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Area Forecast Discussion (AFD)

Issued by Blacksburg, VA (RNK)

FXUS61 KRNK 151922

Area Forecast Discussion
National Weather Service Blacksburg VA
322 PM EDT Wed Aug 15 2018

High pressure will bring fair weather to the Appalachians and
central mid Atlantic region tonight. This high will then slide
off to our east tomorrow and bring a chance for showers and
thunderstorms back to the forecast. Low pressure over the Great
Lakes will track east Friday and Saturday pushing a cold front 
into the mid Atlantic region, with a good chance for showers 
and thunderstorms through the weekend and into the first part of
next week.


As of 245 PM EDT Wednesday...

Radar is essentially devoid of returns early this afternoon but
visible satellite imagery shows some good Cu towers as an
approaching short wave works on some surface instability with
marginal lapse rates aloft. Thus, expect a slight chance for 
widely scattered/isolated showers/thunder along and west of the
Blue Ridge, primarily the southern Blue Ridge into the 
mountains of NC before any convective elements dissipate this 
early evening.

Expect mid/high clouds upstream to move over the region tonight,
but there are indications the clouds will be breaking up a bit
as they go. With this in mind, will go a bit below guidance as
expect there will be a few windows for radiational cooling to
occur. This should also result in another round of valley fog
west of the Blue Ridge tonight. Lows tonight look to be in the
mid/upper 60s east of the Blue Ridge with upper 50s/around 60 to
the west along with some cooler readings in the valleys. If 
clouds come in thicker than anticipated, will have to revisit 
the temperature and fog forecast.

Surface high pressure will be drifting off to the east 
tomorrow, allowing for southwesterly return flow to develop. The
best isentropic lift will follow the strongest 850mb winds up 
into the Ohio valley to our west, but there looks to be enough 
forcing for some scattered/widely scattered showers/thunder 
mainly along and west of the Ridge again tomorrow afternoon.
Highs tomorrow will be a bit warmer with lower 90s east of the
Ridge, mid/upper 80s to the west.


As of 300 PM EDT Wednesday...

Fairly typical summertime pattern with weak flow aloft at the 
southern and central latitudes of the CONUS and stronger activity 
along and north of the Canadian border. Global models in general 
agreement however in trying to reestablish troughing in the eastern 
half of the CONUS as the subtropical ridge expands somewhat across 
the desert southwest. This will allow for increasing southwest flow 
aloft, increasing humidity and higher chances for mainly 
afternoon/evening convection both Friday and Saturday, The best 
chances for showers and thunderstorms will be in the west closer to 
the best upper support and deeper moisture but mesoscale 
interactions likely to support scattered storms elsewhere as well. 
Daytime high temps will be moderated by considerable cloudiness 
while reverse process in effect during the overnight yielding 
generally above normal lows. Humidity will again become more 
noticeable with dewpoints ranging from the mid-60s northwest to low 
70s southeast.


As of 300 PM EDT Wednesday...

Short-wave ridging or least zonal flow builds in for later Sunday 
into Monday as the first trough lifts out. Precipitation chances 
will decline somewhat Sunday and Monday but cannot remove pops 
entirely under a still moist and conditionally unstable air mass. 
Yet another trough is forecast to develop over the central U.S. 
early next week. This will be fairly amplified for what is still mid-
summer, especially as advertised by the 12z ECMWF and CMC which spin 
up a fairly strong surface low (sub-1000 mb) for the season over the 
mid- to upper Mississippi Valley late Monday into Tuesday. This 
feature lifts north into the Great Lakes by Wednesday with a 
potentially broad shield of precipitation along and ahead of a 
frontal boundary over the central states and moving into parts of 
the Ohio valley and the western portions of our CWA Tue/Wed. GFS is 
considerably less aggressive with this feature but general pattern 
is similar. Either way precipitation chances will be on the increase 
early to mid-week. Little significant change in temperatures 
expected with no significant heat but plenty of humidity and fairly 
warm overnight temperatures owing to high moisture and clouds.


As of 245 PM EDT Wednesday...

Medium confidence in sub-VFR tonight. High confidence other

Radar is quite sparse at the moment and believe no TAF sites 
will see convective precipitation before any showers/thunder 
dissipate this evening so will maintain a dry forecast. Expect 
some mid/high clouds to move over tonight but there are 
indications these clouds will break up a bit as they go, 
allowing for some radiational cooling and fog/stratus 
development overnight, mainly in the valleys west of the Blue 
Ridge. Will go with IFR/LIFR vsby/cigs at KLWB and KBCB with a 
tempo MVFR at KLYH and KDAN toward daybreak. Any fog/stratus 
will burn off Thursday morning with VFR expected all sites 
toward the end of the period. Scattered showers/thunder will be 
developing by early Thursday afternoon but best chances appear 
to be later so will stay with a dry forecast through the latter 
portion of this valid period.

Winds will generally be light.

Extended Discussion...

Scattered afternoon and evening showers and storms will be 
possible at airports west of the Blue Ridge Thursday night.
The next cold front and chance for more widespread showers and 
thunderstorms with the associated MVFR conditions arrives on 
Friday and will keep chances of showers and thunderstorms 
through Monday.