NWS Blacksburg, VA wrote its last area forecast discussion before the first event at Jun 21, 12:39 PM EST, 1 hour 26 minutes earlier.
Area forecast discussion, Jun 21, 12:39 PM EST, 1 hour 26 minutes before
Area Forecast Discussion
National Weather Service Blacksburg VA
139 PM EDT TUE JUN 21 2016
.SYNOPSIS...
A cold front will slowly move southeast out of the Ohio Valley
today and tonight and become nearly stationary over the Mid-
Atlantic Region by Wednesday. It will then slowly lift back north
as a warm front late Wednesday, with another stronger cold front
arriving from the west by Thursday night. Expect a daily threat
of showers and thunderstorms through this period.
&&
.NEAR TERM /THROUGH TONIGHT/...
As of 1130 AM EDT Tuesday...
KFCX Radar is expected to be down until Wednesday... see equipment
section of this AFD for details.
Surface cold front extends from along the Mason-Dixon line,
southwest along the Ohio River into Missouri. This front will be
the primary focus for thunderstorm development this afternoon. In
addition to the front, a prefrontal convergence boundary,
associated with an axis of vorticity has moved out ahead of the
front. This axis of lift was producing a band of cloudiness across
our CWA along with some light rain showers.
Concern this afternoon and evening is the development of strong to
severe thunderstorms. Meso-models place the main emphasis for
thunderstorm development along the front, which means areas to our
north, especially northern VA will likely be under the gun for
severe storms this afternoon. Additional activity will likely
develop along the front over the Ohio Valley, and this will move
downstream into WVA later this afternoon and evening. Prefontal
boundary may also become active, but mainly over the western CWA
where low level convergence will allow activity to continue on the
upwind side of the Appalachian Divide.
Big question for our CWA is overall impact. Existing cloudiness
will cut down on insolation and winds behind the early morning
prefrontal boundary may place our foothill counties and NC piedmont
in a void per subsidence from the downslope west-northwest wind.
That said, think the main focus for storms which may require
warnings would be across our northern CWA, storms originating
along the cold front to our northwest and moving across West
Virginia and into areas mainly north of I-64 in Virginia the first
half of the afternoon, eventually making it down to the Highway 460
Corridor for the late afternoon and evening.
We will be doing a special 18z/2PM upper air release to get a
better feel for thunderstorm potential and type, but SPC has most
of our CWA outlooked for severe storms this afternoon, favoring
KY/WV/VA/MD/DE as a target for organized severe thunderstorms. The
expectation is for clouds to erode in advance of the approaching
front with combination of diurnal heating and widespread low level
moisture contributing to MLCAPE of 1500-2000 J/KG. Winds aloft are
forecast to strengthen in associating with the the amplification
of the upper trough with 40-45kt mid-level flow supportive of both
supercell and bowing segments, threats being damaging winds, large
hail, and isolated tornadoes.
Something else to watch is for potential training of storm cells
in the western CWA, west of I-77 on the upslope side of the
Appalachians, where the confluence of the westerly wind may
result in regeneration of storm cells in spite of the fast wind
flow aloft. PWATs are forecast to increase into the 1.5 to 1.7
inch rain within this area, so rain efficiency will increase.
Main storm threat should play out between now and 10PM, storms
weakening after loss of daytime heating. There will continue to be
some upper support for lingering showers vicinity of the cold
front which is not expected to pass through the forecast area.
Models have this front becoming stationary across the region
tonight.
&&
.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
As of 445 AM EDT Tuesday...
On Wednesday the frontal boundary will be just north or perhaps
across part of our area, but by later in the day may begin to lift
northward again as a broad upper ridge of high pressure slowly
slides toward the east and at least by Wednesday night the low
level flow turns more from the southwest. It looks like there will
be plenty of cloud cover in the vicinity of this frontal boundary,
and right now appears doubtful that there will be much instability
to support a lot of convection during the day. Best chances would
be far western portions, and perhaps even late at night if
organized deeper convection well to the northwest slides back in
with upper level westerly flow. Starting to look like Wednesday
could be a bit of a down day in terms of thunderstorm activity,
especially anywhere north of the boundary should it remain a
little farther south.
