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Area Forecast Discussion (AFD)

Issued by Blacksburg, VA (RNK)

FXUS61 KRNK 261701

Area Forecast Discussion
National Weather Service Blacksburg VA
101 PM EDT Sat May 26 2018

High pressure centered offshore will maintain a moist southerly
flow of air into the region for the next several days. 
Subtropical Storm Alberto moving into the Gulf of Mexico this 
weekend will move ashore and into the lower Mississippi Valley 
early next week. Tropical moisture will stream north into the 
region making for very rain efficient showers and thunderstorms.

As of 950 AM EDT Saturday...

Seeing slow moving convection across the far west within an axis
of weak convergence aligned along the western mountains between
upstream troffiness and return of deeper tropical moisture per
morning soundings. Latest analysis also showing a weak mid level
wave riding up from the southwest to the north of the residual
front and surface wave to the south. This supports some slow
expansion of convection to the north and east with coverage
perhaps slowed somewhat by lack of instability due to cloud
cover. However latest short term solutions show quite a bit of
shra/tsra expanding east under increasing southwest flow so
keeping likely/cat pops through the afternoon. Weak steering
still of concern with potential flash flood issues but given
more localized nature and uncertainty including how far 
north/east heavier rainfall might materialize wont go with with
a watch unless things become more widespread. Lowered highs a
little in the west/south per more clouds/showers while keeping
mainly 80s in the east. 

Previous discussion as of 430 AM EDT Saturday...

Unsettled weather is expected through the Holiday Weekend as 
southerly wind flow brings increasing moisture to the 
Appalachians. Surface dewpoints in the 60s to near 70 along 
with PWAT values in excess of 1.50 inches will favor August-
like conditions with high humidity and rain efficient 
showers/storms. The key word is showers, as it will not rain the
entire time, but when it does rain, it will pour. 

Shower coverage today will be the greatest during the peak
heating of the day with convergence along the mountains driving
much of the lift to create the showers and afternoon
thunderstorms. There is a subtle shortwave embedded within the 
southerly flow that may provide enough lift to help focus some 
of the deep convection into clusters, so will have to monitor 
this closely as this may help provide a focus for an area of 
localized heavy rain. The high resolution models suggest threat 
for the heaviest rain today will be just north of our CWA where 
the westerlies across the Ohio Valley and northern Mid-Atlantic 
converge with the southerly winds coming up from the southern 
Mid-Atlantic. For this reason WPC has highlighted areas north 
of I-64 within a Slight Risk for excessive rainfall in their 
Day-1 Outlook. 

There is a weak upper level trough moving east across the Great
Lakes and Ohio Valley. This feature will maintain enough lift 
for showers to persist across the Virginias tonight, and 
especially north of I-64 where wind convergence will continue 
between the two flow regimes.

Temperatures will trend 10-15 degrees above normal for the next
several nights per the significant uptick in dewpoints. 
However, daytime highs will not be as extreme per the abundant 
cloud cover and shower coverage...yielding Tmax values of no 
more than about 5 degrees above the norm.


As of 400 AM EDT Saturday...

The pattern will remain blocky and sluggish into the first part of next 
week with a shallow trof sliding by to our north and Alberto moving 
slowly along the eastern Gulf coast to the Mississippi delta by Monday. 
This will keep the Appalachians and central mid Atlantic region in a 
warm and very moist airmass for the next several days. 

Sunday looks to feature ample instability in a relatively weak wind 
field as the upper trof tries to push a washed out frontal boundary 
into the region from the northwest. This will add some synoptic scale 
lift to air convection and should yield good coverage of showers/storms 
especially across the northwestern two thirds of the area. Any severe 
threat will be small with more of a focus on potential hydro issues 
from training storms in an environment of precipitable water values 
in the upper percentiles.

Sunday night into Monday will see some subsidence around the periphery 
of Alberto push in from the south along with slightly lower theta-e 
air. While there will still be showers/storms around, believe these 
limiting factors will suppress activity a bit and keep the preferred 
area for convection across the southern half of the area. Steering flow 
becomes very weak with little to move precipitation along and will have 
to watch for areas of prolonged persistent rainfall. A better push of 
moisture arrives on Tuesday as Alberto will be moving up into the lower 
Mississippi valley and throwing waves of energy in our direction, but 
the best forcing with this system remains well off to our


As of 400 AM EDT Saturday

By mid to late week, remnants of the tropical system could bring 
tropical moisture and heavy rains to our region. Flooding potential 
will depend on the track of the system and how much rainfall is 
accumulated from the showers and thunderstorms earlier in the week.

Expect Alberto to make its way into the Tennessee valley by Wednesday. 
The timing of the ECMWF was quicker and further north and west (Ohio 
Valley) compared to the GFS which was slower and more compacted (along 
the Appalachians. By Thursday and Friday, the low will open up and 
weaken as it moves northeast. There is a continued threat for showers 
and thunderstorms into the end of the week. Urban and small stream 
flooding risk will be highest early in the week with river flooding 
possibly by the end of the week depending on actual rainfall totals.


As of 100 PM EDT Saturday...

Widespread showers and storms will continue across the mountains
and progress east across the rest of the region through the
afternoon before tapering off this evening. This will lead to
widespread sub-VFR cigs/vsbys over the west and deteriorating 
conditions over the east this afternoon. Localized LIFR along
with mountain obscuration could also occur within the heavier
bands espcly near the Blue Ridge. 

Expect showers to fade with loss of heating and passing of a mid
level wave this evening. This likely to lead to sub-VFR in fog
and low clouds given the widespread rainfall from earlier.
Therefore including sub-VFR most spots excluding KROA with IFR
or worse possible by daybreak on Sunday. 

.Extended Aviation Discussion...

Wet pattern anticipated through the upcoming week with periods 
of at least diurnally driven sub-VFR conditions associated with 
deep convection. Moist low level southerly winds will also favor
lower layers of cloudiness at night, especially along the spine
of the Appalachians with potential for both MVFR Cigs and early
morning MVFR visibilities from mist/haze. The tropical system
over the Gulf of Mexico may begin to impact the region mid-week.