National Weather Service National Headquarters

• Home
• Site Map
• News
• Organization
• Search for: NWS All NOAA 

-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KRNK 140732
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
332 AM EDT Sat Apr 14 2018

.SYNOPSIS...
-- Changed Discussion --
High pressure offshore will continue to extend across the region
today into tonight. A cold front will push east into the area by
later Sunday possibly bringing strong storms with heavy
rainfall to the region Sunday into Sunday night. Cooler than
normal temperatures under gusty northwest winds are expected
much of next week after cold frontal passage.
-- End Changed Discussion --
&&

.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --
As of 300 AM EDT Saturday...

Upper ridging including surface high pressure offshore will remain
dominant today with continued warm southwest flow in place. Some
low level moisture should begin to advect into southwest
sections early this morning before mixing out with mainly
high/mid clouds expected to prevail for much of the day. Cant
totally rule out an isolated shower popping up across the far
west but too iffy to include much pop so basically keeping
things dry/warm through the afternoon. Given such a warm start
with many locations still in the 60s, and expected sunshine
within the warm advection regime, appears will see highs on the
warm end of guidance so bumped up temps a few degrees today.
This should allow values to reach around 80 east and 70s west
outside of the higher elevations.

Front aloft will approach from the west overnight allowing a better
veering profile to take shape with inclusion of a bit more low level
southeast trajectory late ahead of this feature. Models again show
moistening but mostly in the low levels where guidance has been
overdone with moisture depth lately. Think best chances of showers will
come after midnight but mostly southern Blue Ridge westward similar to
a 00z GFS/ECMWF blend. Thus reduced pops some overnight as well with
only isolated pops east and mid range chances along/west of the I-77
corridor. Otherwise should see increasing clouds, but still quite mild
per mixing/clouds with lows mostly upper 50s to mid 60s.
-- End Changed Discussion --
&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
As of 150 PM EDT Friday...

Active weather expected for the latter portion of the weekend.

A closed low will move from the mid Mississippi valley into the
Great Lakes region through the first part of next week. This
will drive a strong cold front through the Appalachians and
central mid Atlantic region late Sunday and Sunday night. This
is a very dynamic system and will bring a threat for both severe
weather and flooding.

Southerly flow starts to amplify ahead of the approaching front
and strong gusty winds will begin to affect the region from the
mountains of NC into the higher elevations of southeast WV, and
especially the Mountain Empire of VA Saturday night into
Sunday. Precipitation will also be on the increase mainly in
upslope flow along the Blue Ridge ahead of the front Saturday
night into early Sunday, along with some rogue showers out
across the piedmont.

Guidance continues to advertise excellent deep layer moisture
transport with GFS synthetic water vapor imagery showing a plume
extending from deep in the Gulf and Caribbean and NAEFS
precipitable water anomalies remaining close to 3 standard
deviations above normal. Widespread rainfall around 1.5 inches
seems reasonable with higher amounts of 2 to 3 inches possible
along the Blue Ridge due to orographic effects even though the
more southerly fetch will be less than perpendicular to the
Ridge. Additionally, Hysplit backward trajectories continue to
indicate that any convection along the front to our south will
not be very efficient at inhibiting moisture transport, and the
slower propagation of N-S oriented convection along/ahead of the
front does not fit the conceptual research model favoring
moisture cutoff so still have confidence in the beefy QPF
numbers. Despite dry antecedent conditions, the expected QPF and
potential for enhanced rainfall rates due to embedded
convective elements and orographic effects may be enough to
produce some localized flooding, and ensemble forecasts are now
showing a couple of river forecast points rising into minor
flood stage within the envelope of uncertainty. Will continue to
highlight the hydro threat in the Hazardous Weather Outlook and
wait to see if any refinement in the timing/location of
expected heavy rainfall can be made before issuing flash/flood
watches. Further details can be found in the hydrology section.

The severe threat will be arriving in a high shear/low CAPE
environment along and just ahead of the front. Expect activity
to enter the region from the west Sunday evening, then be moving
of of the region further east into the piedmont later Sunday
night. Hodographs continue to look long and loopy with generous
low level helicity values, and CAPE values have increased a bit.
Expect the mode of convection will favor a wavy QLCS moving
across the region ahead of the front with the potential for
damaging wind and possibly brief spin-ups. While the best
instability will generally reside east of the Blue Ridge, expect
strong kinematic environment will allow for severe threat
further west/northwest which will encompass locations west of
the Ridge the as well. Unsure if any discrete cells will be able
to form ahead of the main activity where there would be access
to more instability. Will be watching this situation closely for
any changes in timing or location of features that would impact
the region.

Behind the front cold air will pour in on strengthening
northwest winds and expect a ptype transition to a mix of
rain/snow for the higher elevations west of the Blue Ridge.
Accumulations are expected to be an inch or less for the
hilltops.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
As of 150 PM EDT Friday...

The upper pattern looks progressive through the end of the
week. This should yield generally fair, albeit quite windy
weather through Friday.

