Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 190001
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
801 PM EDT Thu Apr 18 2019

.SYNOPSIS...
A strong cold front will approach from the west late tonight
into Friday morning. This cold front will move across the mid-
Atlantic and Southeast states Friday before pushing to the east
Friday night. An area of upper level low pressure lingers into
Saturday resulting in unsettled weather before high pressure
builds back in late in the weekend.

&&

.NEAR TERM /THROUGH FRIDAY/...
As of 300 PM EDT Thursday...

Nice late afternoon across the region with mild temperatures prevailing
within the warm sector as the residual warm front continues to lift
northeast. This should allow the region to remain in a lull with
only some passing mid/high clouds early this evening ahead of
the next round of deep moisture to the west. Models continue to
show an abrupt increase in moisture/clouds espcly later this evening
with enough lift within the warm sector to support some increase
in showers, mainly Blue Ridge after midnight, as the low level
flow turns more southeast. Thus included pops a bit sooner over
the southwest but only spotty showers at this point.

Otherwise main concern will be toward daybreak over southwest
sections as a band of residual deep convection heads east toward
the I-77 corridor. This in advance of a deepening upper low
sliding northeast out of the mid Mississippi Valley and ahead of
an associated cold front dropping in with phasing northern
stream energy. However this band looks to run into less
instability given timing, so appears initial band likely more of
a heavy rain threat with stronger storms just south closer to
the surface wave. Therefore looking at higher likely/categorical
pops southwest late tonight with mainly chance evolving
elsewhere late.

Southerly flow will also be increasing above the surface as a jet
max in advance of the upper low shifts east resulting in a 45-50 kt
850 mb jet across the west after midnight. However forecast
soundings show a strong inversion around 4-5K feet that could limit
mixing before the rain arrives. This may curtail the stronger speeds
across Tazewell/Mercer counties to just a brief period toward dawn
so wont hoist a wind advisory headline at this point but need to
watch.

Lows should be mild although could see values drop to the cooler
side of guidance outside the ridges as winds diminish so included
some 50s with low 60s ridgetops.

Very active day in store on Friday as the upper low deepens and
heads just west of the mountains while the surface cold front
punches slowly east with a possible secondary wave enhancing
convection overhead. Latest guidance just a bit slower but in turn
stronger with instability along/east of the Blue Ridge Friday
afternoon. This beneath very strong wind fields aloft that may tend
to result in more parallel flow to bands later in the day as the
upper low gains latitude. However models still remain inconsistent
with the evolution espcly with timing, and the degree of deeper
convection that should form along the leading edge of the earlier
band once it starts to encounter some increase in instability
beneath the very strong shear heading east of the mountains in the
afternoon. This along with the possible second wave and another
strong shortwave riding up from the Carolinas likely to result
in a possible bowing line/QLCS with embedded supercells espcly if
things do slow up a bit. Thus running with similar pops as previous
including a potential secondary line along the actual cold front
over the far west in the afternoon. Winds may also mix down just
ahead of the convection to produce stronger gusts just ahead of the
showers/storms out east where beefed up gusts to near advisory
levels there as well Friday afternoon.

Highs mostly in the 70s with 60s west where anticipate more
clouds/showers sooner that should taper highs a bit.

Flooding also another aspect pending the degree of bands per higher
rainfall rates needed for flash flooding given some drying the past
few days and green up out east. Latest models have decreased
amounts a bit with the higher threat looking to be from the NC
mountains northeast along the Blue Ridge to south of Highway 460 as
very high PWATS for mid April advect in. However given lack of
confidence in seeing heavy rainfall outside of bands, decided to
forego an areal flood watch at this time and let the mid shift take
another look.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/...
As of 155 PM EDT Thursday...

Upper low starts out in western Tennessee and moves up the western
side of the central Appalachians into eastern Ohio by late Saturday.
Sunday and Sunday night the low moves east toward New Jersey. Colder
temperatures aloft reach southwest Virginia, northern North Carolina
and West Virginia on Saturday with a residual pocket of pocket of
cold air over Virginia and West Virginia increasing lapse rates
lingering into Sunday.

