Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
710 am EDT Thursday Sep 18 2014
high pressure over the area will weaken and slide offshore this
afternoon ahead of a weak cool front that should pass across the
region this evening. Strong high pressure will build to the north
late tonight into Saturday before passing offshore on Sunday. A
cold front will cross the area from the northwest on Monday
bringing a better chance of showers to start the week.
Near term /through tonight/...
as of 235 am EDT Thursday...
Fog and low clouds the main issues early this morning as expect quite a
bit of dense fog coverage espcly mountains given the weak flow and
faint convergence. May need to hoist an advisory pending trends but
will let ride for now with grids/zone forecast product mention and see how widespread
coverage gets toward sunrise.
Otherwise area will be in transition today between the weak wedge seen over
the past 24 hours and the onset of a much stronger cad event that will
unfold late tonight. This scenario to take shape in the wake of the
passing of the center of the upstream 500 mb trough this afternoon and its
associated weak surface trough/front that will pivot through from the
northwest later today. Most guidance now showing an axis of weak low level
convergence just ahead of this feature...mainly along/east of the Blue
Ridge this afternoon...and across the south this evening. However
moisture looks quite shallow for the most part while forecast convective available potential energy
reach 500-1k j/kg under an axis of weak 850 mb Theta-E ridging. Thus
appears some low topped convection possible provided enough heating
with most turning partly sunny after early low deck. Since moisture
depth and lift not conducive to much deep convection...will leave out
thunder mention similar to the shallow nature of rain showers seen off the NCEP
WRF...but cant completely rule out per decent forecast lapses out east.
Otherwise will run with a shotgun...mainly isolated type coverage...Blue
Ridge east into this evening with little pop northwest where drier northwest flow
should prevail. Left highs in line with the latest MOS except perhaps
cooler Blue Ridge as light southeast flow develops including afternoon clouds.
Showers should sink south or SW and fade late this evening as the onset
of a deepening east/southeast flow takes shape in response to strong high
pressure building in from the north late. Most soundings show quite a
bit of moisture banked up along the mountains with depth even enough to
support some spotty -ra and certainly drizzle/fog by daybreak. However
trends appear a bit slower than before so trimmed back onset time a bit
while running with isolated probability of precipitation and most widespread drizzle along the
eastern slopes late. Will be cool/moist with lows mostly in the 50s
except closer to 60 southeast espcly if The Wedge is slower to spill south.
Short term /Friday through Sunday/...
as of 300 am EDT Thursday...
A broad area of high pressure will move over New England and wedge
south down the East Coast on Friday. BUFKIT displaying a saturated
sounding through 10kft and with an easterly flow...areas of fog and
drizzle likely along eastern slopes of the Blue Ridge into the early
afternoon. Drier air begins to push into the area late in the
afternoon but not enough to completely end light precipitation or
remove cloud cover...especially across the North Carolina Highlands.
Although limited...afternoon heating may erase drizzle/fog through
the afternoon...while some light showers may pass over the region
into the evening. Friday afternoon temperatures will be colder than
normal with areas keeping precipitation through the day not likely
to make it out of the 60s.
Low level dry air continues to surge southward Friday night but with
moisture trapped under the inversion...stratus clouds should linger
into Saturday morning. By Saturday afternoon...a disturbance
develops and becomes stronger over the Gulf Stream. Since this
system is to the south...northeasterly flow will help the ridge to
the north pull more dry air into the forecast area. Low clouds are
expected to break in the afternoon with temperatures recovering to
near normal. The Gulf Stream disturbance remains off shore and with
continued subsidence and dry northeasterly flow...Sunday will be
another rain free day with temperatures again topping out near
normal. A cold front is also expected to track over the Ohio Valley
on Sunday. This may turns the winds to the southwest and could push
temperatures warmer than normal on Sunday.
Long term /Sunday night through Wednesday/...
as of 300 PM EDT Wednesday...
A strong fall front will approach the region Sunday night...crossing
over the region on Monday. By the time this front arrives...winds
will become westerly. We should see rain falling along western
slopes...losing their intensity moving to the Blue Ridge early
Monday morning. This precipitation may then jump the foothills as a
weak line of convection develops over the Piedmont where a Lee
trough will reside on Monday. Showers leave the area by Monday
evening with high pressure building in from the west.
A very large 1025mb high will cover much of the United States east
of the Mississippi River Tuesday and Wednesday. Ahead of the front
850 mb temperatures warm to near +16c on Sunday. Behind the front 850 mb
temperatures are around +8c Tuesday and Wednesday. Tuesday and
Wednesday high temperatures will run 5f-10f colder than normal.
Overnight lows are expected to drop into the 40s.
Aviation /12z Thursday through Monday/...
as of 650 am EDT Thursday...
Quite variable ceilings and visibilities early this morning with pockets of
MVFR to LIFR in fog...mainly across the mountains...and widespread
MVFR cloud bases except for periods of VFR where some breaks
occur. Latest infrared pics indicate this scenario will persist until at
least middle morning and perhaps longer Blue Ridge until better
heating/mixing develops. Thus will trend improving ceilings to be a
bit slower and include up and down changeable bases within brief
tempo groups until middle morning.
After 14-15z/10-11am...conditions should improve back into the
VFR category...with most low ceilings lifting into the 40-60 kft
range for this afternoon. However heating may promote some
buildups of cumulus across the mountains where isolated afternoon rain showers
possible but since iffy wont mention at this point. However do expect
localized MVFR near any convection this afternoon. Otherwise looking
at cumulus fields scattering out by evening underneath a leftover middle
deck. This supports mainly VFR into early tonight as the weak
boundary shifts south and high pressure starts to build in from
the north. Flow turns more easterly later tonight as stronger high
pressure builds in with widespread MVFR/IFR ceilings likely developing
after midnight with the onset of stronger wedging. Think some
drizzle also possible along the Blue Ridge espcly kroa/kbcb near
dawn with patchy dense fog mixed in and across the deeper western
Extended aviation discussion...
High pressure will dominate the weather through the end of the
week with little or no precipitation. Expect mainly VFR
conditions outside any fog at night and potential for some low
level stratus along and east of roa/bcb...though the high
pressure should be strong enough to keep moisture limited.
Presence of subsidence inversion aloft will likely result in
cloud layer persisting between 040-060kft...so it will not be
Next upstream trough and front expected to arrive by early next
week with a threat of showers mainly late Sunday night into Monday.
May see some MVFR ceilings/visibilities within a band of showers during that
period but appears most of the time will continue to see VFR.