Severe Thunderstorm Warning 108, NWS Blacksburg, VA

Area Forecast Discussion
National Weather Service Blacksburg VA
131 PM EDT Sun Jul 22 2018

.SYNOPSIS...
Deep low pressure will continue to move south drawing abundant
moisture into the region. Cloudy conditions with scattered to
numerous showers and thunderstorms will persist for the next
several days. Temperatures will be near or just below normal
most of the week.

&&

.NEAR TERM /THROUGH MONDAY/...
As of 130 PM EDT Sunday...

A closed upper level low will continue dropping south during the
forecast period. This should bring a good deal of moisture into the
region providing mostly overcast skies and scattered to numerous
showers and thunderstorms. Any severe threat will be minimized by
the lack of heating, however flash flooding could be an issue as any
thunderstorm that does develop will bring the possibility of a
copius amount of rain in a very short period of time. Southeast
flow around the surface low will enhance the rain amounts along
and near the Blue Ridge from upsloping. Furthermore, despite
the lack of typical summertime heating, expect coverage will
still be greatest during the afternoon to evening hours.

Coverage of storms will wane some going into the overnight hours but
will increase again during the day Monday. Forecast PWATs during the
day will rise to near two inches and WPC has outlooked the
majority of the forecast area in a slight risk for excessive
rainfall. Will need to continue to evaluate this evolving situation
to better ascertain if and where a watch may be necessary, but
will continue to highlight within the HWO.

&&

.SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY NIGHT/...
As of 130 PM EDT Sunday...

Guidance is consistent in advertising a wet scenario for the
Appalachians and central mid Atlantic region through the middle of next
week. Stacked closed low will sink from the Ohio valley to the Gulf
coast before gradually opening up and assimilating into the mean flow
Wednesday night. Backward HYSPLIT trajectories indicate this will
establish persistent southeasterly flow coming off the warm waters of
the Atlantic and allow for deep moisture transport into the region.
This will be occurring as high pressure over the Atlantic keeps a good
wedge signature in place east of the mountains. This will result in
stout isentropic lift generating widespread rainfall as moist
southeasterly flow rides up over the wedge.

Simulates water vapor imagery coupled with low level jet analysis
indicates the best moisture transport will occur generally east of
Interstate 77 where precipitable water values will likely exceed 2
inches and support hefty rainfall totals through Wednesday night with
some orographic enhancement as well. While the deep moisture is
generally not conducive to thunder, believe there will be some
occasional given deep skinny CAPE evident on model soundings and
convective elements will further enhance rainfall.

The southern Blue Ridge into the mountains of North Carolina will be a
bit removed from best deep moisture transport, but the saturated
boundary layer with southeasterly flow interacting with the terrain
will likely yield a prolonged period of persistent lower rain rate
precipitation that will add up to decent amounts over a 48 hour period.

What this all adds up to is a wet forecast with widespread rain that
will be heavy at times through Wednesday night, with some thunder
during the afternoon/evening. Widespread rainfall totals of generally
2 to 5 inches are expected with locally higher amounts possible, and
WPC slight risk for excessive rainfall for most of the area looks on
target.  Given the southeasterly flow and placement of synoptic
features it is expected that there will be lower amounts of 1 to 2
inches through the period well west of the Blue Ridge across portions
of the Mountain Empire and southeast West Virginia as some downsloping
effects lessen precipitation a bit.

For now, will adopt a more short fuse philosophy and hold off on
issuance of any flash/flood watches until more specific threat locations
declare themselves in time.

Given expected rain and abundant cloud cover expect a smaller diurnal
spread in temperatures. Highs through Wednesday will be om the cool
side of normal while mins will be at to slightly above normal.

&&

.LONG TERM /THURSDAY THROUGH SATURDAY/...
As of 130 PM EDT Sunday...

As the dissipating upper low along the Gulf coast rejoins the flow,
another large closed low will reinforce the eastern trof as it moves
across southern Canada. This will gradually shift our winds around to
the southwest as a weak cold front approaches from the west. This will
bring a good chance for showers and Thunderstorms as the front moves
through on Thursday, but it looks like a bit of a break in the
precipitation for Friday. However the weak front will get bent back
into the region on Saturday and bring a return to showers and
thunderstorms for the weekend.

Southwesterly flow will bring a return to warm temperatures with
readings rising to near or above normal by the end of the
workweek.

&&

.AVIATION /17Z SUNDAY THROUGH THURSDAY/...
As of 130 PM EDT Sunday...

Mostly sub-VFR, but highly variable ceilings for the TAF period as
upper level low continues to drift southward. Scattered to numerous
showers and thunderstorms can be expected throughout with greatest
coverage during the afternoon and evening hours. Patchy fog and low
stratus will be possible overnight, particularly in areas that
receive rain during the day. Storm coverage will be reduced during
the night time hours and will increase again during the day
tomorrow.

Extended Aviation:

Wet weather will continue to be in place for most of the week as
closed upper low will weaken slightly, but deep troughing will
remain over the eastern CONUS. Scattered to numerous showers and
thunderstorms, mainly during the afternoon hours will be in the
forecast for the next several days. Expect rather extensive cloud
cover with ceilings varying through all categories will also be in
store through the period.

&&

.HYDROLOGY...
The wet pattern described above prompts a brief discussion of
hydrologic concerns, both flash and riverine. Current guidance
suggests that no flash flood watch appropriate due to the
inherent uncertainty as to location and relatively low overall
probabilities for flash flooding. A short-fused flash flood
watch issued 3 to 6 hours in advance is a more likely scenario
than any extended 24 to 36 hour in advance watch. Looking at the
river forecasts, both deterministic and MMEFS ensembles does
not suggest much cause for concern at this time. Nearly all the
ensemble members showing below flood stage crests. River
flooding under a mostly convective regime in July is simply not
very common. An isolated river flooding threat could develop
where rainfall maximizes over a smaller river basin such as the
upper Roanoke or Clinch River, but larger basins such as the
New, James and Dan should avoid any significant flooding
issues unless coverage is more widespread than anticipated.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...None.
NC...None.
WV...None.

&&

SYNOPSIS...JR
NEAR TERM...JR
SHORT TERM...MBS
LONG TERM...MBS
AVIATION...JR/PC
HYDROLOGY...PC

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