A strong cold front will cross from west to east across the area
Friday night dropping 850MB temperatures to below zero west of the
Blue Ridge by Saturday morning, with NAM running cooler than the
GFS. Temperatures start to moderate on Sunday. Cold front will be
the back edge of the more intense showers and thunderstorms. Models
show a brief dry slot behind the front then showers filling back in
with the upper low from around Bluefield Saturday morning spreading
to the piedmont Saturday evening. Challenging to determine if lapse
rates and instability will support isolated thunderstorms Saturday.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
As of 1245 PM EDT Thursday...

Once upper low exits the northeast United States pattern returns to
broad southwest flow and weak 500MB ridging over the east through
Thursday. Consensus is fair in the long range guidance with no
significant synoptic features impacting the Mid Atlantic region this
time frame.

High pressure following the deep surface low fills in from the
southwest with the high over centered over the Carolinas by Monday
night. 850 temperatures warm back into the +10 to +14 range on
Monday and remain similar through Thursday. Temperatures will be
above normal through the period.

Low level moisture trapped under the upper low will just be exiting
southwest Virginia Monday morning. Baroclinic zone from the
northeast United States across the Ohio Valley may get close enough
by Wednesday morning to increase cloud cover north of I-64. Cold
front may drop into northern Virginia Thursday with more clouds and
a chance of precipitation but confidence is low at this time.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
As of 750 PM EDT Thursday...

VFR conditions remain in place at this hour and that should hold
for another six hours or so before conditions begin to
deteriorate rather quickly. For now, just high/mid clouds with
ceilings AOA 100 through the evening with no visibility
restrictions. Immediate aviation concern would be the wind with
gusty southerly winds lingering into the evening hours more than
usual as a strong storm system and the tightening gradient
approaches from the west. Look for SSE-SSW 10-15kts with gusts
20-25kts through at least 04Z. Some locations may not even see
much decrease in winds after 04Z because of low-level jet
increasing overnight.

Main concern for the rest of the TAF valid period will be the
deep upper low and associated occluded front approaching from
the west and then moving over the region slowly from
west/southwest to east/northeast Friday. Expect multiple rounds
of convection given good dynamics and reasonable thermodynamics.
The first line of convection should reach the western border of
the CWA around 08Z or so and impact KBLF, KBCB, and to a lesser
extent (because of weakening extent northward) KLWB. Possible
brief period of gusty winds on leading edge of convection and
brief heavy rain. This line will be in a diminishing state due
to early morning timing. Note, there may also be some southeast
upslope induced showers along the Blue Ridge from KTNB to
KROA/KMTV overnight/early Friday before the convection arrives
bringing MVFR ceilings.

During the day, with the upper low overhead and its associated
cold pool and dynamics, redevelopment of bands or parallel lines
of convection is likely, especially east of the Blue Ridge.
Locally heavy rain will be the main threat, but the potential
for stronger cells and QLCSs remains with the threat for
damaging wind gusts and isolated tornadoes, mainly east to
southeast of the Blue Ridge. The greatest threat for severe will
remain east and south of our CWA it appears at this point. Have
included thunder at all TAF sites during the afternoon Friday,
but the greatest concern for severe thunderstorms would be KROA,
KLYH, and especially KDAN in the 18Z-00Z time frame. Ceilings
and visibilities during the day, and especially in rain showers
and thunderstorms, will average mostly MVFR with brief periods
of IFR in heavier rain and associated fog. Winds will remain
SSE-SSW until late in the day at speeds of 8-12kts with gusts
20-25kts.

/Confidence in Forecast Parameters/
Ceilings - Moderate,
Visibilities - Moderate,
Winds - Moderate to High,
Thunderstorm Threat - Moderate to High.

EXTENDED AVIATION DISCUSSION...

The upper low will slowly circulate and drift northeast of the
area Saturday. This will bring a deformation zone over the area
leaving Saturday with scattered to numerous showers and isolated
thunderstorms with periods of MVFR ceilings and visibilities
for the most part, especially west of the Blue Ridge. Winds will
be gusty from the west and northwest behind this front on
Saturday and Saturday night. VFR conditions will return Sunday
into Tuesday under weak high pressure and a relatively flat flow
aloft.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...None.
NC...None.
WV...None.

&&

$$

SYNOPSIS...JH
NEAR TERM...JH
SHORT TERM...AMS
LONG TERM...AMS
AVIATION...JH/RAB


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