Area Forecast Discussion
Issued by NWS Charleston, WV

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FXUS61 KRLX 211047
AFDRLX

Area Forecast Discussion
National Weather Service Charleston WV
Issued by National Weather Service Pittsburgh PA
647 AM EDT Fri Sep 21 2018

.SYNOPSIS...
Summer-like heat ridge breaks down today. Strong cold front
arrives tonight, and then stalls near the region over the
weekend. Another strong cold front possible middle of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 645 AM Friday...

Forecast on track early this morning, as the mountain valley fog
thins out.

As of 400 AM Friday...

Fog was less prevalent early this morning, owing to stronger
boundary layer winds between the departed high, and a cold front
approaching from the northwest, albeit well upstream,
approaching a St Louis to Detroit line. Dense fog did manage to
form in the deeper mountain valleys, but ongoing strengthening
of the boundary layer winds should eradicate this fog promptly
around sunrise, if not before.

Stratus was banked up along the eastern slopes of the northern
WV mountains, on moist south to southeast low level flow. This
was part of a massive stratus and stratocu fest across the
interior middle Atlantic and much of the northeast. For our
eastern slopes, the stratus will start mixing out by midday,
aided by veering of the low level flow to the south to
southwest.

Otherwise one last very warm and humid day is on tap in deep
layer warm and moist southwest flow today. Showers and
thunderstorms fire up in the heating of the day, with isolated
to scattered coverage anticipated in the mountains early on, and
eventually in the middle Ohio Valley.

Instability will increase most over the middle Ohio Valley today,
in the axis of greatest low level moisture fed by the low level
jet to 35 kts. H85 theta e values are progged to climb as high
as the mid 340K range, to go along with surface dew points in
the lower 70s. CAPE climbs to around 1500 KJ/KG as a result.

An axis of effective layer shear reaches this moist, unstable
axis ahead of the then approaching cold front, west of the Ohio
River late this afternoon, and then crossing the Ohio River
this evening. Flow is progged to be sufficiently strong enough,
especially with the aid of the organized, linear synoptic
forcing ahead of and near the cold front, for convection to
organize into a line that may evolve into a QLCS. This is
reflected in the SWODY1 slight risk to the Ohio River, with
damaging winds the greatest concern.

The thunderstorms get out ahead of the best forcing tonight as
they cross the Ohio River, as the NAM12 depicts the shear axis
getting out ahead of the instability axis. With the loss of
heating, convection will move into increasingly stable air
tonight, and the severe threat diminishes east of the Ohio
River, as depicted in the narrow marginal risk area east of the
Ohio River. The severe threat wanes altogether overnight, as
the band of precipitation pushes further southeast.

The southeast moving front is progged to get about half the way
through the area by dawn Saturday, with showers likely still
ongoing southeast of it, and drier and cooler air pushing into
the middle Ohio Valley behind it. This is faster than previous
runs. Stratus or low stratocu is likely in a band behind the
front, before the low level frontal inversion starts to deepen.
Post rain fog may form as temperatures and dew points falling
in tandem, but boundary level flow of 15 to 20 kts should
prevent a widespread dense fog in the wake of the cold front.

Kept high temperatures near the high end of the guidance envelop
today, while the temperature and dew point gradient associated
with the cold front reflect the faster timing, with lower values
in the middle Ohio Valley, compared with previous packages.

&&

.SHORT TERM /SATURDAY THROUGH MONDAY NIGHT/...
As of 430 AM Friday...

Saturday morning, a cold front will be located across southern
WV extending into eastern Kentucky. This serves as the focus for
precip across SWVA and the southern WV mountains/coal fields
during the morning into midday, particularly in the upslope
areas as it pushes southward. Storm activity would be mainly
relegated to areas near and on the warm side of the front, as
well as into parts of the southern mountains.

Then, an elongated band of mid-level vorticity beneath
favorable jet dynamics/divergence aloft traverses the Ohio
Valley into the Mid-Atlantic and serves as the focus of
precipitation through the remainder of the weekend. Medium and
long range models generally agree that widespread amounts of
0.5-1" are possible through Sunday where the corridor of
showers sets up, and that locally higher amounts in the 1.5-2"
are possible. Locations favored for higher rainfall amounts
include the Mountains and along an axis extending from the
Tristate area up through north/central WV. Models do disagree on
the start time of showers further north with solutions
generally in the Saturday afternoon camp or after midnight
Sunday morning camp. Have erred toward the former solution given
consistency with those models.

High moisture content in the 1.6-1.8" PWAT range keep heavy rain
risk through the weekend, and thus the flash flood threat.
Moderate flow aloft, parallel to any boundary that may set up
with disturbances, leaves the door open for cell training. This
morning WPC has included roughly the southeastern half of the
CWA in a marginal risk for excessive rainfall Saturday with an
expansive slight risk painted across much of that same area for
Sunday.