Lingering upslope rain/snow showers will end Tuesday morning as
high pressure briefly builds over the southeastern US. A fast
moving low will move into the Great Lakes region on Wednesday,
pushing a cold front into the mountains Wednesday with a chance
for showers Wednesday afternoon into Wednesday night. Upslope
rain/snow showers will linger across the high elevations west of
the Blue Ridge through Thursday before high pressure returns to
the eastern US on Friday.

Temperatures will generally be a bit below normal, though a
rebound to above normal is expected Wednesday ahead of the cold
front. The very gusty conditions will make it feel notably
cooler.

&&

.AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...
As of 110 AM EDT Saturday...

Overall VFR conditions will continue overnight into Saturday
under mainly high clouds. Some low stratocu may move north to
the NC-VA border in the early morning hours but this remains
iffy and mainly supported off the latest Nam. Clouds look to be
scattered out enough to continue forecasting for VFR, however
some uncertainty does exist and occasional conditions for MVFR
BKN cigs could be possible espcly east early this morning.
Daytime heating should lift any cloud cover to above 3kft
tomorrow. These clouds may also delay mixing, which will
postpone gusty winds into the afternoon. Does appear that winds
could again gust to 20-25 kts Saturday afternoon espcly over the
mountains.

Extended Aviation Discussion...

Saturday night into Sunday, sub-VFR conditions will become more
common with the approach and passage of a potent cold front.
Isolated to scattered thunderstorms will be possible on Sunday.

Late Sunday night, winds become gusty from the northwest in the
wake of the front. Flight conditions trend to VFR east of the
Blue Ridge with lingering sub-VFR across the mountains into
Monday.

Gusty to very gusty northwest winds will continue into Monday
night through Tuesday.

Expect overall VFR on Wednesday with a gusty southwest wind
likely ahead of yet another approaching cold front.

&&

.FIRE WEATHER...
-- Changed Discussion --
As of 330 AM EDT Saturday...

Still no wetting until Sunday, which may come in the form of downpours
of heavy rain associated with showers/thunderstorms from a strong cold
front. Dewpoints will be significantly higher from this afternoon into
Sunday giving the air a more summery feel with higher humidity. However
could still see areas across the northwest where humidity levels remain
below 35 percent and coincide with gusty winds to produce a low end
enhanced fire weather threat. Thus will headline within the Fire
Weather Forecast but run without an Enhanced fire danger statement for
now. Winds will also remain gusty on Sunday, with some strong gusts
with the frontal passage.

Ventilation and dispersion will generally be good to excellent limiting
smoke management issues.
-- End Changed Discussion --

&&

.HYDROLOGY...
As of 150 PM EDT Friday...

Hydrologic ensembles from GEFS and North American (CMC-GFS)
models continue to advertise rapid rises on nearly all the major
rivers in the CWA in response to the Sunday rain event. Most of
the rises are within-bank to near Action stage with possible
Minor flood stage on a few of the more susceptible locations,
mainly the Dan River and lower Roanoke. As usual there is
considerable run-to-run variation in the QPF driving these
forecasts but most are clustering around 2 inches storm total
with the usual terrain enhancement effects which hits the upper
Dan River basin (VA Blue Ridge) most effectively. SREF ensembles
are not yet capturing the event with QPF just beginning to
appear in the model runs. The RFC deterministic forecasts
available on the AHPS web pages are showing no rises in the
Yadkin, Roanoke or Dan basins as the QPF falls outside the 48-
hour window for SERFC. OHRFC forecasts (which are using QPF from
the entire event) for the New and Greenbrier are showing rises
to near flood stage on the New at Radford and just over flood
stage on the Greenbrier at Alderson. The James River with only
48-hours of QPF shows a sharp rise within-banks right at the end
of the forecast period. Later forecasts should clarify the
effects of the forecast rainfall.

Then there is the National Water Model (NWM) which uses the
single run QPF solutions from the GFS, the latest available from
06z run which focuses most of the heavier rainfall east of the
Blue Ridge. These simulations suggest the potential for flood
stage to be exceeded along the Dan River and lower Roanoke
(below Smith Mountain Lake) with rises to below bankfull along
the James, New and upper Roanoke rivers. The response of some
rivers in the NWM appears to be highly sensitive to QPF
variations as the 00z GFS input drove stages along the Dan to
levels which are very unlikely to be realized. It remains to be
seen how well the model performs with the actual rainfall which
continues to be problematic with potential CSTAR effects and
local convection likely playing a role.

In addition, the flash flood threat cannot be discounted as
high rainfall rates are likely with convective elements but with
a fairly progressive system it will take some persistent
training to cause flash flooding. WPC is currently showing a
slight risk for excessive rainfall (exceeding flash flood
guidance) on Sunday across much of the CWA. The 3-hour flash
flood guidance currently ranges from about 1.75 inches in the
wetter mountain areas up to 3 inches or more in parts of the
drier VA piedmont where moderate drought conditions continue to
exist. Given these fairly high numbers, some advisory level
flooding such as ponding of water and minor runoff issues seems
likely with this event, but not flash flooding.

&&

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

&&

$$
SYNOPSIS...JH
NEAR TERM...JH
SHORT TERM...MBS
LONG TERM...MBS
AVIATION...JH/RCS
FIRE WEATHER...JH/PM/WP
HYDROLOGY...PC