-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KRLX 220953
AFDRLX

Area Forecast Discussion
National Weather Service Charleston WV
553 AM EDT Sat Jul 22 2017

.SYNOPSIS...
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Strong to severe thunderstorms are possible this weekend with
risk for heavy rainfall and potential flooding. Fronts cross
late Sunday and again late Monday. High pressure crosses
midweek. Next front late week brings more relief next weekend.
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&&

.NEAR TERM /THROUGH TONIGHT/...
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As of 600 AM Saturday...

Another batch of convection has developed north of CRW into
s-central OH. Therefore made some grid adjustments to account
for this activity early this morning.

There remains some uncertainty regarding timing and evolution
of convection later today. However, the latest HRRR/HRRRx are in
general agreement with other convective-allowing model (CAM)
guidance in depicting a re-intensification of remnant MCS
activity spreading east-southeast from OH across at least the
northern 1/2 of WV by 18z. Any outflow to the south of the more
organized activity moving into PA could trigger additional
diurnally driven storms from the Tri-State area eastward into
the central Appalachians. Deep moisture will remain in place
along with very unstable conditions which should promote more
storms into tonight.

SPC has maintained a SLGT risk across all but the far southern
portion of RLX CWA. High PWs along with increasing westerly flow
aloft suggests damaging winds are the primary severe threat.
Other concern is for rounds of heavy rain and risk for flash
flooding. Confidence in QPF details is lower with time but
potential is there for some training cells with local rain rates
exceeding 2 in/hr. A flash flood watch was considered in
collaboration with ILN but thinking this would be a more short-
fused watch situation with higher confidence in convective
rainfall trends.
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&&

.SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/...
As of 420 AM Saturday...

Convectively active, wavy, roughly west to east oriented
front, right over the area first thing Sunday morning, sinks
southward through the area during daylight the morning hours.
However, with the cold front still west of the Ohio River,
models insist the atmosphere easily recovers, with CAPE values
climbing to 4 kj/kg over the middle Ohio Valley Sunday
afternoon, to go along with modest deep shear into the 30s.
Models do show hints of the atmosphere being worked over, with
holes in the 2 inch PW air, where values are progged as low as
1.5. With a flat pressure gradient per disorganized low
pressure along and south of the cold front, mesoscale processes
will be dominant.

All things considered, the Sunday forecast is complicated, but
likely to be active, especially with forcing being provided by
the front, and a digging upper level short wave trough
approaching from the northwest. This is reflected in SPCs slight
risk for damaging wind, and perhaps even large hail, given
thick CAPE through the hail growth level and the modest deep
layer bulk shear, although the discussion suggests that the
forecast area may be between complexes to the southeast and
northwest.

The marginal risk for excessive rainfall reflects the variable
PW distribution, and the weak low level inflow, but still
copious amounts of moisture in place. Flood watch issuance
would likely follow a shorter summer fuse similar to convective
watches, as the convective complexes evolve and track.

With loss of heating, and the ring of fire shifting south as
the upper level trough digs in, convection should gradually
wane Sunday night. The upper level short wave trough pushes a
second cold front through on Monday into Monday night, but
moisture will become a bit more limited by then, and storm
coverage should be limited and the storms on the move. This
should limit any further water concerns, but daytime heating
driving CAPE values to perhaps 3 kj/kg in said narrow band could
still lead to strong storms in modest deep layer bulk shear.

Canadian high pressure brings cooler and less humid air in for
Tuesday and Tuesday night.

Convection spells high bust potential on highs Sunday or Monday,
but lows and dew points will be typical of the summer muggies.
Central guidance Monday night through Tuesday night reflect
values returning to near normal in the fresher canadian air.

&&

.LONG TERM /WEDNESDAY THROUGH FRIDAY/...
As of 420 AM Saturday...

High pressure exits Wednesday, and afternoon thunderstorms are
possible in the mountains on return southerly flow.

A cold front approaches Thursday, and moves through Thursday
night, with a round or two of showers and thunderstorms likely.
Model divergence suggests it may take until sometime Friday to
get the front, and its associated precipitation, entirely south
of the area.

A large canadian high pressure system will bring fresh, cooler
and less humid air by next weekend, as the pattern amplifies
with a deepening trough in the eastern U.S., and a building
ridge in the west.

Central guidance reflects warm weather Wednesday through
Thursday, followed by near, or even slightly below normal
temperatures in the drier air for next weekend.

&&

.AVIATION /09Z SATURDAY THROUGH WEDNESDAY/...
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As of 600 AM Saturday...

Big change with the 22/09z TAF update was to improve visibility
at most terminals based on observational trends. More isolated
convection has developed over the northern 1/2 of the airspace
with KPKB most likely to be briefly impacted. The other
significant change with the intermediate update was to adjust
timing of thunderstorms expected to spread east-southeast from
OH and re-intensify later this morning into the early afternoon.
The northern 1/2 of the airspace is the area most likely to
experience thunderstorm impacts between 14-18z and added 2hr
IFR vis TEMPO groups to KPKB, KCKB and KEKN. Convection may be
a bit more isolated over the southern airspace and will probably
stick with VCTS at KCRW, KHTS and KBKW. Not quite sure how to
play the later periods in the wake of the early-day storms with
thunderstorm risk lasting through tonight. There may also be
some areas of fog but later issuances can address this better
going forward.

FORECAST CONFIDENCE AND ALTERNATE SCENARIOS THROUGH 06Z SUNDAY...

FORECAST CONFIDENCE: Medium.

ALTERNATE SCENARIOS: Limited impacts from upstream thunderstorms
early today and no fog formation tonight into Sunday morning.

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.

DATE               SAT 07/22/17
UTC 1HRLY       09   10   11   12   13   14   15   16   17   18   19   20
EDT 1HRLY       05   06   07   08   09   10   11   12   13   14   15   16
CRW CONSISTENCY  M    M    M    M    H    H    H    H    H    H    H    M
HTS CONSISTENCY  M    M    M    M    H    H    H    H    H    H    H    M
BKW CONSISTENCY  M    M    M    M    H    H    H    H    H    H    H    M
EKN CONSISTENCY  L    L    L    L    M    M    M    H    H    H    H    M
PKB CONSISTENCY  L    L    L    L    M    M    H    H    H    H    H    M
CKB CONSISTENCY  M    L    L    L    M    M    M    M    H    H    H    M

AFTER 06Z SUNDAY...
IFR possible Saturday night and Sunday in heavy showers and
thunderstorms.
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&&

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

&&

$$
SYNOPSIS...TRM/99
NEAR TERM...99
SHORT TERM...TRM
LONG TERM...TRM
AVIATION...99