Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 220105

Area Forecast Discussion
National Weather Service Blacksburg VA
905 PM EDT Sat Apr 21 2018

High pressure will stay anchored from the Great Lakes to the
Middle Atlantic through Sunday night, while low pressure shifts
from the southern Plains and Texas to the lower Mississippi
Valley. This low will then track to the North Carolina and
Virginia coast by Wednesday.


As of 910 PM EDT Saturday...

At 01Z (9 PM EDT), surface high pressure was located across far
southeast VA/northeast NC with a dry airmass still firmly
entrenched across our region with the 00Z RNK sounding showing
0.35" PW values.

A canopy of high clouds will continue to drift over our area,
especially north of highway 460, keeping low temperatures
slightly warmer compared to the past couple of days. However,
with such a dry air mass in place and thinning cirrus in the
southern portion of our forecast area, brought down low temps a
few degrees closer to colder NAM MOS and HRRR guidance values.

For Sunday, an upper low will be in Arkansas, while an inverted
through extends northeast into KY, from a surface low over MS.
Anticipate some lowering and thickening of cloud cover through
the day, but no rain. With the clouds and southeast winds have
temps at or below mos, with upper 50s to lower 60s NC mountains
through the New River Valley into the Alleghanys/Greenbrier,
with mid to upper 60s elsewhere.


As of 245 PM EDT Saturday...

This portion of the forecast is looking more and more likely to be
one with plenty of wind and plenty of rain. A slow moving upper
level low pressure system will progress from the Lower Mississippi
River Valley northeast into parts of the Tennessee and Lower Ohio
River Valleys. The result for our region will be a developing
southeast flow into the region that will be tapping the Atlantic
Ocean for moisture, all while an in situ cold air damming wedge
develops. The rain, clouds and strong inversion will help keep
daytime temperatures on the cool side, and overnight lows near
normal for most of the region.

The southeast flow will be persistent during this period, and peak
in speed at 850 mb around 45 to 50 kts across the mountains Monday
night. The area defined by an area of between Bluefield, WV to
Marion, VA to Richlands, VA will likely be very gusty as this
orientation of a flow yields downsloping, mixing conditions
there outside the influence of the wedge. Currently, forecast
gusts in this area, and neighboring higher terrain, are in the
range where a wind advisory would be warranted, but we are too
early for any type of official product at this point. However,
the concern will be mentioned in the Hazardous Weather Outlook.

Another concern during this portion of the forecast will be the
potential for localized flooding Monday through Tuesday night and
then . Please see the HYDROLOGY section of this discussion for

The precipitation will start to exit the region Tuesday night, but
not by a significant amount. Rather than having the region with
categorical POPs, we will start trending to more regions of
likely and chance POPs as the upper low continue a northern trek
into the Great Lakes region. This will help start veering winds
more southerly, and we will start to lose the strong Atlantic
moisture fetch.


As of 320 PM EDT Saturday...

The upper low over the Great Lakes will get caught up in the
northern stream jet, open as a wave, and eject eastward with the
prevailing flow. Its associated cold front/trough axis will cross
our region, and help bring winds around the the northwest Wednesday
into Wednesday night. This will result in our winds shifting
northwest, and allowing for upslope rain showers across the west,
and decreasing coverage of precipitation east of the Blue Ridge.

Our weather pattern will remain quite progressive through the
remainder of the forecast period. Two additional northern stream
shortwave troughs are expected to zip through the region. Each will
bring a return of showers to the area, with the greatest
concentration across the mountains. The first will be Wednesday
night into Thursday. The second will occur either Friday or
Saturday, as model agreement isn`t as strong on this second one. Our
forecast will reflect the quicker of the two solutions with enough
colder air arriving in its wake for the potential for some snow
showers along the highest peaks and ridges of southeast West
Virginia, south into the Northern Mountains of North Carolina.

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


As of 730 PM EDT Saturday...

VFR conditions should be the rule through the forecast period as
high pressure is firmly in control. Expect increasing high
clouds into the overnight hours and tomorrow ahead of a low
pressure system moving in from the west. Over the course of the
day increasing moisture will lead to a thicker cloud deck and
some lowering. Winds will mainly be variable and light or even
calm through the nighttime hours. During the day Sunday, winds
should predominantly be from the southeast, however speeds
should remain light.

There exists some possibility for isolated precip in the
southwest section of the forecast area late in the TAF period,
but should remain far enough away to impact any of the TAF

High confidence for VFR conditions through the bulk of the TAF
period, with uncertainty growing near the end for small
potential to introduce MVFR conditions in the mountains.

Extended Aviation Discussion...

Clouds will thicken and lower by Sunday night in advance of an
approaching low in the Lower Mississippi River Valley. However,
cloud bases should remain VFR until deeper moisture arrives
Monday. SCT MVFR showers are possible in southern Virginia and
northern North Carolina Sunday night with rain becoming
widespread across the Mid Atlantic region on Monday. MVFR/IFR
ceilings and visibilities are expected Monday night and Tuesday
due to widespread rain and an upslope southeast wind. SCT MVFR
showers will continue into Wednesday.


As of 310 PM EDT Saturday...

Monday through Tuesday night, rainfall amounts over one inch are
forecast for the vast majority of the area, with locations along and
near the NC/VA border nearing two inches. Along and upstream of the
crest of the Blue Ridge, the strong southeast flow will yield
strong upslope enhancement with rainfall totals here possibly in
the two to three inch range with some isolated spots nearing
four inches. Flooding will be concern in the near term,
especially along the crest of the Blue Ridge within our forecast
area, and across parts of Southside Virginia and neighboring
sections of the Foothills and Piedmont of North Carolina. In the
slightly longer term, Tuesday into Wednesday, the GEFS based
SERFC Ensemble River Forecasts is offering a more likely than
not chance of some minor flooding along the Dan River as runoff
from this event travels downstream. While this statement does
not constitute an official forecast, it is a heads-up for an
above average potential for flooding within this river basin.




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