If storms remain to the northwest of our area overnight as they
approach from the west, we will have another chance by late in the
day Thursday or into the evening as a strong upper wave and
associated cold front move in from the northwest. This pattern has
some similarities to last Thursday, with increasing thicknesses
and very warm temperatures assuming good heating and drier air for
much of Thursday, allowing an organized line of convection along
the front to survive even if arriving into the evening. At this
time, it does look like this system would give many parts of our
CWA the best chances of the week for severe weather, mainly in the
form of damaging winds. Much like last week however, there remains
plenty of uncertainty about this scenario and will likely remain
that way until at least the early part of Thursday. More certainty
that many locations, especially in the Piedmont, will experience
another hot day with temperatures likely getting into the low to
mid 90s.
&&
.LONG TERM /FRIDAY THROUGH MONDAY/...
As of 300 PM EDT Monday...
Broad upper ridge is forecast to remain over the the southern United
States, no surprise since its now officially summer. Models differ on
magnitude and overall spacial coverage, but I think its a given the
desert southwest will continue to bake. In the southeast CONUS the
upper ridge seems to go through more of an ebb and flow cycle with
heights rising and falling over the Mid-Atlantic states in between
the larger short wave troughs which have been more cyclic over
eastern Canada, Great Lakes and New England.
Attm it appears the ridge will build north across the Mid Atlantic
Region for the weekend promoting warmer temperatures. The increasing
subsidence should also limit the chance for showers for all but the
mountains, the mountains serving as a forcing mechanism for widely
scattered late afternoon and evening pop-up activity (nothing
severe). The ridge is forecast to weaken again for early next
week, another short wave dipping through the Great Lakes and
sending a surface front our way for Monday and Tuesday. If this
plays out as forecast then POPs would trend upward.
&&
.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
As of 750 AM EDT Tuesday...
A cold front continued to track south of the Ohio River by mid
afternoon Tuesday, spreading scattered showers and isolated
thunderstorms from western Virginia to far eastern Kentucky.
Storms are expected to continue to fire and move into the western
slopes of the Appalachians during the early afternoon hours,
impacting first KBLF and KLWB, then KBCB and KROA by early
evening. Some storms could reach KLYH and KDAN, but much less
confident on coverage east of the Blue Ridge where downsloping
and persistent cloudcover will be limiting factors into the early
evening hours. Wind gusts of 25-45 kts are possible with any
storms, and would be most likely impact western TAF sites, but the
strongest winds gusts will remain isolated. However, even outside
of any storms, winds could gust this afternoon from 20-25 kts.
Otherwise, expecting alternating VFR/MVFR conditions today based
on VCTS/TS. The only exception being any heavier storms that may
pass directly over an airport, which would could result in very
brief IFR. Behind the front, expect partial clearing which could
result in some patchy fog to form late tonight, most likely at
KLWB and possibly at KBCB depending on location of afternoon
rainfall.
Low to medium confidence on the cigs and vsbys due to the
uncertainty of shower/storm location this afternoon and evening.
Medium to high confidence in winds through the TAF valid period.
Extended discussion...
The cold front approaching our area today will stall near the
VA/NC border Wednesday through Thursday. This boundary will
provide the focus for several rounds of thunderstorms through the
week, as disturbances flow along the track from the northwest to
southeast across the region. The most favorable period for
thunderstorms at this time appears to be Thursday
afternoon/evening, but showers/thunderstorms will also be possible
Wednesday late afternoon and evening and even into Wednesday
night.
Some strong to severe thunderstorms will be possible Thursday
evening into the overnight hours. Periods of mostly MVFR+ cigs
can be expected through much of the TAF valid period. Lower
category CIGS and brief periods of IFR-LIFR vsbys can be expected
in showers/thunderstorms. In addition, late night/early morning
fog will become increasingly common through the period with moist
ground and increasing RH levels.
&&
.EQUIPMENT...
As of 1045 AM EDT Tuesday...
The KFCX Radar is down. Bad power supply. A replacement has been
ordered but turn-around time is 24 hours. Estimated back in
service time is Noon Wednesday. Please refer to adjacent radars
at KLWX, KGSP, KMRX, KAKQ, KRAX AND KRLX.
&&
.RNK WATCHES/WARNINGS/ADVISORIES...
VA...None.
NC...None.
WV...None.
&&
SYNOPSIS...PM/SK
NEAR TERM...PM
SHORT TERM...SK
LONG TERM...PM
AVIATION...JM/PH/SK
EQUIPMENT...PM