Area Forecast Discussion
Issued by NWS Blacksburg, VA-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS61 KRNK 291126
Area Forecast Discussion
National Weather Service Blacksburg VA
726 AM EDT Sat Apr 29 2017
High pressure offshore will maintain a warm and humid southerly
flow of air across the region through the weekend. A strong
cold front will move through the area Monday with showers and
thunderstorms, followed by some cooler, but near seasonal
temperatures through the first part of next week.
.NEAR TERM /THROUGH TONIGHT/...
As of 222 AM EDT Saturday...
Overall convection continues to skirt by just to the north early this
morning associated with weak impulses riding along a weak front stalled
near the Ohio River. May see density help drag an isolated shower or
storm toward the far northwest counties early this morning, so included
a token mention mainly western Greenbrier given lingering instability,
otherwise keeping things dry to start. Degree of cloud cover/fog the
main concern early on given debris canopy over the north this morning
and potential for warm advection strato-cu to fill in across the
south/east. Thus bumped up sky cover to start before seeing any leftover
clouds erode by late morning as upper heights build through the
afternoon in response to strong high pressure offshore. This
combined with a very warm 850 mb southwest trajectory should
interact with better afternoon insolation to push highs into the
80s most spots, with perhaps a few low 90s east given such a
warm start. This may push record highs in spots despite lack of
good downsloping per the climate section below, unless clouds
are a bit more and linger longer.
Heating and moist dewpoints will also allow for very strong afternoon
instability although convective focus limited to mainly the higher
terrain where orographics and differential heating aided by some early
cloudiness could spark at least isolated storms. However most models
have backed off on coverage over the west this afternoon given the
cap/warmth aloft and best forcing shifting off well to the west. Think
degree of heating including southwest flow along the mountains still
enough for spotty coverage espcly if get an outflow or two
around within the weak convergence field. Therefore keeping in
low pops mainly southern Blue Ridge to start with a gradual
northward shift later in the day perhaps aided by a faint wave
spilling over the ridge.
Expect any convection to fade shortly after loss of heating this
evening given lack of much support as the main area of lift shifts back
to the west with the next wave along the front. However guidance shows
some instability lingering overnight in the warm/moist environment
south of the residual front just to the north. This along with
continued southwest flow and possibly residual outflow could keep an
isolated storm or two going into the overnight but not enough to
mention after midnight for now. Otherwise more patchy fog likely under
partly cloudy skies early with perhaps more stratus around late. Lows
mostly in the 60s although a few spots out east may struggle to fall
below 70 as appears guidance too quick to drop dewpoints off late.
.SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/...
As of 215 AM EDT Saturday...
Sunday through Monday, expect a strengthening southerly low level
jet, thanks to an increasing pressure gradient between stationary
high pressure off the coast of the Carolinas and a deepening area of
low pressure ejecting northeast from the Central Plains states. The
low`s associated cold front is progged to cross the region on Monday.
We can expect a continuation of above normal temperatures and dew
points values heading into Monday thanks to the southerly jet.
Although, as we experience more cloud cover and chances of
precipitation, daytime highs are not expected to be as extreme as
those forecast today. Sunday into early Sunday evening, convection
will be primarily diurnally and orographically based. By late Sunday
night, coverage is expected to start increasing again with the
approach of a strong cold front. This front will cross the region on
Monday with generous shear along and just in advance of it. Storms
on Monday may be on the strong side with gusty winds, especially
during the peak heating of the afternoon across the Piedmont.
Model guidance has trended quicker with the passage of the front, so
that limited shower and thunderstorm activity will still be across
the far eastern sections of the area Monday evening. Tuesday night,
winds become gusty from the northwest behind the front with some of
the higher elevations experiencing gusts of 35 to 45 mph, with
weaker gusts at lower elevations, and even weaker in the Piedmont.
As drier air races into the region, anticipate skies to clear
rapidly with limited cloud cover across the region on Tuesday.
However, gusty winds will continue during the course of Tuesday. By
Tuesday night, winds will weaken, but high level cloud cover will
start to increase as a disturbance makes headway eastward through
the Lower Ohio Valley.
Temperatures during this portion of the forecast will be above
normal, but trend cooler each day. Readings on Sunday will average
10 to 15 degrees above normal with values by Tuesday above 5 degrees
.LONG TERM /WEDNESDAY THROUGH FRIDAY/...
As of 240 AM EDT Saturday...
A high amplitude eastern trough gets reinforced Wednesday through
Thursday. 00Z ECMWF closes off a low over the Gulf Coast states by
Friday morning, while the GFS is much deeper in the northern stream.
Surface boundary remains east-west oriented on Wednesday. Weak waves
of low pressure will track east along the front. Will carry a dry
forecast Wednesday with near normal temperatures.
The 00Z ECMWF brings one of these waves and the associated
probability of precipitation into the Mid Atlantic region on
Wednesday night and Thursday. WPC leaned toward the ensembles in
this time frame, tracking a low from the Tennessee Valley Thursday
through the Mid Atlantic states on Friday. This pattern favors below
normal temperatures for both of those days.
.AVIATION /12Z SATURDAY THROUGH WEDNESDAY/...
-- Changed Discussion --As of 710 AM EDT Saturday...
Area remains in between deeper moisture with the front just to
the northwest and return low level moisture around high pressure
to the southeast early this morning. This should allow for
mainly VFR conditions under high/mid clouds this morning with
some patchy valley fog making for brief sub-VFR in spots mainly
across the western TAF sites. Only exception likely around KDAN
and perhaps KLYH where have seen MVFR low deck slide north east
of the Blue Ridge. Latest Nam suggests that some of this
stratus/strato-cu could linger across the south/east through mid
or late morning so including a prevailing lower cig at KDAN,
while leaving out at KLYH for now given uncertainty. Also could
see an isolated shower work toward KLWB along the southern end
of the main convective band to the north. Thus may also include
a VCSH mention for a few hours pending release coverage.
Combination of deeper moisture lifting back to the north and
strong heating should make for a return to widespread VFR
heading into this afternoon across the region. However lingering
low level moisture in conjunction with the heating will result
in increasing instability by mid to late afternoon. This could
be enough to produce isolated mainly mountain convection but
coverage at this point too iffy to make mention in any of the
TAF locations. Otherwise looking at some developing CU fields
with a few buildups likely under south to southwest winds at
7-15 kts with possible gusts to around 20 kts this afternoon
under the strong heating.
Expect any isolated evening convection to fade shortly after
dark making way for overall VFR into the overnight. However
more potential for patchy dense fog in spots espcly valleys
late, as well as possible stratus along/east of the Blue Ridge,
including any location that sees an afternoon shower today.
Therefore will include brief MVFR vsbys in a few of the valleys
and eastern sections where low level moisture will be a bit
Extended Aviation Discussion...
High pressure should continue to linger Sunday ahead of a cold
front to the west resulting in widespread VFR conditions after
early fog/stratus fade. As a low pressure system approaches
from the west late Sunday night, the potential for showers and
thunderstorms will increase over the western mountains by early
Monday. The cold front associated with this system should reach
the Appalachians Monday and depart offshore by Tuesday morning.
As such, expect the best chance of MVFR conditions during the
frontal passage from west to east during Monday. VFR conditions
return behind the front on gusty westerly winds Tuesday followed
by continued good flying weather under high pressure Tuesday
night into much of Wednesday. However moisture may return to the
mountains by late in the day Wednesday with local MVFR possible
in isolated showers by the end of the day.
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Potential for high temperature records to be broken Saturday,
April 29, as well as record warm lows for the 29th