isolated showers and a few
storms are possible.

As we head into and through the overnight hours, anticipate a
trend towards worsening conditions. Ceilings will trend to
IFR/MVFR conditions with visibilities trending to LIFR/IFR by
daybreak. Precipitation will be on the increase by late tonight
in the form of scattered showers, with the best coverage across
western sections of the area.

During Monday, look for coverage to continue to increase,
especially in the west and south. A summertime, rain-cooled,
wedge scenario will help maintain cloud cover across the region,
with limited improvement through the morning hours. Visibilities
will improve through the morning, but not very rapidly.

Extended Aviation Discussion...

The wedge may be slow to erode Monday with the potential for
widespread low-mid clouds and a few showers Monday resulting in
periods of sub-VFR ceilings and visibilities. Late in the day
Monday, a cold front will move into the region from the west
resulting in a better chance for showers and thunderstorms and
accompanying sub-VFR conditions. The front will stall, as the
past several have done so, and linger just to our south
throughout much of the Wed-Fri period until another frontal
system arrives and potentially finally dries out the region.
All-in-all look for unsettled week with scattered to numerous
showers, with thunderstorms possible during the afternoon and
evening hours.

&&

.EQUIPMENT...
As of 755 PM EDT Saturday Aug 12th...

KFCX doppler radar will continue to be down for the rest of the
month due to a failing bull gear. It will only be operational
during this time frame for brief periods if an imminent
significant widespread severe weather and/or hydrologic event
occurs. Operating the system for any amount of time until the
bull gear is replaced risks a catastrophic failure that would
further extend system downtime. From the 17th through the 27th
the radar will most likely be down completely as the repairs
are expected to be made during this period.

&&

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

&&

$$
SYNOPSIS...RAB
NEAR TERM...DS/RAB
SHORT TERM...RCS
LONG TERM...NF
AVIATION...DS/RAB
EQUIPMENT...RAB/WERT



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