Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 302314

Area Forecast Discussion
National Weather Service Blacksburg VA
714 PM EDT Sun Apr 30 2017

A large low pressure system was over Missouri with a warm front
extending across the Ohio Valley and a cold front trailing into
the Gulf of Mexico. This system will move northeast tonight and
Monday, pushing a strong cold front through the eastern United
States. Weak high pressure follows the front with drier weather
for Tuesday and Wednesday.


As of 703 PM EDT Saturday...

Satellite/radar showing limited coverage for showers, much less
thunder in our forecast area early this evening. Small area of
low level convergence situated over southside VA helping with a
few showers there. Otherwise will wait until midnight or so to
see better lift/enhancement across the southern Blue Ridge for
more shower development.

Previous discussion from mid afternoon...
Deep upper closed low over western Oklahoma this morning will
track northeast to southeast Iowa by Monday morning then to
northeast Wisconsin by Monday evening. Monday in good agreement
with the timing and track of the low.

Challenging to pinpoint where isolated to widely scattered storms
will develop this afternoon. Some of the Hi-Res guidance including
SPC-HRRR suggests favored areas north of Lynchburg and Lewisburg and
another in southwest North Carolina with the storms tracking to the
north and northeast. Any thunderstorms will dissipate shortly after
sunset with loss of heating.

Winds increase from the ssw, especially just above the surface
tonight. Low level is around 60 knots at Boone and Bluefield by
Monday morning. Enough low level moisture again tonight for a layer
of low clouds. Band of deeper moisture along the front crosses west
to east over the region roughly between 15Z/11AM through 03Z/11PM,
with the GFS still a couple hours faster than the NAM. This slows
down the higher probability of precipitation on Monday morning. Wind
shift to the west will behind the front. This looks like late in the
day for southeast West Virginia.

Still not clear how much pre-frontal precipitation there will be.
Questionable how much instability will increase on Monday if most of
the mountains start out cloudy and the clouds spread east during the
day. Temperatures at mid level not quite as warm just ahead of the
front. Bufkit forecast soundings showed enough slow level heating to
break the cap by 20Z/4PM Monday across southwest Virginia, northern
North Carolina, and southeast West Virginia. Off the 12Z NAM and GFS
the forecast Lifted Index values are -4 to -6 with Convective
Available Potential Energy around 1500 J/KG with the push east of
more stable air in the late afternoon and evening from west to east.

Similar to past couple of nights, forecast area will remain in the
mild humid air mass and with increasing cloud cover late tonight
minimum temperatures will be above normal. Also slightly increased
the maximum temperatures for MOnday since front is coming through a
little later in the day.


As of 310 PM EDT Sunday...

Forecast models shift a cold front east of our forecast area around
06z (2 AM EDT) Tuesday. In its wake, a brief period of 40-50kt 850mb
winds are expected, but pressure rises diminish during the pre-dawn
hours Tuesday and the forecast inversion height should keep the
strongest winds aloft. Therefore will continue to highlight gusty
winds in the mountains in the HWO, but not issue headlines.

Breezy conditions continue on Tuesday, and despite cooler 850mb
temps, downsloping wind should push temps up to near normal values
for early May.

Leaned toward the cooler guidance values for lows Wednesday morning
as a large high noses into the western potion of the area. A weak
return flow Wednesday will bring clouds and an increase in dew
points to the southern potion of the forecast area late in the day.
Despite weak low level convergence late in the southern Blue Ridge,
forecast soundings show a healthy amount of dry air in the low
levels, therefore kept a dry forecast in place.


As of 245 PM EDT Sunday...

This period of the forecast will be dominated by a high amplitude
upper trof/cut off low that lifts out of the southern plains early
in the period, and into New England by the end of the weekend. While
there are still some differences in the models with regard to
intensity and timing, there is growing consensus that widespread
showers and thunderstorms impact our area sometime from Thursday
into Friday with upslope showers lingering into Sunday as
reinforcing shortwaves rotate southward on the west side of the upper
low. Ensemble members seem to be centering around late
Thursday/early Friday morning for the best chance for precipitation.

The 12Z GFS is showing better run to run consistency, but also
aligning more with the 00Z ECMWF with a slightly less amplified
upper trof and more northern placement of the upper low. For our
area, chances for strong storms appear best when instability and
lift align on Friday, but a greater concern may be for the potential
for flooding, as NAEFS and GEFS ensembles off the meteorological
model ensemble river forecasts show some confidence in the potential
for flooding in our area late Thursday/Friday. Will include the
mention for possible flooding during this time in the HWO.

After being wedged in Thursday, warmest temps for this period will
be Friday before below normal temps arrive Saturday into Sunday
under the influence of the upper trof.


As of 705 PM EDT Sunday...

Should stay VFR this evening at all sites, then low levels
moisten up as southeast to south flow increases with most sites
falling to MVFR or lower after 06z, especially near the Blue
Ridge, while BLF stays VFR.

Timing of front and pre-frontal showers and thunderstorms looks
to be starting around 15z BLF/LWB to 17z BCB/ROA and 19-20z
LYH/DAN. Ending time will be by 20z BLF/LWB to 23z ROA/BCB and
beyond this taf period out east.

Thunder chances are good, but could be limited by low level
cloud cover. For now will keep VCTS in the TAFs with moderate
rain showers with MVFR cigs, with good possibility for IFR
cigs/vsbys in heavy downpours.

SPC has marginal to slight risk of severe storms, so flying
conditions will be turbulent and poor in the region tomorrow.

Winds will also be strong from the south tonight into Monday,
then stay gusty behind the front Monday afternoon/evening.

Extended Aviation Discussion...

The cold front will move east of KLYH and KDAN by 03Z bringing
the winds around to the southwest then west. Southeast West
Virginia may have MVFR upslope clouds into early Tuesday. VFR
conditions return Tuesday and will continue under high pressure
Tuesday night and Wednesday.

A slow moving low pressure system will bring widespread
precipitation and sub-VFR conditions to the Mid Atlantic region
Thursday and Friday.




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