Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 202341

Area Forecast Discussion
National Weather Service Blacksburg VA
641 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
Alleghenies 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 640 PM EST Monday...

High confidence in light winds, medium confidence in VFR cigs
this TAF period.

Latest satellite images show a decent canopy of cirrus overhead
with more upstream. While guidance indicates this cirrus will
remain over the region through tonight, there are some notable
thin spots. Believe the overall coverage of high clouds will be
enough to hinder radiational cooling and prevent another round
of late night fog development. If thin spots become more
pronounced this may have to be revisited, especially at KLWB.

GFS is very keen to develop some low clouds associated with the
wedge as it surges in overnight. The NAM and most other guidance
is not nearly as bullish with low level moisture and late
afternoon visible satellite images revealed little in the way
of low clouds upstream. Believe the low levels will moisten a
bit overnight and some orographic effects on Tuesday as flow
becomes more southeasterly may aid low cloud development, but
will play more of a compromise and keep all sites with VFR cigs,
save for a tempo to MVFR at BCB. It will be quite difficult to
discern any low clouds beneath the cirrus canopy tonight so will
have to monitor obs to keep tabs the situation.

Expect winds to remain light through the period with no
significant gusts.

Extended Aviation Discussion...

A continued lowering/thickening of cloud cover expected through
Tuesday 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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