Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 240510
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
1210 AM EST Thu Jan 24 2019

.SYNOPSIS...
A low pressure system will cross the Ohio Valley today and will
push a cold front through the Mid Atlantic region tonight into
early on Thursday. This front will be followed by a reinforcing
cold front Thursday night. The area will continue within a
pattern that features quick moving, but weak, disturbances into
early next week.

&&

.NEAR TERM /THROUGH TODAY/...
As of 915 PM EST Wednesday...

Precipitation is slowly spreading into the region from the west
ahead of the approaching strong cold front, while lighter
rain/drizzle develops in the isentropic lift further to the
east. The flood watch looks on target encompassing the region
either side of the Blue Ridge where the heaviest rain will fall
as the front draws closer. Guidance has been fairly consistent
with location and timing of features and do not see anything in
new data to indicate any significant deviations.

Winds that have been blowing at the higher elevations along and
west of the Blue Ridge are expected to slacken a bit after the
onset of precipitation. However in the east, the low level flow
will really ramp up and generate some gusty conditions a bit
later tonight. Also expecting a surge of strong gusty winds
associated with the actual frontal passage. While no new wind
headlines are currently planned, with 50kts of wind just above
the surface the situation will be monitored closely throughout
the night.

Previous discussion...

Overall a pretty busy near-term period, as a strong cold front
approaches and eventually moves through the region overnight tonight
into early Thursday, with the weather pattern transitioning toward
colder northwest flow into Thursday.

PoPs increase toward Categorical for a large part of the
overnight as we await the passage of the cold front for at
least a period of steady moderate rains. Recent runs of the
HRRR, 3-km NAM and the NMM-ARW however depict a fine line of
enhanced rain showers along the front. Though an exact timing of
this potential fine line still is pretty hazy as conveyed in
guidance, this period of briefly heavy rains associated with the
front should occur toward midnight into the pre-dawn hours.
Models show nil to very little instability above wedge
inversion, instability that presently doesn`t look likely to be
realized so I`m not envisioning there being any thunder from
this line. Given the strong jet aloft, could see some short-
fused SPS potential along portions of this enhanced fine line in
regions of stronger echoes later tonight, which could focus
stronger gusts down to the surface. As the frontal boundary runs
parallel to anomalous south/southwest flow aloft, it may be a
little slow to progress eastward which will only help to aid in
producing a generous amount of rainfall. Little significantly
changed in terms of storm total rainfall, between 1-2 inches. On
that basis, no further extension/addition to the Flood Watch in
effect for tonight was warranted. See the Hydrology section for
more on the hydrologic details. Low temperatures tonight should
occur very early tonight with temperatures generally rising in
southerly warm advection into the 40s to mid 50s areawide by
early Thursday morning.

Into Thursday morning, we should see the cold front continuing to
move across our far eastern zones (roughly near or just east of
Highway 29). At the same time, temperatures will be falling quite
sharply from the 40s into sub-freezing to around freezing along/west
of the Blue Ridge as strong cold advection occurs. Upslope rain
should begin a transition over to snow showers toward mid-morning
west of the New River Valley into southeast WV and into the
mountains of northwest NC. Snow amounts don`t appear to be too
substantial though as there isn`t a great moisture connection
to the Great Lakes and less than ideal saturation in the snow
growth layer. Have shown snow amounts from a coating to up to
two inches, highest toward northwest Greenbrier County. It is
possible that roads could become slick by afternoon in the
favored upslope areas given the falling temperatures with any
rain on pavement potentially freezing. Confidence on there being
slippery roads isn`t overly high but is something that will
bear monitoring. By late afternoon, temperatures should range
from the mid 20s to near freezing from the NRV
westward/southwestward, the mid/upper 30s from the Roanoke
Valley into the foothills and the upper 30s-low 40s into the NC
Piedmont into Southside. Northwest winds will increase with
gusts potentially as high as 40 mph by afternoon associated with
good pressure rises and good mixing. Think northwest winds will
stay just below Wind Advisory levels though as would like to
see a stronger northwest low-level jet than the 30-35 kts that
is progged.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY NIGHT/...
As of 302 PM EST Wednesday...

During this portion of the forecast our region, and much of the
eastern and northeastern portions of the nation will have a broad
upper level trough positioned overhead. Within the flow of this
broad trough, a couple weaker shortwave troughs will spiral across
our region. Each of these will have their origins from south-central
Canada, and contain only the limited moisture they can advect
southeast into our area. The result will be some isolated to
scattered snow showers across mainly parts of southeast West
Virginia, south into the the Northern Mountains of North Carolina
during two distinct time periods. The first of these is forecast for
late Thursday night into early Friday morning. The second is
forecast for Saturday night into Sunday.

