Area Forecast Discussion
Issued by NWS Blacksburg, VA-- Highlight Changed Discussion --
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FXUS61 KRNK 301454
Area Forecast Discussion
National Weather Service Blacksburg VA
954 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/...
-- Changed Discussion --As of 950 am EST Wednesday...
Morning update will reflect lower chances of precipitation across
the southeast portion of the forecast area until the mid-afternoon
hours. This delay in arrival of the bulk of the precipitation
should allow for high temperatures to be a degree or two warmer in
this region. Latest radar trends show the ongoing precipitation
along or west of the crest of the Blue Ridge. Latest guidance does
well with matching the current returns with limited, if any,
coverage in the far southeast until the afternoon. No other
significant changes will be made at this time.
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.
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.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
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
.LONG TERM /SATURDAY THROUGH TUESDAY/...
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.
.AVIATION /15Z WEDNESDAY THROUGH SUNDAY/...
As of 635 AM EST Wednesday...
Trying to zero in on the variably cigs/vsbys across the area,
which are more up and down east of the Alleghanys as showers are
intermittent. Appears enough mixing to take place through late
morning to eventually scour out the lower clouds and vsbys, but at
least until 14-15z will see IFR or lower at all sites. Steadier
showers will stay west of ROA/BCB through early afternoon then
spread east. Will see moderate showers at least in the mountains
this morning, then BCB/ROA in the 21-23z time frame and 22-00z
time frame LYH/DAN.
Model show cigs staying mostly MVFR or worse, but could see VFR at
times this afternoon in the piedmont/foothills.
Look for showers to end in BLF/LWB by 0z, and by 01-03z ROA east.
South to southwest winds will gust up to 20kts at times in the
mountains this morning, then winds shift to the west after fropa
this afternoon/evening from west to east.
As we head into the overnight skies will be stay bkn/ovc in the
mountains with MVFR to low end VFR expected while it clears out
from Roanoke, east. Winds should stay breezy especially in the
mountains and Roanoke tonight, with some gusts to 30 kts.
Extended aviation discussion...
VFR thru Saturday, with high pressure in control. Another storm
system enters the area the 2nd half of the weekend and should
bring more precip and lower cigs/vsbys.