Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 202354

Area Forecast Discussion
National Weather Service Blacksburg VA
654 PM EST Fri Jan 20 2017

Weak surface high pressure will lead to a brief period of dry
weather tonight. A warm frontal boundary situated across the Gulf
Coast states starts to increase the moisture and rainfall again by
Saturday night into Sunday. Periods of steady moderate to at times
heavy rain are possible at various points this weekend, resulting in
gradual within-bank river rises. The low with this system will move
to Virginia by Monday. High pressure then builds into the region,
with more seasonal temperatures and a period of gusty northwest
winds Monday night into Tuesday.


As of 620 PM EST Friday...

Main problem tonight will be forecasting where areas of fog
develop or do they just remain as low clouds. Current cloud deck
east of the Blue Ridge is around 500 feet. Locations under 500
feet may see some patchy fog, while others above 500 feet will
be stuck with low visibilities and patchy drizzle.

Skies were mostly clear before sunset across the mountains.
Some high clouds will invade this evening, but think radiational
cooling should allow valleys to reach cross-over temperatures
with patchy dense fog developing during the pre-dawn hours.

Previous forecast...

For Tonight: In general, I think what we see is what we`ll get
for tonight...that being continued cloudy skies, with patchy
areas of mist/light fog scattered about. Most guidance maintains
a saturated, shallow inversion around/below 900 mb per BUFKIT
profiles that probably stands to get reinforced as southwest
flow aloft increases temperatures in the low to mid- levels.
Inversion should at least maintain broken to overcast cloudiness
and result in only a small fall in temperatures from current
values tonight. Given light wind conditions, there probably will
be areas of patchy fog, and while some of the 4-km NAM/SREF
visibility progs would argue for dense fog I`m not convinced
that visibilities will drop all that low for a prolonged period.
Prospect of fog is probably more limited along the Mountain
Empire up into southwest West Virginia given that winds should
be sufficiently strong enough to keep visibility from dropping
much should any fog develop there. Kept temperatures mainly in
the mid to upper 40s for tonight.

For Saturday: Aforementioned inversion should hang tough until at
least mid-morning, though when we can break free of the inversion is
somewhat of a question mark. Once that happens, with 850 mb temps of
around +9 to +11C, temperatures should rise more quickly into the
50s to near 60. In mid-levels though, we`ll start to see increasing
500 mb height falls starting by the afternoon. A returning warm
front begins to increase precipitation chances from south to north
during the afternoon hours. However the trend in most model guidance
today has been to slow the speed at which the front and its QPF
shield lifts north. So I`ve essentially produced a gradient in PoPs
for the afternoon, with slight chance along a line from Bluefield to
Roanoke to Appomattox and high chance to low Likely across the
southside of Virginia into the North Carolina foothills and
Piedmont. Modest elevated instability advects into our North
Carolina counties late Saturday afternoon (SI`s nearing -1 and
roughly 75 to 150 J/kg elevated CAPE); I couldn`t discount the idea
of a rumble of thunder but it may be the exception vs the rule and
areas in central/southern NC probably stands a better chance of
seeing any thunder though early Saturday evening.


As of 245 PM EST Friday...

Complex low pressure system to evolve during the weekend into early
next week as a very strong upper low across the southern Plains
Saturday night slowly works east into the region on Monday. Should
initially start out in a weak wedge Saturday night, in between a
warm front to the south including a weak area of low pressure over
the Carolinas, and weak ridging to the north. This has trended to
slow down the onset of more widespread rainfall working up from the
south on latest models with best insentropic lift more confined to
possible convection along/south of the warm front.

However as shortwave energy lifting north around the upstream upper
system heads north, expecting an axis of light to moderate rainfall
to work north into southern sections early Sunday and across the
region during the day. Could even see some heavier rain southwest
sections where a period of strong upslope may combine with the
passage of the weak surface low to result in even some elevated
thunder. Latest guidance even showing some weak instability and
strong shear as the wedge erodes so included isolated thunder across
much of the south/west Sunday. Appears a good inch or more of QPF
possible from late Saturday night through Sunday with rainfall
likely tapering from south to north by late Sunday. Otherwise mainly
chance to low likely pops south Saturday night, then categorical
Sunday all sections. Highs very trick on Sunday with potential to
zoom well into the 60s south/west, while the north could linger
closer to 50.

