Area Forecast Discussion
Issued by NWS Blacksburg, VA-- Remove Highlighting --
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FXUS61 KRNK 300952
Area Forecast Discussion
National Weather Service Blacksburg VA
452 AM EST Wed Nov 30 2016
A cold front slides east to the mountains this afternoon, and east
of the piedmont this evening. High pressure will track from the
lower Mississippi Valley Thursday to the Southern Appalachians
Friday morning. Expect drier and cooler weather for the end of
.NEAR TERM /THROUGH TONIGHT/...
As of 340 am EST Wednesday...
Radar has been pretty active this morning, with elevated instability
leading to thunderstorms moving across the New River Valley/Mountain
Empire and Alleghany Highlands, as well as portions of the piedmont.
The latest RAP showed this elevated instability lingering into mid
morning. There will be a break in the showers across the southeast
CWA with main axis of lift and moisture transport staying with the
front and upper dynamics across the TN Valley into the Central
Appalachians. Therefore will have higher pops in the mountains this
morning then models shift the axis east into the piedmont by late
There is a decent amount of lift and wind energy to give most of
not all the forecast area a good rain. Unstable air is limited by
clouds and showers but with increase in the jet should be enough
dynamical forcing to string some thunder in the forecast, especially
in the foothills/piedmont. The Storm Prediction Center shows our
eastern 2/3rds of the forecast area in a marginal risk, mainly
concerned about winds mixing down with line of showers and isolated
storms. Will mention this in the HWO, but more along/east of the
Blue Ridge as think the west will be stable enough to keep stronger
winds from mixing down past 4000 ft. Some of the higher terrain this
morning could see some gusts to 45 mph, although not enough to have
a wind advisory.
Temperatures despite the clouds and showers will warm into the lower
to mid 70s east to mid to upper 60s west.
Tonight, the front moves east of the forecast area by midnight with
showers ending. There will be clearing skies as we head into late
tonight in the east while upslope keeps clouds in the mountains,
with slight chance of showers over western Greenbrier County. Colder
air will star to track in overnight west of the Blue Ridge with lows
in the upper 30s to lower 40s, while the foothills/piedmont slide
into the mid 40s to around 50.
There will be a gradient behind the front tonight but 8h winds are
around 30 kts or less, so winds should be under 30 mph in the gust
dept, except possible gusts to 35 mpg across the Alleghany
Highlands, higher ridges.
.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
-- Changed Discussion --As of 400 AM EST Wednesday...
The final and most significant short wave of the slowly filling eastern
U.S. low will be exiting to the east at the beginning of this period.
The pattern will transition back to a cooler/seasonal pattern.
Upslope trajectories are not overly favorable, generally a WSW-W
flow across the Alleghanys. This in combination with the lifting
out/filling upper low and the gradual return to southwest flow
aloft does not lead to a very favorable setup for snow. In
general, the GFS is the only model indicating much threat for any
upslope rain/snow showers. Will hold onto some slight chance
-SHSN across western Greenbrier through Thu morning, then drop.
For Friday, there is a very weak clipper evident along the bottom
of the departing upper low. This may induce a brief period of snow
showers across the northern Alleghanys again, mainly north of the
RNK CWA, but again the GFS remains one of the few models to depict
any QPF with this feature as far south as western Greenbrier. Will
carry a low slight chance pop to account for this feature. Both
events are barely worth mentioning from my analysis.
Otherwise, the main story this period will be much cooler
temperatures. 850mb temps will fall back below 0C across most of
the CWA during the day Thu and remain in this range well into the
weekend, bottoming out Saturday in the -6C range north to around
0C south. Temperatures will trend back toward seasonal levels with
lows in the 20s and 30s and highs in the 40s and 50s, colder
-- End Changed Discussion --
.LONG TERM /SATURDAY THROUGH TUESDAY/...
-- Changed Discussion --As of 430 AM EST Wednesday...
The focus during this period will be on a southern latitude system
originating from a closed upper low over Mexico. There remain
significant differences in how this system will evolve with the
GFS still slow to bring the parent system out of Mexico, but now
on board with what the ECMWF has been showing for several days,
namely a northern stream system traversing the region Sunday into
Monday. Models have been trending toward a weaker closed low and
greater potential for northern stream energy to pick up the system
sooner, thus forecast trends lean toward the faster ECMWF
solution. These changes will result in chance pops being
introduced in the southwest part of the CWA Saturday night/early
Sunday, increasing to likely Sunday, but then decreasing quicker
back to chance or less by Monday.
P-type may be a concern early Sunday with surface temps hovering
in the 29F to 32F range and 850mb temps struggling to rise above
zero. This would especially be true for areas where the
precipitation arrives late Saturday night/early Sunday. At any
rate, especially per ECMWF, warm air will begin to surge northward
during the day changing any frozen/freezing precipitation to
liquid. The caveat may be the potential for an "in-situ" wedge to
develop and keep the threat for winter precipitation in the area
longer, quite common at this time of year, and something the
models are poor at resolving.
Uncertainty continues for the remainder of the period as the GFS
still has the parent upper low coming out of Mexico and bringing
another swath of precipitation across the southeast/Mid-Atlantic
Monday into Tuesday, while the ECMWF has moved the entire system
out of the region, leaving dry weather in place beyond Monday
night. Have nudged the forecast toward the preferred ECMWF.
After a seasonably chilly/slightly below normal period through
the weekend, expect a return to milder temperatures early next
week as heights build aloft across the southeast U.S. in the wake
of the weekend storm system.
-- End Changed Discussion --
.AVIATION /09Z WEDNESDAY THROUGH SUNDAY/...
As of 1145 PM EST Tuesday...
Expecting MVFR to IFR clouds to fill in over southwest Virginia,
northwest North Carolina and southeast West Virginia tonight. LIFR
fog developed at locations that had rain Tuesday afternoon and
stay cloud free longer. Winds will pick up overnight, especially
at higher elevations as the low level increases from the
southwest. Low confidence on the timing for when visibilities will
improve as fog erodes.
Area of upper diffluence that was providing lift for the showers
and thunderstorms over Mississippi and Tennessee will move into
the region late tonight. Guidance was in good agreement keeping
much of the area dry until after 06Z/1AM. Some warm air advection
also in the region tonight and Wednesday morning which will aide
Next band of heavier showers and will cross the region from west
to east Wednesday afternoon allowing for widespread sub-VFR cigs
and vsbys until the front crosses Wednesday evening. Shallow
convection may enhance the winds. High confidence on the passage
of the front and associated moderate showers but lower confidence
on the exact timing.
Extended aviation discussion...
Improving conditions expected after the upper low and final short
wave move east of the area later Wednesday night. Conditions
should become VFR east of the Blue Ridge into the weekend.
Upslope clouds and potentially a weak clipper system could bring a
low chance of rain or snow showers into the western sites Thursday
and Friday along with lingering MVFR cigs before conditions
improve there as well over the weekend.