Area Forecast Discussion
Issued by NWS Wilmington, OH

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FXUS61 KILN 271107
AFDILN

Area Forecast Discussion
National Weather Service Wilmington OH
707 AM EDT Sat May 27 2017

.SYNOPSIS...
With warm and moist air over the Ohio Valley, there will be
occasional chances for showers and thunderstorms through the
weekend. After a cold front moves through the region Sunday
night, drier conditions are expected for Monday.

&&

.NEAR TERM /THROUGH TONIGHT/...
Pseudo-zonal / slightly ridged mid level flow currently exists
over the Ohio Valley. The latest surface observations indicate
that there is a weak surface low over central Indiana. One
shortwave has helped spark the convection over the ILN CWA early
this morning, and GOES-16 (preliminary / non- operational)
water vapor (CH09/CH10) imagery suggests there is another weak
vorticity maxima over northern Illinois, likely somewhere in the
925mb-850mb layer. At the surface, there is a boundary
stretching across the region roughly from west-to-east, though
it is somewhat wavy (in part due to the surface low). A well
advertised low level jet has been helping to bring increased
theta-e and moisture convergence just off the surface, but this
influence is going to continue to shift ESE with time. Thus, as
the lead shortwave continues to move east as well, convective
potential will wane over the next few hours, leaving most of the
ILN CWA dry by daybreak. Over the boundary that will remain in
place, isentropic ascent may lead to some showers continuing for
the next few hours, primarily in the northern half of the CWA.
However, the stable near-surface air north of the boundary is
obvious, as the convection moving across the northern tier of
ILN counties has produced several ripples / gravity waves that
are evident on mosaic radar imagery.

As the Indiana surface low moves east today, the surface
boundary will remain in place across the ILN CWA, causing a
gradient in theta-e across the forecast area. In subsidence
behind this early morning shortwave, generally dry conditions
are expected for the first part of the day. The forecast is less
clear going into the afternoon. What appears fairly certain is
that increasing heat (temperatures near 80 in the southern CWA)
and humidity (dewpoints in the mid to upper 60s in the southern
CWA) will combine with steepening mid-level lapse rates
(eastward-advecting EML) to produce significant instability --
CAPE values as high as 2000-3000 J/kg in the southern half of
the forecast area. As is usually the case, big numbers are not
the whole story, as the biggest negative factor for convection
appears to be the presence of a 750mb-800mb cap. It seems that
the consensus of model soundings would support this cap largely
holding north of the boundary, but there is more of a range of
possibilities near and south of the boundary during peak
heating. Forcing is also a concern, as the boundary can
certainly act as a focus of convergence, but mid/upper support
appears to be lacking. Nonetheless, the environment will be
supportive of strong to severe storms, based almost on the
instability values alone. There does appear to be sufficient
deep-layer WNW shear to allow for some organization as well,
but wind speeds aloft actually begin to decrease after 21Z. That
lays out the conditional threat -- if the cap breaks, and if
the mid-level flow has not begun to weaken too much, some
organized severe storms will be possible (mainly in the southern
third of the CWA). Looks mainly like it would be a
wind/hail/rain threat -- even though supercell structures could
be possible, low level flow is too weak for more than a very
isolated tornado threat. However, as the upper flow is kind of
parallel to the boundary, there is at least some concern for
training storms and maybe flash flooding. A flash flood watch
was considered, but would like more confidence in the cap
breaking and storm coverage before issuing.

&&

.SHORT TERM /SUNDAY/...
After any storms along the boundary have dissipated or moved SE
of the ILN CWA this evening, another break in the action is
expected. From here, attention will turn to the severe
(potentially very severe) convection occurring well upstream
across Missouri and southern Illinois. This activity is not even
expected to reach the lower Ohio Valley until late evening, and
thus, any impacts on the ILN CWA will be well into the overnight
hours. 00Z runs across the suite of models have been fairly
consistent in depicting this activity propagating into the
instability / greater theta-e air mass, roughly along an ESE
trajectory. The ILN CWA will be on the northern fringe of this,
still in a favorable area for convection to develop ahead of a
mid-level shortwave, but removed from the best thermodynamic
environment at a sub-optimal time of day. A severe threat will
exist -- mainly a damaging wind threat -- but should not be as
significant as areas further south and west. The main focus here
will be in the overnight / early Sunday morning hours.

Once the morning activity has passed east, another break is
expected through the first half of Sunday. Additional storms
associated with the cold front will be discussed in the Long
Term AFD section below. With less certainty in the pattern, will
not allow a completely dry forecast for this time period, as
there should still be some isentropic ascent with continued warm
advection aloft.

The temperature forecast through the first three periods of the
forecast is one that is largely dependent on convection and
clouds, so perhaps it would be better read as a mid-point of the
expectations -- get some clearing and it will be too low, get
some convection and it will be too high. There will be a general
SW-to-NE gradient in temperatures across the ILN CWA through the
period, and depending on the position of the surface boundaries,
a tighter gradient may exist than what is currently in the
forecast.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
A cold front will move east through the region Sunday night.
Scattered showers and thunderstorms are expected along and ahead of
the front. A few strong to severe storms will be possible late
Sunday afternoon into the evening hours with damaging winds and
large hail the primary severe weather threats. Precipitation should
exit east of the eastern CWFA after midnight, with clearing expected
behind the front.

The first half of the week will feature a large closed upper level
low which is forecast to rotate slowly east across Ontario. Embedded
disturbances rotating around the low, along with weak sfc trof axes,
will bring a low chance threat of a few showers from time to time,
mainly during the peak heating of the day. Otherwise, partly cloudy
conditions with seasonable temperatures in the 70s are expected.

As the upper level low finally rotates farther east by the end of
the work week, we should finally see a distinct dry period as
surface high pressure settles over the Ohio Valley. Again,
temperatures will be near seasonal normals.

&&

.AVIATION /11Z SATURDAY THROUGH WEDNESDAY/...
Some patchy MVFR ceilings remain in place across the region, but
these should lift to VFR over the next few hours, and VFR
conditions are expected to prevail through the day today.
Chances for precipitation will be limited to passing light
showers this morning, before dry conditions are expected late
morning into the afternoon. Winds will be westerly to
northwesterly at generally around 10 knots.

During the mid to late afternoon, there will be a chance of some
showers and storms developing near Cincinnati. However,
confidence remains too low to include specifically, so a VCSH
will be used in the TAFs.

Additional chances for showers and storms will be possible late
Saturday night into Sunday morning, along with MVFR ceilings.

OUTLOOK...Intermittent chances of thunderstorms along with MVFR
ceilings will be possible from Saturday night through Monday
morning.

&&

.ILN WATCHES/WARNINGS/ADVISORIES...
OH...None.
KY...None.
IN...None.

&&

$$
SYNOPSIS...Hatzos
NEAR TERM...Hatzos
SHORT TERM...Hatzos
LONG TERM...Hickman
AVIATION...Hatzos


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