Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 220941

Area Forecast Discussion
National Weather Service Blacksburg VA
541 AM EDT Sun Apr 22 2018

High pressure will stay anchored from the Great Lakes to the
Middle Atlantic through Sunday night, while low pressure shifts
from Arkansas to eastern Tennessee. This low will then track
over the Ohio Valley on Wednesday.


As of 300 AM EDT Sunday...

For today, an upper low will be in Arkansas, while an inverted
trough extends northeast into Kentucky, from a surface low over
Mississippi. Meanwhile, over the Mid Atlantic, two surface high
pressure systems will join forces to keep rain out of the area
for one more day. We will see an increasing in high clouds as
moisture from the upper low rides over the merged surface ridge.
Despite thickening high clouds, temperatures across the
mountains will be slightly warmer than normal with highs in the
mid to upper 60s, with the exception of higher ridges being in
the upper 50s. The foothills and piedmont counties will
experience slightly cooler than normal temperatures today. With
a cool light southeast breeze, highs in the east will range in
the upper 60s to lower 70s.

Clouds will continue to lower tonight as the upper low moves
over eastern Tennessee and closer to the area. Saturation of the
environment will take some time as the dry high over the Mid
Atlantic holds off rain from entering the area until later
Monday morning. Some light rain from southeast upslope flow may
fall by sunrise, but the bulk of the heavier rain is not
scheduled until later in the day. With moisture continuing to
pump into the region overnight, temperatures will remain mild
with lows ranging in the 40s.


As of 150 AM EDT Sunday...

Upper low becomes vertically stacked and drifts from the
Tennessee Valley into the Ohio Valley during this time frame. At
the surface a triple-point low forms Tuesday then moves
northeast to the Virginia coast Tuesday night and Wednesday.
Upper pattern is somewhat blocky with a ridge preventing much
eastward movement of the upper low. But timing of precipitation
similar to last several runs with steady rain filling in from
southwest to northeast across southwest Virginia, southeast West
Virginia and northern North Carolina on Monday.

This a favorable scenario for a classic upslope heavy rain
event along the Blue Ridge and foothills to the east. With deep
and very strong isentropic lift and upslope bringing
precipitable water values around 1.5 inches into the region,
anticipate 0.5 to 1.0 inches of rain every 6 hours from 18Z/2PM
Monday through 18Z/2PM Tuesday.

Will stay on the cool side of guidance for maximum temperatures
on both Monday and Tuesday due to very limited solar insolation
and a strong in-situ wedge. Once the surface low reaches the
Virginia coast, surface winds will become west then northwest
eroding the wedge. At this time, models have this occuring on
Tuesday night. Not particularly meaningful cold air advection
behind this system but there may be enough of a downslope
component to the wind along with the deeper moisture pulling out
of the area, that there may be some breaks in the clouds east
of the Blue Ridge on Wednesday. Breaks or not, will still have
Wednesday warmer than Tuesday.


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 130 AM EDT Sunday...

VFR conditions should be the rule through the forecast period as
high pressure is firmly in control. Expect increasing high
clouds into 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 light and variable or even calm through the morning
hours. During the day, winds should predominantly be from the
south-southeast, however speeds should remain under 8 knots.

There exists some possibility for light rain 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 locations.

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 NW NC mountains.

Extended Aviation Discussion...

Clouds will thicken and lower by sunrise Monday in advance of
an approaching low moving across Tennessee. However, cloud
bases should remain VFR until deeper moisture arrives later Monday.
SCT MVFR ceilings are possible in southern Virginia and
northern North Carolina overnight Sunday with rain becoming
widespread across the Mid Atlantic region by Monday afternoon.
MVFR/IFR ceilings and visibilities are expected Monday night and
Tuesday due to widespread rain and an upslope southeast wind.
The lower pressure system is expected to track north into the
Ohio Valley Wednesday. Low clouds and showers are possible
across the mountains. A cold front is forecasted to move across
the region Thursday, bringing rain to the entire forecast area.


As of 530 AM EDT Sunday...

Collaborated with WPC on covering the southern Blue Ridge in a
Marginal/Slight Risk of Excessive Rainfall for Monday through
Tuesday night. Total rainfall amounts will range from 1.5 to 3
inches with locally higher amounts along the Blue Ridge. If this
amount of rainfall in realized some small streams and creeks
will likely flood, then mainstem rivers will rise.

Based on the expected rainfall the lower reaches of the Dan and
Roanoke Rivers and along the James River from Lick Run to
Scottsville have to potential to rise to Action Stage. While the
official river forecast does not yet contain the expected
rainfall, this discussion highlights an above average potential
for flooding within these river basins.




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