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FXUS61 KRNK 250825 CCA

Area Forecast Discussion
National Weather Service Blacksburg VA
358 AM EDT Sun Jun 25 2017

Weak high pressure will linger over the region through Monday.
An upper trough and associated cold front will sweep through the
area Tuesday with a few showers followed by a cooler, drier air
mass. Toward the end of the week, high pressure will move east
of the region into the western Atlantic, allowing much warmer
and more humid air to return to the region along with increasing
chances for showers and thunderstorms.


As of 330 AM EDT Sunday...

A diffuse frontal boundary is evident across the extreme
southern parts of the CWA or toward the I-40 corridor in North
Carolina. There is quite a range in dewpoints this hour from the
mid 50s across the Alleghanys and the northwest part of the CWA
to near 70 yet across the southeast part of the CWA, say
Caswell/Halifax areas toward Yadkin. A broad upper trough will
continue to deepen across the Great Lakes today pushing the
baroclinic boundary further south, such that by evening, it
should be located from east central NC into central SC, then 
west into central GA. Any showers or thunderstorms today will be
near and south of the front, leaving our entire CWA rain free 
today with lower humidity levels and slightly cooler 
temperatures, since not only do the dewpoints drop into the 50s
north to 60s south by evening, but the 850mb temperatures show 
a marked drop from +18C of the past few days to around +8C by 
Monday morning. Some cirrus can be expected as a result of the 
prevailing southwest flow aloft as we remain on the east side of
the broad upper trough anchored over the Great Lakes, but SC/CU
development becomes less of a concern with time, even in the 
upslope areas of eastern WV.

As noted above, temperatures will be on a cooling trend thanks
to cold advection at 850mb and the deepening upper trough. High
temperatures today will range from the 70s in the northwest to 
lower 80s in the southeast, on average about 5 degrees cooler
than what we saw Saturday, with elevations above 3000 ft. likely
only seeing high temperatures in the 60s today. Lows tonight
will be very pleasant for the end of June with widespread lows
in the 50s. A few 40s for lows in places like Lewisburg, Burkes
Garden, and similar across the mountain valleys in the west
would not be unprecedented at all. However, the coolest air of
the week will come after a secondary front moves through the
area Tuesday.


As of 330 AM EDT Sunday...

An upper level low over the Great Lakes will bring an increase in 
clouds Monday afternoon and a chance for rain Tuesday. This low will 
open into a trough Tuesday night as it continues to track east and 
off the New England coast. Cool high pressure will settle over the 
region on Wednesday.

Models vary greatly for Tuesday's rain chances in timing, confidence 
and rainfall amounts. The most agressive is the ECM which has rain 
entering the mountains before sunrise Tuesday. The GFS has rain in 
the mountains during the morning while the NAM holds everything off 
until late in the afternoon. The ECM is very confident on rain 
chances Tuesday with PoPs 80-90%. The GFS confidence is 30-50% for 
the mountains and zero east of the Blue Ridge. The NAM has likely 
PoPs (60%) for only the mountain in the afternoon. Model rainfall 
amounts range from half an inch (ECM) to a couple of hundredths 

We favored the GFS solution for a few reasons. First, as the upper 
level trough axis pivots over the Ohio Valley and Mid Atlantic 
region, the associated short waves track across central VA and not 
down into NC. This pattern and northwest downsloping flow will limit 
rain chances east of the Blue Ridge. Second, while the upper level 
trough is in play, our flow will be from the northwest. The moisture 
source will be limited coming from either the Great Lakes or 
residual soil moisture from TS Cindy's track across WV/OH. There is 
no return flow from the Gulf or the Atlantic for rainfall amounts to 
match that of the ECM.

While under the influence of an upper level trough and cool 
northwest flow, temperatures Monday and Tuesday will run 5F-10F 
cooler than normal. Wednesday's temperatures will also be slight 
cooler than normal but will begin a warming trend for the end of the 


As of 230 PM EDT Saturday...

Trough lifts out midweek with height building across the southern 
U.S. Surface high will slide east of the region Wednesday night. 
Digging trough over the upper midwest to start to build a little 
more of heat ridge in the southeast with 591 heights. Will stay 
mainly dry but heating up with threat of typical summertime 
convection as early as Thursday over the WV mountains, with better 
threat Friday-Saturday. Southwest flow aloft increases by Saturday 
with a frontal boundary slowly moving toward the Ohio Valley. Should 
see an uptick in coverage in convection over the weekend across
the mountains.

Thursday will see moisture increase with highs bumping up another 5 
degrees with upper 70s/lower 80s west, mid to upper 80s east. These 
warmer temperatures which are close to normal for the end of June 
will stick around into July 1st, but the humidity will be more 
noticable as well.


As of 155 AM EDT Sunday...

Moving into an extended period of mostly VFR conditions across
the region over the next few days. The weak front has pushed
southeast of the area down toward the I-40 corridor taking the
showers and thunderstorms with it. Flow aloft is still broad
west-northwest under the influence of a broad upper trough
deepening across the Great Lakes. This will continue until
Tuesday when a short wave rounds the base of the upper trough
and following its passage, the flow begins to relax, become more
zonal and eventually southwest as the trough weakens and lifts

The only potential through the period for any non-VFR conditions
would be during the late night/early morning hours when any
spots that decouple and radiate effectively will see fog
development. Given the moist ground from recent rainfall and the
cooler/drier air mass moving into the region for the first half
of the week, this seems like a reasonable possibility at sites
such as KLWB AND KBCB. Not overly confident about such tonight
because air mass has not fully transitioned yet and there are
some high clouds skirting the area. Have advertised MVFR-IFR
conditions briefly in fog at these two sites in the 09Z-13Z time
frame, otherwise will monitor. Potential would seem less Monday
morning due to drier air.

Winds will be WNW-NW at speeds of 3-6kts before 13Z, then
increasing to 6-11kts with low end gusts at most locations
during the 14Z-22Z time frame, diminishing or becoming calm
after 00Z.

Medium to high confidence in ceilings through the TAF valid
Medium to high confidence in visibilities through the TAF valid
Medium to high confidence in wind speed/direction through the
TAF valid period.

Extended Aviation Discussion...

A weak disturbance dropping in from the northwest may bring a
period of MVFR cigs along with a few showers to the mountains 
late Monday night into Tuesday. Otherwise expecting overall VFR
for Monday through Thursday with some brief late night/early 
morning mountain and river valley fog possible.

On Friday, the potential will increase for some brief, localized
sub-VFR conditions as showers and thunderstorm chances 





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