FXUS61 KRNK 300720

Area Forecast Discussion
National Weather Service Blacksburg VA
320 AM EDT Sun Apr 30 2017

High pressure offshore will maintain a warm and humid southerly
flow of air across the region through tonight. A strong cold
front will then move through the area Monday with showers and
thunderstorms, followed by some cooler, but near seasonal
temperatures through the first part of next week.

As of 240 AM EDT Sunday...

Upper ridging offshore will remain in control today but gradually
weaken into tonight as height falls slowly push east ahead of
the complex upper trough to the west. This will again keep much
of the area in overall subsidence into the afternoon with very
little support aloft for much more than isolated to widely
scattered convection given weaker instability and still some
capping aloft. Latest guidance again remains quite limited with
any shra/tsra coverage later on espcly given early clouds and
more of a southerly trajectory today. Appears best areas of
progged weak convergence along the southern Blue Ridge/foothills
where more differential heating possible per morning low clouds,
and over the far northwest just south of a passing faint wave
aloft. This closer to the latest Cams ensemble and SPC WRF
solutions so after trimming back coverage early, keeping 20/30
pops in these spots for mid to late afternoon pop up storms.
Otherwise expecting residual clouds to fade to more sunshine
with heating, but may get stuck southern/eastern sections a bit
longer espcly given current satellite trends. Expect this along
with the more southerly flow and cooler temps aloft to keep highs
lower, mainly 80-85, although mostly 70s at elevation.

Upper low will pass well to the northwest late tonight in turn
pushing a trailing cold front toward the region by daybreak
Monday. Models show a steady increase in the jet aloft ahead of
the boundary overnight with low level convergence developing
southwest sections within a surface southeast trajectory that
will quickly veer southwest with height. This in conjunction
with an axis of higher PWATs should spell a redevelopment of
shallow convection southern Blue Ridge after dark with coverage
spreading north overnight as weak lift/moisture advection
increase along a north/south theta-e lobe. However soundings
show moisture quite shallow, so appears mainly showers vs much
thunder, initially driven by more of an upslope type setup with
instability fading shortly after sunset. Thus only running with
low to mid range pops over all but the extreme eastern sections
late as guidance usually a bit fast and overdone with this
regime espcly given much deeper convection to the west near the
front. Otherwise will see warm advection driven southerly winds
ramp up over the western ridges overnight, but appears not
enough for a headline across the Mtn Empire region for now per
limited mixing via clouds. Elsewhere another very mild overnight
with clouds increasing and lows mostly in the 60s.

As of 210 AM EDT Sunday...

A cold front will cross the area on Monday. Run to run model
solutions have been fairly consistent with the timing and the
associated convective parameters. Surface based cape increases to
just shy of 2000 J/KG and surface based LI's drop to around -5 or -6
by the afternoon. A very strong southerly 850mb low level jet may
maximize around 40 to 50 kts just in advance of the front, prompting
notable low level shear, at least in terms of low level speed shear,
if not low level veering shear. The Storm Prediction Center is still
prompting a marginal risk of severe weather in association with this

By Monday evening, most of the area will be on the back side
of the front, with lingering isolated to scattered showers and some
storms along and east of Lynchburg to Danville line. These too
should exit the region by the late evening. Northwest winds behind
the front will be on the gusty side through the night and into
Tuesday. Some of the higher elevations could experience gusts on the
order 35 to 45 mph, especially during the Monday night time period.

Dry weather is expected into Tuesday night as high pressure build
into and then over the area. Winds will weaken as the high moves

On Wednesday, wind will back to the south in advance of an
approaching area of low pressure through the western Gulf Coast
states. Enough daytime heating and moisture convergence near the
crest of Blue Ridge may prompt a few showers from near Galax, VA,
southwest into the Northern Mountains of North Carolina.

By Wendesday night, a warm front will have developed over the area
and lifted north into the Ohio Valley. Look for patchy light rain to
develop, especially over western and northern sections of the
forecast area.

Temperatures during this portion of the forecast will start about 10
degrees above normal on Monday, and be near normal by Wednesday.

As of 250 AM EDT Sunday...

Larger differences in the synoptic pattern show up in the different
models for this portion of the forecast. WPC leaning heavily on the
ensembles with keeps a progressive amplified eastern trough.

Will have highest probability of precipitation on Thursday through
Thursday night. Then with lower confidence due to the large spread
in the guidance will have lower probability of precipitation by
Saturday and Sunday. Will be keeping maximum temperatures below
normal for Thursday through Saturday.

As of 105 AM EDT Sunday...

Quite varied conditions likely into the overnight as an axis of
residual moisture from earlier convection to the south lifts
northeast across the area. This will result in a mix from mainly
mid/high clouds over northeast sections to periods of lower
strato- cu over the south and west where moisture may
eventually lower into stratus late. Where clouds are less also
expecting to see locally dense fog develop with MVFR vsbys or
worse in the valleys espcly around KLWB, and perhaps KLYH
toward daybreak. Otherwise followed closer to most guidance in
bringing MVFR to ocnl IFR cigs into KDAN overnight and possibly
KBCB/KROA/KBLF as low level moisture spreads north. Elsewhere
keeping cigs mainly VFR but low confidence in just how much low
cloud canopy will spread up espcly east of the Blue Ridge by

Lower cigs may again linger southern and southwest sections
through midday Sunday under more of a low level south/southeast
trajectory before heating/mixing helps scour out most low cigs
during the afternoon. Convective chances again appear quite
isolated Sunday afternoon and certainly not enough to include
any mention in the TAF forecast for any particular airport at
this point. Otherwise a bit more breezy with southerly winds
perhaps gusting to 20 kts or better Sunday afternoon espcly along
the ridges.

Extended Aviation Discussion...

As a low pressure system approaches from the west late Sunday
night, the potential for showers and thunderstorms will increase
over the western mountains by early Monday. The cold front
associated with this system should reach the Appalachians Monday
and depart offshore by Tuesday morning. As such, expect the
best chance of MVFR conditions during the frontal passage from
west to east during Monday. VFR conditions return behind the
front on gusty westerly winds Tuesday followed by continued good
flying weather under high pressure Tuesday night into much of
Wednesday. However moisture may return to the mountains by late
in the day Wednesday with local MVFR possible in isolated
showers by the end of the day.

By Thursday next system should be impacting us with showers and
a few storms, so sub-VFR is likely.




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