Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 250000

Area Forecast Discussion
National Weather Service Blacksburg VA
800 PM EDT Mon Jul 24 2017

A weak cold front will slide southeast through the area tonight
before stalling over the Carolinas Tuesday into Wednesday. High
pressure builds north of the region behind the front bringing a
return to more seasonal temperatures later tonight through mid
or late week. Another cold front approaches from the northwest
on Friday and works slowly through the area by the weekend.


As of 745 PM EDT Monday...

Line of showers and thunderstorms extends very near I-81 this
hour from JFZ northeast to DCA. This is associated with last
short wave of northeast U.S. trough. This should be the last
round of convection for a couple of days of any significance.
HRRR indicates that this convection will dissipate quickly with
the loss of daytime heating, roughly in the 02Z-03Z time frame.

Otherwise, looking for partly cloudy skies overnight, patchy
dense fog development near LWB and BCB possible later tonight.
Made some minor tweaks to temperatures and dewpoints, mainly
cooling areas a tad quicker west of the Blue Ridge, and not
quite so quickly to the east.

As of 210 PM EDT Monday...

Convection continues at a minimum this afternoon as the region
remains in a somewhat drier westerly flow between the exiting
surface trough to the east and the cold front along the Ohio
River. This front should finally slide southeast and across the
region later this evening as the upper trough to the north
swings by to the northeast. Latest guidance still showing some
degree of at least shallow convection just ahead of the boundary
with the Nam quite agressive, while other solutions only show
widely scattered coverage within the instability axis along our
northwest slopes. Since some potential for a band of broken
convection to move into the northwest sections, have included
low to isolated pops later this afternoon into early evening for
locations mainly west of the New River Valley. Otherwise should
see clearing take shape overnight as the front sags to the
south allowing for a bit more comfortable air to advect into the
mountains where expect lows well into the 60s. Moisture may
linger across the south/east so kept lows mainly upper 60s to
lower 70s there. Also despite good drying today, appears some
fog/stratus development possible espcly valleys given heavy
rain from last night and better radiational cooling west

High pressure builds in from the north on Tuesday with weak
northerly flow resulting in enough dry advection to limit most
convection to across far southern sections at best. This seen
via model lowering of PWATS from north to south and axis of
deeper moisture well to the south closer to the residual surface
front. Therefore reduced pops to slight chance mainly across
northwest NC where perhaps enough low level moisture/instability
may linger to pop a shower or two but iffy. Otherwise enough
subsidence for overall mostly sunny skies and not quite as
hot/humid with highs low/mid 80s mountains to 87-92 east of the
Blue Ridge.


As of 1245 PM EDT Monday...

Little change in the weather pattern and no synoptic scale forcing
Wednesday and Thursday. Will keep minimal probability of
precipitation, mainly in southern Virginia and northern North
Carolina each afternoon and evening. Persistence combined with bias
corrected MET/MAV guidance will be reasonable for daily highs and


As of 1245 PM EDT Monday...

Upper pattern amplifies starting on Friday with the upper ridge over
the western United States slightly retrograding and a trof digging
in the east. An upper low may eventually close off somewhere over
the Tennessee Valley or Mid Atlantic region by Sunday or Monday.
This closed low solution was not supported by a majority of the
extended guidance. Probability of showers and thunderstorms will
increase on Friday and Saturday with some vorticity advection and
lowering heights. Air mass ahead of the trough stays in the +16 to
+20 range for temperatures at 850MB and will be a little cooler on
Sunday and Monday.


As of 745 PM EDT Monday...

Line of showers and isolated thunderstorms stretches from DCA to
JFZ, very close to the I-81 corridor at this hour. Activity
associated with final short wave tracking through the base of
the northeastern U.S. upper trough. Latest HRRR has this
activity persisting for another couple of hours, especially in
the far southwest VA/northwest NC area, then dissipating quickly
toward 03Z. Have indicated this trend in the grids and TAFs,
with the only TAF to include VCTS at this time BLF due to a
nearby TSRA in Tazewell county. Have included VCSH at BCB and
LYH because of nearby activity that could affect these sites
before they dissipate in the next couple of hours. Ceilings
associated with these showers are mostly in the 040-050 range.

Concerns overnight mainly focus around fog development and
possible low cloud development along the Alleghany front. Will
discuss the later first as that possibility seems limited due to
decreasing upslope W-NW flow becoming near calm or light and
variable, possibly even light northeast. Will include for now as
it remains a possibility per model soundings. Fog development at
LWB and BCB especially seems a better possibility due to recent
rainfall, hence moist ground in the area, clearing skies, and near
calm winds. Have continued earlier thinking on this and
continued potential for IFR-LIFR FG at BCB and LWB in the 08Z-
12Z time frame. Will hold off on fog elsewhere, although MVFR BR
conditions at LYH and DAN do not seem out of the question, just
not enough confidence to include at this time.

Winds will be light WSW-WNW 4-6kts this evening, becoming calm
overnight at most sites. Winds will be light and variable
Tuesday, but good potential for light northeast winds east of
the Blue Ridge.

Medium confidence in ceilings through 13Z, then high confidence
in VFR ceilings.
Medium to high confidence in visibilities through 13Z, then
high confidence in VFR visibilities.
Medium to high confidence in wind speed and direction through
the TAF valid period.

Aviation Extended Discussion...

A frontal boundary will remain stalled to our south through Wed,
with high pressure wedging southward on Wed from the northeast
U.S. Overall VFR, but with flow turning more east, this could
bring some MVFR/IFR ceilings, which should quickly dissipate by
Thursday. Any convection also likely to remain isolated and
south of the TAF sites again later Wednesday. Next best threat
for showers and storms arrives late Thursday as a new front
approaches from the north with more widespread activity in the
area Friday as the front drifts slowly southeast through the




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