Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 271556

Area Forecast Discussion
National Weather Service Blacksburg VA
1156 AM EDT Thu Apr 27 2017

Chances of showers and thunderstorms begin this afternoon as a
low pressure system passes to the north. High pressure will
build off the Southeast coast during the next few days to bring
well above normal temperatures.


As of 1155 AM EDT Thursday...

A low pressure system centered just north of the Great Lakes is
stretching a cold front southward toward the Ohio River Valley.
An outflow boundary from convection the previous day in this
area should move ahead toward the Appalachian Mountains by the
afternoon. The latest high resolution models show scattered
showers and thunderstorms developing over the Tennessee River
Valley and spreading eastward toward locations west of the Blue
Ridge, but the timing has slowed down by a couple hours and is
reflected in this latest update.

The Storm Prediction Center has the entire CWA outlooked for a
marginal risk of severe thunderstorms today. Model CAPE is
forecast into the 1000-1500 J/kg range this afternoon, but
diurnal gains in buoyancy may be muted by increasing cloud
coverage ahead of the cold front. If sufficient solar heating
occurs, low to mid level flow will encourage momentum transport
capable of supporting locally damaging wind gusts.

Loss of daytime heating will result in a demise of the shower
and thunderstorm activity before midnight. The actual surface
front should dissipate overhead as the building upper ridge of
high pressure acts as a road block to prevent any sort of change
in air mass to the forecast area.


As of 400 AM EDT Thursday...

A broad and strong h5 ridge will be centered over the southeastern
U.S. in this period with 588 dm heights over Florida building to
591+ dm just off the Carolina coast by Saturday. This will provide a
very warm and fairly humid air mass across the southern Appalachian
region, especially for late April. High temperatures will run 10 to
15F above normal both Friday and Saturday with highs in the upper
70s to low/mid 80s Friday to mid/high 80s Saturday. Not really
threatening records but still unseasonably warm. Precipitation-wise
do not expect to see much, if any. A strong upper trough will be
digging southward into the southern high plains Friday/Saturday
setting the stage for a heavy rainfall event well to our west,
across parts of the southern and central Mississippi Valley. Forcing
for precipitation in our area will be weak with only a chance for
diurnally driven convection which will be widely scattered and not
hydrologically significant. This will allow the last of our flooded
rivers to continue receding. Models indicate decent ML CAPE of 1-2K
Saturday afternoon but only modest shear but could allow a few
strong to near-severe pulse thunderstorms in the afternoon.


As of 400 AM EDT Thursday...

Very warm and humid conditions continue Sunday with the chance for
showers and thunderstorms, especially along the Blue Ridge. By
Monday, the vertically stacked upper closed low over the Midwest
will move into the Great Lakes region. This will push a vigorous
cold front into the area from the west with a good chance of showers
and thunderstorms through Monday afternoon into Monday night. High
pressure will then build on for Tuesday with cooler weather and
windy conditions. Moisture increases on Wednesday, with better
chances of rain Wednesday night into Thursday.


As of 745 AM EDT Thursday...

VFR conditions at all sites through 18z/2PM today before
frontal showers and a few storms arrive along the western slopes
of the Appalachians. Increasing clouds/showers are the result
of an approaching cold front. Ongoing line of deep convection
over the TN valley is forecast to weaken this morning before
redeveloping along the outflow boundary as it moves into the
western Appalachians this afternoon. Storm coverage the
afternoon is expected to be scattered in nature...thunderstorm
clusters as opposed to a solid line. Southerly winds will become
a bit gusty from mid morning on per 35 to 40 knot LLJ ahead of
the approaching front.

Showers/Storms are forecast to linger until loss of daytime
heating. The actual surface front is expected to wash out and
dissipate as it crosses the mountains, so in spite of a wind
shift as it crosses the mountains this evening, very little if
any wind shift is expected over the piedmont east of the
mountains where the winds will remain predominantly out of the
south or southwest.

Extended Aviation Discussion...

Mainly VFR flying weather is expected for Friday and into the
weekend as high pressure builds over the southeast states
promoting temperatures which will be much above normal. Warming
surface temperatures also means increasing atmospheric
instability. As such, potential will exist for cloud build-ups
and the risk for isolated to scattered thunderstorms along the
spine of the Appalachians during the peak heating hours each




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