Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 211033

Area Forecast Discussion
National Weather Service Blacksburg VA
533 AM EST Tue Feb 21 2017

High pressure along the Mid-Atlantic coast will shift east and
offshore today ahead of a weak cold front approaching from the
west. This front should slide east into the mountains overnight
into early Wednesday before dissipating. High pressure will
strengthen offshore for the end of the week ahead of a stronger
cold front that looks to arrive from the west on Saturday.

As of 240 AM EST Tuesday...

Strong upper ridging across the region will gradually get dampened a
bit this afternoon as shortwave energy passes to the north later today
in advance of the main upstream split flow troughiness tonight. This
will continue to keep rounds of high/mid clouds advecting in from the
west today overtop weak low level wedging that looks to linger espcly
Blue Ridge east into tonight despite high pressure exiting offshore.
However weak low level southeast flow around the surface high the main
concern in regards to possible coverage of developing low clouds this
morning and whether or not they persist given lingering dry air aloft.
This could make for large temp differences as more insolation similar
to the drier Nam would push highs well into the 60s most spots while
the cloudier GFS looks a bit more realistic under easterly flow in
holding spots near the eastern slopes a category or so cooler. Thus
trended in that direction while bumping up highs slightly from previous
with warmest values far west and across the southeast. Otherwise
appears enough dry air in between the low level moisture and higher
cloud canopy to preclude any pops today espcly with better lift still
to the west per model consensus.

Complex system to the west will basically split overnight with the main
upper low heading across the Gulf coast states leaving a weak front and
associated mid level shortwave to track toward the Appalachians late.
Although widespread deep moisture limited, guidance showing a swath of
higher PWATS and upper diffluence pushing east to just east of the Blue
Ridge late. Appears this in combination with deeper southwest flow
aloft and thinking that any deep convection with the upper low to the
south should be far enough removed to avoid a cutoff of moisture
advection ahead of the front late. Therefore running with a gradual
increase in pops from chance to low likelys far west after midnight and
overall chance coverage Blue Ridge and less out east. QPF still looking
rather light heading east although could see a tenth or two western
mountains where better southeast flow around the perimeter of the
residual eastern wedge/subsidence could help lift a bit more. Expect
overall uniform lows mainly in the 40s given clouds and light showers
that should hold readings up some under the weak warm advection regime.


As of 240 AM EST Tuesday...

A northern stream cold front, coming across the Ohio Valley, will
washout by the time it reaches the Appalachian Mountains Wednesday
morning. However, prefrontal showers and short waves being ejected
north by a southern stream low will be able to advance east of the
mountains as they get caught up in zonal flow Wednesday. The
intensity of these showers will weaken as they move east and over a
surface wedge. Showers moving east of the Blue Ridge will enhance
the wedge over southern and southwestern Virginia and into the
Carolinas. The parent high of this insitu wedge will drift east, but
linger over the region into Thursday night. Rounds of warm moist air
and insentropic lift coming over the wedge will keep low clouds and
the chance for light rain/drizzle for area along and east of the
Blue Ridge Wednesday night into Thursday night. With more clouds and
precipitation expected and the models have a warm bias during insitu
wedges, we have cooled temperatures just under guidance for
Wednesday and Thursday afternoons. Even though we are dealing with a
cold air damming event in late February, temperatures will run 10F
to 15F warmer than normal. Also during these insitu events, models
are to quick to have winds coming out of the south, therefore back
them to more east-southeast Thursday and Thursday night.


As of 344 PM EST Monday...

As noted by WPC long range models were in decent agreement at
the synoptic scale Friday and Saturday with the GFS becoming the
outlier Sunday and Monday. West Virginia, Virginia and North
Carolina will be in the warm sector with well above normal
temperatures Friday and Saturday ahead of the strong cold front.

850MB temperatures Saturday night in the 0 to -8 range but back up
just above zero by Monday morning. Low level jet and cold advection
will lead to gusty winds behind the front but neither contributes
enough that gusts would approach 40 kts.

Pattern remains active Monday with a surface low over the southern
plains that moves east and brings a chance of precipitation back to
the Mid Atlantic region.


As of 520 AM EST Tuesday...

Overall VFR looks to continue this morning with lack of any
lower cloud canopy attm. However will need to watch over the
next few hours to see if southeast upslope generates any strato-
cu within the developing wedge as seen via the latest GFS
solution. For now plan to bleed in VFR strato-cu cigs from
southwest to northeast through the day with perhaps a period of
MVFR along the Blue Ridge around KBCB/KROA later this morning.
Otherwise keeping overall VFR in place under mainly mid/high
clouds and southeast flow that could produce gusts to 20 kts at
times over the mountains, espcly KBLF.

Moisture will deepen from west to east overnight with a continued
lowering/thickening of cloud cover expected through this evening.
Showers and likely sub-VFR to develop later this evening over
far western sections and spread east into early Wednesday
preceding a weakening cold front arriving from the west. Best
shot at seeing lower IFR conditions will be across the mountains
into tonight, then southern sections into early Wednesday where
low cigs and at least MVFR vsbys in showers/fog are most likely.
Otherwise MVFR cigs to VFR/ocnl MVFR vsbys overnight through
Wednesday morning.

Expect rainfall to decrease Wednesday afternoon into Wednesday
night while leaving residual sub-VFR cigs, and ocnl MVFR vsbys
in place until possibly seeing some slow overnight improvement,
but quite uncertain at this point.

Extended Aviation Discussion...

Weak warm front lifting back north Thursday into Thursday
night may lead to added shower redevelopment before the residual
wedge exits on Friday. This could cause sub-VFR to persist in
low clouds and spotty rain into Thursday night.

Area will slide in between weak low pressure off the southeast
coast Friday and the next stronger upstream cold front that
looks to arrive by midday Saturday. This should allow for some
improvement in cigs/vsbys within the warm sector Friday as the
low level jet increases. Expect a mix of MVFR and low end VFR
ceilings and mainly VFR visibilities during the day with surface
wind gusts of 20 to 30 kts possible across the higher terrain.
Band of showers including a period of sub-VFR to arrive late
Friday night into early Saturday with the front, followed by
drier weather and increasing northwest winds by afternoon behind
the boundary.




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