The remainder of the forecast region will experience limited cloud
cover and little if any precipitation, especially the farther east
you are in the forecast area.

Temperatures during this portion of the forecast will trend colder
with readings averaging about five degrees below normal by Saturday.

Forecast confidence during this portion of the forecast is high.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
As of 302 PM EST Wednesday...

During this portion of the forecast this region of the country will
will continue to remain under the influence of periodic shortwave
troughs spiraling through the region within the axis of a broad
longwave trough pattern. Each of these features will be relatively
moisture-starved with the result being primarily upslope cloud cover
and isolated to scattered snow showers across parts of southeast
West Virginia, south into the Northern Mountains of North Carolina.
The remainder of the forecast area would have less cloud cover from
west to east, and limited, if any, precipitation.

The exception to this scenario may come Tuesday night into
Wednesday. Guidance is hinting that a shortwave trough during this
time period may make a more southerly swing through the Ohio Valley
and deepen into a closed low. This may be enough to temporarily back
winds to the south or southwest to advect a greater amount of
moisture into the region, and have a period of insentropic lift
precipitation ahead of the associated cold front. This would allow
for better coverage of precipitation across the entire forecast
area, and a better chance of accumulating snow across the mountains.

Temperatures during this portion of the forecast are expected to
trend milder Sunday into Monday when values are expected to be about
five to ten degrees above normal. Look for cooling Tuesday into
Wednesday with temperatures adjusting to roughly five degrees below
normal.

Forecast confidence during this portion of the forecast is
moderate.

&&

.AVIATION /05Z THURSDAY THROUGH MONDAY/...
As of 1210 AM EST Thursday...

Strong cold front will pass across the Appalachians between now
and daybreak, yeilding widespread rain, poor vsby, and low cigs.
FROPA is expected to bring improving conditions for all but the
western slopes where low level moisture is advertised to
persist throughout the day.

Strong gusty winds will also be affecting TAF sites. Prefrontal
winds aloft are strong and will result in LLWS due to surface
based inversion which is preventing mix down into the valleys.
After FROPA, all TAF sites are expected to experience moderate
to strong surface gusts with upwards of 30-35 knots across the
ridge crests. After sunset winds are expected to gradually
diminish.


Extended Aviation Discussion...

Still will be contending with northwesterly wind gusts in the
western mountains but a general decrease in magnitude Thursday
night into midday Friday. There also will be isolated to
scattered snow showers across parts of southeast West Virginia.
The snow showers or flurries will also come to an end by mid-
day Friday.

For Friday night and Saturday, anticipating VFR conditions.

A weak clipper system may bring isolated snow showers to
southeast West Virginia on Sunday along with patchy sub-VFR
conditions. However, the vast majority of the forecast region
will remain VFR.

&&

.HYDROLOGY...
As of 303 PM EST Wednesday...

A Flood Watch remains in effect for tonight into Thursday morning
near the Blue Ridge where the greatest potential for heavier
stratiform rainfall exists associated with a cold frontal passage.

While rainfall rates are not expected to exceed Flash Flood
Guidance, the ground was still saturated from the rain last weekend.
There will be a solid 12 hour period of moderate to heavy rain with
1 to 2 inches still being expected through Thursday morning. In that
window, it appears as though the greatest potential for heavier
rains may exist overnight tonight. The Arctic air our area is
recently recovering from has likely led to frozen ground, which
should foster more efficient rainfall-to-runoff potential than last
weekend`s rain.

In terms of hydrologic impacts, may see some minor areal
flooding and/or poor-drainage flooding overnight tonight into
Thursday morning. There will also be rises on mainstem rivers,
including the Greenbrier, Roanoke and Dan Rivers. A Flood
Warning has been posted for the Dan River at South Boston where
there`s greater confidence on achieving minor flood stage. Other
river forecast points currently expected to reach Action Stage
levels per RFC forecasts which may require closer watch include
the Dan at Danville and Paces, the Greenbrier River at Alderson,
and the Roanoke River at Randolph. Tributaries in these river
basins may also see a decent rise as the rain runs off from the
mountains into the main stem channels.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...Flood Watch until 11 AM EST this morning for VAZ012>020-
     022>024-032>035-043.
NC...Flood Watch until 11 AM EST this morning for NCZ001>004-
     018>020.
WV...None.

&&

$$
SYNOPSIS...AL
NEAR TERM...AL/MBS
SHORT TERM...DS
LONG TERM...DS
AVIATION...MBS/PM
HYDROLOGY...AL/AMS


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