Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 202054

Area Forecast Discussion
National Weather Service Blacksburg VA
354 PM EST Mon Nov 20 2017

High pressure over the Carolinas will move east and off the
southeast coast tonight. A cold front will move through the
eastern United States Tuesday night, followed by high pressure
on Wednesday and Thursday. Another cold front will cross the
region this weekend bringing the next round of colder air.


As of 200 PM EST Monday...

Large high pressure area centered over the southern Appalachians is
the main weather feature affecting weather across the Piedmont and
into the Appalachians this afternoon. Temperatures have rebounded
nicely back into the 40s to mid-50s under plentiful sunshine. A
split flow regime was evolving aloft, with a southern stream trough
over the southern Plains spreading an area of high clouds across the
lower Mississippi Valley in a lower-tropospheric warm advection

Clear skies will initially predominate the region for the rest of
the afternoon and into a good part of tonight. This should lead to
rather strong radiational cooling areawide with a decoupled boundary
layer leading to light surface winds. Radiational cooling should be
enhanced in areas where light snowpack exists per satellite imagery
across western Greenbrier County. Moderately strong surge of warm
advection begins after midnight on a southwesterly 850 mb jet from
30-40 kts per 12z NAM-3km output, greatest south. Will also see an
increase in high clouds across the southern third of the CWA. Given
the low-level jet on the ridges and the aforementioned radiational
effects, this should lead to low temperatures being the warmest at
elevation with values in the mid 30s along the Blue Ridge and
western side of the Appalachians, with lows in the lower valleys in
the upper 20s to near freezing.

For Tuesday, look for variable high cloudiness to exist early in the
day. As the day progresses, weak shortwave energy rounding the base
of the southern stream trough in diffluent flow should contribute to
an increase in cloudiness mainly from the Blue Ridge foothills in NC
and advancing into the Piedmont of NC and Southside VA. This appears
to be more probable in the afternoon hours. Guidance has generally
shifted the chances for (very) light rain into the Tuesday evening
period, but left open the door for rain starting early as the
afternoon with 15-20% probabilities along or south of Route 52. Rest
of the region should be dry. Though temperatures should climb
following typical diurnal trends pretty much everywhere, I`ve
started to slow temperature climb by afternoon in areas under
thickest cloud cover/chances for rain. A little less confident on
temperature trends in these southern areas depending on timing
thickest cloud cover. I may still not be low enough and the cooler
12z MET values (e.g. 51 at Winston-Salem, 54 at Mt Airy) may prove
more correct if thicker clouds develop earlier in the day. Official
forecast calls for highs in the 50s, in the lower 50s further south
with warmer values in this range to the north and west.


As of 200 PM EST Tuesday...

High pressure slides offshore Tuesday night in advance of both
northern/southern stream shortwaves within the lingering split flow
regime. These features still remain out of phase with most solutions
showing only a few light showers across the south/east with the
surface low offshore, as well as isolated rain/snow showers far
northwest with the cold front. Thus will keep in low end pops mainly
Tuesday night and dry elsewhere for now. High pressure following
these systems to bring drier weather including a period of weak
cool advection into Wednesday night and Thursday, but with less
wind than previously thought given a weaker post frontal jet
aloft. Otherwise after more clouds Tuesday night through midday
Wednesday, should see skies clear Wednesday into Thanksgiving
before more clouds shift north ahead of the next southeast
offshore wave Thursday night.

Low temperatures mainly Tuesday evening before rising along the
ridges overnight per warm advection/mixing and clouds. This
supports a range from low/mid 30s valleys and east to 40s at
elevation. Cool pool at 850 mb looks to only bleed into the west
Wednesday and across the area on Thursday. Latest guidance even
giving around 60 out east Wednesday afternoon ahead of the
lagging cool advection with mainly 50s east so trended warmer
with highs. Overall thickness in line with cooler highs mostly
in the 40s on Thursday with lows 20s to around 30 overnight.


As of 130 PM EST Monday...

Mean upper troffiness to prevail through the period with
reinforcing energy arriving late in the weekend that looks to
carve out an even deeper 500 mb cold pool into Monday. Otherwise
should see mainly dry weather under weak high pressure and
slightly below seasonal temps until a strong cold front arrives
later Saturday. This front will again be followed by colder
temperatures Sunday with cooler air lingering into early next
week under chilly high pressure. Appears a few rain showers will
be possible northwest ahead of the front Saturday afternoon,
followed by western snow showers early Sunday, but little more
than continued very low pops for now. Highs mostly 50s
Friday/Saturday, cooling to mainly 40s Sunday-Monday, with
perhaps 30s in spots across the mountains behind the front
similar to this past Sunday.


As of 1243 PM EST Monday...

High-confidence VFR conditions through the 18z TAF period.
Broad surface high pressure continues to migrate eastward to our
south, leading to a light northwest component roughly east of
the Blue Ridge contrasting with a light west to southwest
component. Expect a subtle backing to west-southwest in the
Piedmont with all TAF sites eventually going to a southwesterly
wind 3-5 kt by 00z Tuesday.

Clear skies should support an eventual decoupled boundary layer
and lighter wind flow by overnight. May have some pockets of
low-level wind shear as 925-850 mb flow increases to around 35
kts by early-morning Tuesday associated with warm advection.
Accompanying that will be an increase in high clouds from
southwest to northeast and predominantly over our
Piedmont/Southside counties.

Still could see some marginal low-level wind shear into the
morning hours but less confident on this so left out of the
TAFs for now. Expect more variable cloudiness into Tuesday,
with the greatest cloudiness to the south and east. This will
mostly affect Danville with VFR mid- clouds. It`s not until
after 18z Tuesday when degradation in ceilings then becomes a
possibility at Danville. Southwest winds increase Tuesday to
around 6-8 kts.

Extended Discussion...

Look for lowering ceilings along the southern Blue Ridge
foothills in NC and into the Piedmont Tuesday afternoon into the
evening. Though VFR ceilings should be common outside of these
areas, MVFR bases appear possible at Danville Tuesday afternoon
and evening, along with limited chances for light showers.

High pressure then establishes itself for the Wednesday through
Friday Thanksgiving period, with an expectation of good flying
conditions for Thanksgiving travel in the RNK airspace.

Next chance for sub-VFR conditions is with a cold front on




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