Area Forecast Discussion
Issued by NWS Blacksburg, VA-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS61 KRNK 201800
Area Forecast Discussion
National Weather Service Blacksburg VA
200 PM EDT FRI MAY 20 2016
Low pressure over the lower Mississippi Valley will deepen and
move northeast into the Tennessee Valley today and tonight. At the
same time, another low develops off of South Carolina and tracks
northeast along the coast. By Sunday, these systems transition to
a strong low well off of the New Jersey coast.
.NEAR TERM /THROUGH TONIGHT/...
As of 1250 pm EDT Friday...
Light rainfall continues to make steady, but slow, progress toward
and into our region. Most of the activity was still scattered near
the NC/SC border with limited progression into our immediate area.
The earlier update to slow the progress has not been slow enough.
The update this afternoon will reflect approximately a four hour
slow down compared to the previous. This update not only recognizes
the trends of the area radars, but is in alignment with the new
12Z/8am runs of both the GFS and NAM, as well as the 14Z/10am run
of the HRRR.
With less than expected precipitation across the region,
temperatures naturally have been climbing more quickly. The update
will reflect about a three to five temperature increase in the
high temperatures for most locations compared the prior forecast.
As of 950 am EDT Friday...
Looking at the latest radar trend and the latest updates of the
HRRR and RNK WRF ARW guidance, have made some adjustments to the
timing of the arrival of precipitation into the area today. Have
delayed the push of the likely probabilities into the southwest
portion of the region to the middle of the afternoon. Also, have
maintained no precipitation, or at best, a slight chance of light
rain in the northeast portion of the area. Arrival time in this
region being the later portion of the afternoon.
As of 430 AM EDT Friday...
Low pressure will develop over the Tennessee Valley today. This
will enhance a wedge of high pressure over the forecast area as a
second low forms off the Carolina coast. Models were in good
agreement spread clouds northeast across the region followed by
rain. Have slowed the onset of the rain in the northern and
eastern county warning area until late afternoon. Best forcing and
isentropic list as well as strong upslope will be from 00Z/8PM
through 06Z/2AM. 850MB southeast low level jet increases to
around 40KTS which will enhance the upslope and precipitation
amounts on the eastern slopes of the Blue Ridge. Have raised wind
speeds and gusts in the western downslope area overnight. Expect
the smallest rise in temperatures in the southwest county warning
area, especially once the rain begins. Took a blend of guidance
for lows tonight.
.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
As of 330 am Friday...
A Miller B type system expected with parent low weakening over the
Ohio Valley and the energy is transferred to the coastal low. The
upper level trough will move out of the Great Lakes and push the
moderate to heavy rain east of the area Saturday. The challenge in
the forecast is how long does it rain hard before the secondary low
becomes the primary low Saturday afternoon, pulling bulk of the
precipitation out to sea. Several shortwaves will rotate around the
upper trough generating scattered convection afternoon into
Saturday evening. High temperatures Saturday will range from the
upper 50s in the northern mountains to the lower 70s in the
Piedmont. We may see warmer than expected temperatures Saturday
depending on when the bulk of the rain exits.
The Coastal surface low lifts northeast along the coast Saturday
night. The upper level trough will pivot over the region Saturday
night, keeping the chance for rain showers especially over the
mountains. A few showers may overcome northwest flow and push east
of the Blue Ridge overnight. Low temperatures Saturday night will
drop into the mid 40s in the mountains to the mid 50s in the
Cold pool remains over the region Sunday and with afternoon heating,
scattered showers and isolated thunderstorms are possible into the
evening. Sunday afternoon high temperatures will remain below normal
with readings from lower 60s in the west to the lower 70s in the
The upper trough becomes a closed low Sunday night off the Mid
Atlantic coast. As the low moves away, the chance of rain will
decrease with our area in a subsidence region. The locations of this
feature is similar on both the ECMWF and GFS. Low temperatures on
Sunday night into Monday morning will generally be from the mid 40s
in the west to the lower 50s in the east.
.LONG TERM /MONDAY THROUGH THURSDAY/...
As of 330 AM EDT Friday...
An upper level low in the vicinity of the Mid Atlantic will keep a
chance for showers and a few afternoon thunderstorms in the area
Monday. If this low wobbles offshore, the area will be in the
subsidence zone and with high pressure building east, the chance for
rain will decrease for a few days. Once this low departs, upper
level ridging will increase, which means warmer temperatures are on
the way. However, with warmer temperatures, the threat for
thunderstorms will increase which looks to begin Wednesday. The
threat of convection may hang around through the end of the week.
Temperatures will moderate to near normal on Monday, then above
normal for the remainder of the workweek.
.AVIATION /18Z FRIDAY THROUGH THURSDAY/...
-- Changed Discussion --As of 135 pm EDT Friday...
Through Saturday afternoon an upper trough of low pressure will
approach the area. A generous stream of moisture will be advected
north through the area in advance of this system overtop of an
existing lee side wedge of high pressure. The associated surface
low pressure will enter the western parts of the area by mid-day
VFR conditions exist still generally along and north of a KBLF-
KROA-KLYH line. South of this line, MVFR clouds and a few pockets
of light rain were progressing into the region. As the remainder
of the afternoon progresses, look for the MVFR ceilings to
continue spreading north with pockets of light rain. Visibilities
for the most part will remain VFR.
This evening and overnight, look for a broad area of rain to
overspread the region. Visibilities will trend to MVFR with
ceilings trending to IFR at most locations. Conditions will start
to improve late tonight as the heaviest rain moves northeast, out
of the region.
Saturday morning, most visibilities will be in the VFR range by
14z/10am. Ceilings for the most part will improve only to MVFR
through the end of the TAF forecast period. Scattered light rain
showers will be possible across primarily the western parts of the
area, and an isolated thunderstorm or two cannot be ruled out
toward the end of the TAF forecast period.
Saturday afternoon, scattered showers and isolated thunderstorms
will still be possible, mainly in the west. VFR visibilities are
forecast except for brief sub-VFR conditions under any heavier
showers or storm. Ceilings will improve to VFR in the east, but
remain mainly MVFR in the west.
Saturday night into Sunday night, mainly VFR conditions are
expected along with isolated to scattered showers under the
passing upper level trough axis. Northwest upslope flow may
maintain some MVFR ceilings in the west. Surface gusts of 20 to 25
kts will be possible across the mountains during this time frame.
850 mb winds are progged to increase to 25 to 35 kts, and
gradually veer northwest to north.
On Monday, winds will start to decrease as the center of the upper
low continues to exit the region. Isolated showers will still be
possible, especially in the east, related to this feature.
Monday night through Tuesday, expect mainly VFR conditions as high
pressure settles over the area.
Wednesday into Thursday, anticipate a return of scattered light
precipitation, and pockets of sub-VFR conditions, returns to the
area as a frontal boundary establishes itself north of the region.
-- End Changed Discussion --
As of 450 AM EDT Friday...
Consistent forecast from the models and the Weather Prediction
Center with one to two inches of rain expected across the forecast
area from this afternoon through late in the day Saturday. A bulk
of this precipitation will fall this evening into Saturday
morning. There is still a potential that this longer duration
rainfall will lead to localized minor flooding along streams and