Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 290043

Area Forecast Discussion
National Weather Service Blacksburg VA
843 PM EDT Wed Jun 28 2017

High pressure offshore will continue to drift east of the region
overnight. This will allow a southerly flow of warmer and more
humid air to return to the area starting on Thursday. Showers
and storms will be on the increase Friday into the weekend as a
weak cold front stalls out near the region into next week.

As of 840 PM EDT Wednesday...

Evening raobs and surface obs show moisture increasing a bit
across the region this evening as weak south/southeast flow
develops in the wake of high pressure offshore. This should keep
low temps warmer overnight despite mainly clear skies with only
the deeper valleys dropping well into the 50s and perhaps upper
40s northwest. May even see parts of the east including near
the Blue Ridge only fall to around 60 per latest guidance so
bumped up lows a little. Otherwise another quite night in store
under subsidence with only passing high clouds over perhaps a
little trapped strato-cu in spots. Also should see river valley
fog become a little more widespread with some bump in low level
moisture as dewpoints/temps saturate by daybreak.

Previous discussion as of 300 PM EDT Wednesday...

High pressure was centered over our region. Most cloud cover
was limited to cumulus production near and just west of the
crest of the Blue Ridge from roughly Floyd, VA southwest into
the Northern Mountains of North Carolina. These will dissipate
with the approach of sunset. Other cloud cover was some dirty
cirrus spilling into the region in association with a storm
system moving through the Midwest. The vast majority of this
cirrus is on the thin side, and its presence will continue
through the overnight hours.

The center of the high pressure will head east overnight. This will
have the impact of a trend of low level flow becoming more
southerly, and allowing for higher dew point values to creep back
into our area. Currently values in the upper 40s to lower 50s are
common. By tomorrow morning we expect readings in the lower 50s to
upper 50s. This will have the impact of preventing temperatures from
falling as much as they did last night. Lows tonight are forecast to
range from the full range for 50s across the mountains to the mid
50s to near 60 across the Piedmont. With the increase in moisture,
light winds, and a best thin cirrus, fog will be more abundant late
tonight into early Thursday morning as compared to this morning.

On Thursday, the trend towards increasing low level moisture
will continue with gradually increasing dew points. Today the
atmosphere was conducive for some cumulus development along the
southern half of the area near the crest of the Blue Ridge.
Tomorrow, with the increased moisture and instability, this same
region will be a target for actual isolated shower development,
and will be reflected as such in the forecast. High
temperatures are expected to be at least a category warmer than
those of today. Anticipate highs around 80 to the lower 80s
across the mountains with mid to upper 80s across the Piedmont.


As of 315 PM EDT Wednesday...

Southerly surface winds on the backside of a large high pressure
system well off the mid Atlantic coast expected to slowly replace
very dry airmass currently over the forecast area (dew points in the
40s) with dew points more typical of early summer (mid 60s to lower
70s) by the end of the short-term period.

The threat for at least widely scattered mainly diurnally-driven
shower/thunderstorm activity will also increase through the period
as a weak disturbance drifts northeast from out of the northern Gulf
Coast states toward and through the mid-Appalachian region by late
in the period. This system will likely significantly weaken and/or
shear out by the time it arrives on Friday or Saturday, but weak
forcing combining with increasing lower and mid tropospheric
moisture supports most mountain areas receiving at least some rain
Friday into Saturday, with a slightly lower threat across the
Piedmont which will be a bit further removed from the weakening

With relatively lackluster thermodynamic instability and only weak
dynamic forcing, the threat for severe weather Friday into Saturday
appears to be minimal at the present time.


As of 315 PM EDT Wednesday...

Although initial weak short wave trof will be moving away from the
forecast area on Saturday night, additional upstream weak
disturbances and associated weakening fronts are expected to move
east through nearly zonal upper flow during the long-term period.

First wave expected to move into/through the area on Sunday, which
may lay down a weakening cool front that will stall out over/near
the area by Sunday - with additional weak disturbances moving east
and through the area on almost a daily basis through mid week.

Exact timing of each wave is still somewhat problematic, but daily
proximity of waves combining with a moist airmass and an old frontal
boundary warrants a continuation of at least a widely scattered
mainly diurnally-driven shower/thunderstorm threat throughout the
long term period.

Overall forcing and thermodynamics will remain lackluster through
this period, so severe weather threat expected to remain extremely


As of 120 PM EDT Wednesday...

Expecting generally quiet weather this period at TAF sites as
high pressure moves off to our east and return flow on the back
side starts to bring increasing moisture up from the south as
seen in satellite images of Cu field this evening. Expect the Cu
to have a strong diurnal component and mostly dissipate with
loss of heating tonight, though some remnant patches may stew
around overnight. We should be able to radiate reasonably well
with lower clouds decreasing and just some patchy cirrus above,
and with dew points a bit higher than last night expect a
little more fog/stratus development toward daybreak. Believe it
will be mostly confined to the traditional valley locations and
expect some obvis at KLWB and KBCB. Any fog/stratus will
dissipate Thursday morning and expect VFR conditions through the
end of the period with some Cu redeveloping especially from the
southern Blue Ridge down through the mountains of NC where an
isolated shower is possible. Expect generally light winds
through the period.

Extended Aviation Discussion...

Friday, the flux of moisture into the region from the south
will continue. Anticipate even better chances of showers and
storms with localized, brief sub-VFR vsbys/cigs under the
heavier cells. Friday night, the potential for IFR/MVFR fog will
be greater, along with some areas with sub-VFR stratus

Saturday into Sunday, coverage of showers and storms increase
with the approach and arrival of a cold front. Sub-VFR
conditions will be more probable for a longer period of time,
especially during the overnight hours.

By Monday, the bulk of the precipitation will be south of the
region. However, a wet surface and boundary layer will more
easily prompt overnight fog and stratus.




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