Low level winds turn southerly ahead of an approaching short
wave trough rotating up the Ohio Valley Sunday night into
Monday. A few thunderstorms are possible as 20-30 kt mid level
flow pumps warmth and moisture in, but chances quickly wane
after nightfall as the wave jettisons off to the northeast and
heating is lost.

&&

.LONG TERM /TUESDAY THROUGH THURSDAY/...
As of 450 AM Friday...

A deepening upstream trough promotes deep southwest return flow
Tuesday, further destabilizing the atmosphere and increasing
potential for widespread scattered thunderstorms during the
afternoon. Major long range models are in good agreement, though
they have together been trending quicker, for a strong cold
front to move across the region Wednesday. Early thoughts are
that the system could potentially have great dynamics (read:
wind fields) to produce widespread impactful weather but will
have to wait until other temperature and moisture variables are
refined in the forecast to peg actual severe weather potential.

Models agree that troughing aloft will persist over the center
of the continent after frontal passage Thursday and beyond, and
that it will in turn continue to subject this area to
alternating SW return flow and cold fronts. Beyond lending to
the generally unsettled pattern, though, models diverge on
timing of various surface features with perhaps another wave
moving across the remnant cold front Thursday or perhaps another
cold front pushing through by Friday.

&&

.AVIATION /12Z FRIDAY THROUGH TUESDAY/...
As of 645 AM Friday...

Fog in the deeper mountain valleys was starting to thin out, and
should be gone by 13Z,

Stratus banking up against the eastern slopes of the higher
northern mountains of WV was already showing signs of thinning
out, per GOES-R nighttime microphysics channel, and will mix
into a higher stratocu deck by late morning.

Otherwise, today will be a VFR day, in a very warm, humid and
gusty southwest flow ahead of a cold front approaching from the
northwest. Thunderstorms may fire up early in the afternoon
heat over the higher terrain, and the middle Ohio Valley, with
coverage expected to be isolated to scattered, so no mention was
coded in the TAFs.

As the cold front approaches from the northwest tonight, a line
of showers and thunderstorms is likely to move southeastward
into the middle Ohio Valley, with strong wind gusts and brief
IFR conditions. There is a slight risk for severe wind gusts
north and west of the Ohio River. The thunderstorms will weaken
with the loss of heating tonight, with just a line of showers
settling into the central Appalachians overnight, bringing MVFR
conditions.

Ceilings will lower to MVFR in heavier showers, and behind the
cold front, which reaches the Ohio River around 04Z, from
central WV through the northern WV mountains 05-07Z, and the
central and southern WV mountains 08-10Z. By that time,
ceilings will already be improving back to VFR in the middle
Ohio Valley,

Light south to southeast surface flow early this morning will
become southwest and gusty today, and then diminish tonight,
before shifting to northwest behind the cold front, which should
be just about through the WV mountains by the end of the TAF
period. Light southwest flow aloft early this morning will
become moderate west to southwest today, and then to light to
moderate northwest behind the cold front overnight tonight,
with timing very close to that of the surface front.

FORECAST CONFIDENCE AND ALTERNATE SCENARIOS THROUGH 12Z SATURDAY...

FORECAST CONFIDENCE: High

ALTERNATE SCENARIOS: TAFs may eventually need amended for
thunderstorms later today into tonight, especially over the
middle Ohio Valley. Low MVFR to IFR ceilings overnight tonight
may vary.

EXPERIMENTAL TABLE OF FLIGHT CATEGORY OBJECTIVELY SHOWS CONSISTENCY
OF WFO FORECAST TO AVAILABLE MODEL INFORMATION:
H = HIGH:   TAF CONSISTENT WITH ALL MODELS OR ALL BUT ONE MODEL.
M = MEDIUM: TAF HAS VARYING LEVEL OF CONSISTENCY WITH MODELS.
L = LOW:    TAF INCONSISTENT WITH ALL MODELS OR ALL BUT ONE MODEL.

UTC 1HRLY       11   12   13   14   15   16   17   18   19   20   21   22
EDT 1HRLY       07   08   09   10   11   12   13   14   15   16   17   18
CRW CONSISTENCY  L    L    L    L    L    L    L    L    L    L    L    L
HTS CONSISTENCY  L    L    L    L    L    L    L    L    L    L    L    L
BKW CONSISTENCY  L    L    L    L    L    L    L    L    L    L    L    L
EKN CONSISTENCY  H    L    L    L    L    L    L    L    L    L    L    L
PKB CONSISTENCY  L    L    L    L    L    L    L    L    L    L    L    L
CKB CONSISTENCY  L    L    L    L    L    L    L    L    L    L    L    L

AFTER 12Z SATURDAY...
IFR at times in showers and thunderstorms in the central
Appalachians Saturday afternoon through Sunday night, and then
in the middle Ohio Valley and central Appalachians into the
early part of the next work week.

&&

.RLX WATCHES/WARNINGS/ADVISORIES...
WV...None.
OH...None.
KY...None.
VA...None.

&&

$$
SYNOPSIS...TRM/MC
NEAR TERM...TRM
SHORT TERM...MC
LONG TERM...MC
AVIATION...TRM


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