Stacked upper system with associated surface reflection will likely
slide into western sections Sunday night and then perhaps linger
before shunting slowly east/northeast on Monday. However lots of
uncertainty about how far north the system will get and where bands
of heavier rainfall might set up by then. Does appear that another
round of showers and even shallow convection will take shape by
later Sunday night given strong progged upper diffluence, with
focus now appearing over the west per latest EC/GFS mix. Therefore
pushed higher pops farther west into Monday morning with
another 1 to 2 inches possible before things taper off to only
far western mountain showers by later Monday. This may be enough
to result in localized flooding issues espcly Blue Ridge but
given possible rounds of heavier rain vs a prolonged period,
appears overall threat may be a bit less. However will still
need to watch given wet soil conditions and possible primer
heavy rainfall on Sunday. Highs Monday under the evolving cool
pool mainly 45-50 west to mid 50s east at this point.


As of 215 PM EST Friday...

Very wet period to start the week will be winding down Monday night as
the upper low begins to lift off to the northeast making for mainly
wrap around/upslope driven precip into Tuesday morning. Colder air
aloft will be trailing the main axis of showers overnight with any
below freezing temps aloft not arriving until very early Tuesday
morning. However the boundary layer still looks rather warm so thinking
mostly liquid outside of the higher northwest elevations. Otherwise
likely pops to initialize northwest sections Monday evening with mostly
chance elsewhere, lowering to low pops western slopes by midday Tuesday
and dry all other sections. Strong northwest jet in the wake of this
system likely to push gusts to near advisory levels west overnight into
early Tuesday across the higher elevations so may mention in the HWO.
Highs Tuesday still above normal with 40s mountains to 50s east.

Shortwave ridging develops later Tuesday and continues into Wednesday
night with weak surface high pressure extending north into the region.
This should result in a period of dry/mild weather for midweek with
highs well above normal and likely 60s east of the mountains.

This will begin to mark the end of the period of above normal temps
and bring the reality of winter back as a longwave trough pushes
southeast and across the region by the end of the week. Cold front
preceding this upper trough will cross the region on Thursday which
will be the transition day to colder air that arrives on Friday.
Weak impulse sliding up ahead/along the boundary could produce a
band of rain/showers by Thursday morning before things switch to a
mainly mountain upslope snow shower regime under cold northwest flow
within the cyclonic trajectory by Friday. Expect highs by then to
only range from 30s west to low/mid 40s east under blustery
northwest breezes.


As of 645 PM EST Friday...

This evening, shallow saturated subsidence inversion develops
and stays in place through Saturday morning. This lends itself
to relatively high confidence in prolonged degraded flight
conditions through the overnight in prevailing IFR to VLIFR
conditions for most of the TAFs (except mainly VFR at BLF) from
stratus and fog/mist.

Subsidence inversion begins to erode late Saturday morning
which should allow for a slow improvement in flight categories
through 18z Saturday. Rain begins to approach from the south
very late in the TAF period associated with next frontal system,
but unlikely to affect any of the TAFs through 18z Saturday.

Extended Aviation Discussion...

A slow-evolving but potent frontal system affecting a good part
of the southeast CONUS will lead to a potentially long period
of sub-VFR conditions this weekend due to low clouds/stratus
and rain. Rain may at times be steady to heavy and may include a
rumble or two of thunder, particularly Saturday night into
Sunday. Precipitation begins to trend more showery/intermittent
on Monday before trending VFR/MVFR for Tuesday.


As of 305 PM EST Thursday...

Record warm Mins for January 21

Blacksburg....42 in 1954
Bluefield.....47 in 1999
Danville......56 in 1954
Lynchburg.....51 in 1927
Roanoke.......51 in 1959

Record highs for January 21

Blacksburg....59 in 1954
Bluefield.....60 in 1999
Danville......68 in 1959
Lynchburg.....72 in 1932
Roanoke.......74 in 1932




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