Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
735 am EDT Tuesday Sep 30 2014
weak high pressure builds into the area today. A back door cold
front will slide south into the area Wednesday. A strong cold
front arrives Friday.
Near term /through tonight/...
as of 720 am EDT Tuesday...
Regional observation/webcams and satellite showing dense fog across the
New River valley south and east into the foothills and Piedmont
with more cloud cover in the Piedmont. Anticipate fog to lift to
scattered stratus around 10 am when the dense fog advisory expires...then
skies staying mostly sunny for the rest of the day.
Previous valid discussion...
A shortwave moving across the Great Lakes will shift southeast
toward the middle Atlantic tonight. Appears most of the area will
stay dry with little cloud cover. Exception will be late tonight
when shortwave pivots southeast into northern Virginia...may see a scattering of
showers around Amherst to Buckingham...along with more clouds east
of the Blue Ridge in Virginia.
Temperatures today will be milder with more sunshine with highs ranging
from the lower to middle 70s mountains...to upper 70s/near 80
Tonight...should see some middle to upper 40s sheltered valleys in the
southwest with less cloud cover while most experience lows in the
lower to middle 50s.
Short term /Wednesday through Thursday night/...
as of 250 am EDT Tuesday...
Upper low passes well to the NE early Wednesday dragging a weak surface
front through from the northwest during the day before the boundary stalls
just to the south by Wednesday night. Appears just enough deep moisture
along the trailing front/vorticity tail to warrant a small chance of showers
extreme NE sections closer to the core of the upper system into
Wednesday evening. Otherwise partly sunny and quite mild Wednesday given
weak westerly flow aloft and nose of 850 mb warming along the Blue Ridge.
Next wedge set up starts to take shape later Wednesday night as high
pressure over New England noses south allowing a more easterly low
level trajectory to develop. However how fast this unfolds still iffy
given shortwave ridging increasing while the main axis of the surface
ridge is farther east. Since the trend looks slower to see low clouds
develop...delayed the onset of drizzle until late...and trimmed
coverage to mostly the NE and Blue Ridge sections into Thursday
Wedge will start to fade from west to east during Thursday as return
southerly flow kicks in ahead of the next upstream cold front. However
flow looks weak enough in the east to cause drizzle/fog to possibly
linger into the afternoon as most of the warm advection moves over the
cool pool. Elsewhere expecting increasing sunshine with highs bumping
well into the 70s...and close to 80 southeast...while then north/NE sections
may only reach the lower 70s pending timing of cloud exodus.
Cold front approaches from the west very late Thursday night preceded
by increasing moisture advection and SW winds aloft. However guidance
suggests that the low level wedge may flop back in areas east of the
mountains as the best low level warm advection GOES up the western
slopes. Given uncertainty wont include any added drizzle/fog Thursday
night but will add in some warm advection showers SW given the typical
return flow convergence seen on the periphery of a wedge in this type
scenario. Otherwise low temperatures to remain quite mild with readings perhaps
rising late ahead of the front.
Long term /Friday through Monday/...
as of 345 PM EDT Monday...
A cold front will sweep through the region on Friday...clearing the
area Friday evening. Models are even faster today with the front
with rain over the mountains in the morning...drying by early
afternoon. Showers move east of the Blue Ridge in the afternoon
clearing the Piedmont by 08p/00z. Since models have a tendency to
run fronts fast in the fall...will maintain current forecast timing
of showers Friday and Friday evening. Will increase probability of precipitation as models
have been consistent with the frontal passage on Friday. Following
the front and a passing of a short wave axis Saturday morning...a
pressure gradient will bring breezy and gusty conditions to the
region Saturday afternoon. These winds will relax Saturday night as
high pressure builds in from the southwest. With high pressure in
the region...Sunday and Monday will remain dry and less breezy.
Temperatures Friday will depend on timing of showers rolling across
the area. If showers can hold off until late afternoon...Southside
could see 80f before the day is out. Since rain will arrive early
across the west...temperatures should not make it to 70f. Following
the front and over the weekend...temperatures will run 5f-10f cooler
than normal. Temperatures will begin to moderate towards normal
early next week.
Aviation /12z Tuesday through Saturday/...
as of 725 am EDT Tuesday...
Visibilities have been variable early this morning but most sites
are sub VFR...with the vlifr at bcb/lwb. Still expecting the fog
to lift/burn off by 15z. Could be a period of fog at blf as the
moisture lifts from the valleys through 14z.
VFR conditions are expected this afternoon. A shortwave moving
into the middle Atlantic by 12z Wednesday may bring some scattered-broken strato-ac
to lyh with any showers staying north. With not much cloud cover
anticipated typical areas like lwb/bcb/lyh/Dan will have MVFR
visibilities with fog...more dense in the lwb area.
Extended aviation discussion...
VFR ceilings and visibilities can be expected Wednesday and
Thursday...although period of sub VFR likely to occur for our
Mountain Valley airports...kbcb...klwb again Thursday morning. The
next threat for sub VFR ceilings/visibilities will come with a frontal
passage on Friday. This front will likely be associated with a
line of showers and few thunderstorms...sweeping west to east
across the region.
Northwest winds increase behind this front Friday night into
Saturday. Saturday should be VFR although some upslope MVFR ceilings
are possible at lwb/blf.
Virginia...dense fog advisory until 10 am EDT this morning for vaz009-010-
NC...dense fog advisory until 10 am EDT this morning for ncz001>006-