Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 202045

Area Forecast Discussion
National Weather Service Blacksburg VA
345 PM EST Mon Feb 20 2017

High pressure will build over the region today and remain in
place through at least Tuesday. Next chance for rainfall is
later Tuesday night into Wednesday as a weak cold front
traverses the region. Relatively mild but unsettled weather
continues until late in the weekend into early in the weekend
as a stronger cold front from the Ohio Valley moves in.
Temperatures will continue to trend above normal for much of
the workweek.


As of 344 PM EST Monday...Beautiful late-February sensible
weather conditions this afternoon across the central
Appalachians and NC/VA Piedmont Region featuring wall-to-wall
sunshine, light southerly winds and dry air. That has led to
rather dry RH`s in the 15 to 20 percent range being common.
Subsidence exists both at the surface and aloft, with surface
ridge extending from a broad anticyclone centered over western
Quebec with pronounced mid-level ridge aloft over the
Alleghanies and eastern Cumberland Plateau region. Well to the
west over the southern/central Plains into the Mississippi
Valley is a deep mid-level trough and associated surface cold
front, leading to a meridional feed of deep moisture primarily
focused along/west of the Mississippi River Valley.

For Tonight: Deep-layer ridge axis to build over the forecast area
this evening. Intervals of high clouds will advect northeast across
the forecast area as the deeper moisture remains well to the west.
At the surface though, it appears an northeasterly onshore flow/weak
CAD signature develops for our Piedmont/foothills counties later
tonight per indications from the 12z NAM and the 12z GFS. I`ve
introduced a slightly greater degree of cloudiness in these areas,
more for the after-midnight period, and also reduced low
temperatures a little more in these areas as well. Lows project a
bit milder (in the mid/upper 40s) for the Mountain Empire into
Mercer County in more of a southerly return regime, with lows
dropping into the low 40s out in the Piedmont and eastern Southside
areas due to the aforementioned CAD potential.

For Tuesday: Look for a continued increase in mid to high clouds
areawide as mid-level ridge axis shifts eastward and an increase in
moist southerly low-level flow. Given the amplified meridional
nature of the mid-level flow regime, in addition to how dry the low-
level air mass is, any preceding showers ahead of the weakening cold
front hold off until the evening at earliest. So I`ve maintained a
dry forecast. However, given likely cold air damming signature that
only moderates slightly with a light southerly flow, it introduces
some question as to how mild especially eastern and southern
sections get. For these areas - lower NC piedmont, central VA
Piedmont into the Southside -  I have trended toward the lower end
of the guidance temperature envelope, in the mid 50s to near
60. With more of a southerly flow and milder 850 mb temps (+8 to
+10C vs +4 to +6C further east), forecast temps in the mid 60s.


As of 344 PM EST Monday...

We will be dealing with a wedge of high pressure banked up against the
east slopes of the Appalachians through the middle of the week.
Synoptic forcing is weak, but as weak isentropic lift rides over the
wedge with some upper short wave energy from a zonal northern stream
and a closed low slowly migrating eastward along the Gulf coast, we can
expect a good amount of cloudiness along with some showers. The best
chance for precipitation will be Tuesday night into Wednesday as the
main vorticity axis moves over the wedge.

Temperatures on Wednesday will be cooler with precipitation in the
wedge, though still well above normal. The surface high responsible for
the wedge will drift eastward and off the coast for Thursday, giving a
southerly component to our winds and boosting temperatures further
above normal.


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 1230 PM EST Monday...

VFR through the TAF period. SKC conditions with light and
variable winds through the afternoon, give way to increasing
field of mid to high-level cloud cover from southwest to
northeast tonight. Low confidence on prospects for river valley
fog development tonight given that crossover temperatures may
not easily be met, very dry soils from this afternoon and there
appears to be just enough of a southerly boundary layer flow to
mitigate much development.

A continued areal coverage/thickening of mid to high level
clouds anticipated through 18z Tuesday as cold front remains
slow to advance east. If there is an area where low clouds may
develop Tuesday, it would be for the eastern TAFs given onshore
flow/CAD signature. But confidence on that happening is low
based on little indication from model RH profiles. Winds
initially southerly western TAFs with light easterly winds for
LYH/DAN, but become southerly around 4-7 kts all TAFs ahead of
aforementioned front.

Extended Aviation Discussion...

A continued lowering/thickening of cloud cover expected through
the rest of the Tuesday afternoon and early evening hours.
Showers and likely sub-VFR to develop Tuesday night into early
Wednesday with an axis of showers preceding the weakening cold
front arriving from the west. Expect showers to decrease
Wednesday afternoon and night while leaving residual MVFR cigs
in place. Later Thursday into Thursday night, low confidence of
patchy light rain returning to the area, with cigs and vsbys
remaining mainly VFR except for ocnl MVFR across southern and
western sections.

Friday, moderate confidence in precipitation trending more
showery, as the low level jet increases in advance of a strong
cold front. Expect a mix of MVFR and low end VFR ceilings and
mainly VFR visibilities. Surface wind gusts of 20 to 30 kts
possible across the higher terrain. Band of showers with the
front to arrive Friday night into early Saturday followed by drier
weather and increasing northwest wind by afternoon behind the




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