Area Forecast Discussion
Issued by NWS Wilmington, OH

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FXUS61 KILN 241021
AFDILN

Area Forecast Discussion
National Weather Service Wilmington OH
621 AM EDT WED AUG 24 2016

.SYNOPSIS...
With upper level ridging over the southeastern states, warm and
moist air will begin to flow into the region today. Embedded waves
in the mid-level flow will bring chances for showers and storms
today and Thursday, in the northern Ohio Valley and southern Great
Lakes. High pressure over the Great Lakes will bring drier
conditions for Friday and Saturday, before chances for storms
increase slightly on Sunday and into the new week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
As surface high pressure continues to move away to the east, an
increasing gradient is setting up over the Ohio Valley. Ridging
over the southeastern states is centered due south of the Ohio
Valley, meaning that the mid-level flow is more westerly than
southwesterly. Nonetheless, flow from 850mb and lower will have a
significant southwesterly (theta-e advection) component, with both
temperatures and dewpoints forecast to increase today. The
atmosphere will be increasingly moist (precipitable water of
around 1.8-1.9 inches) and unstable (MLCAPE of 750-1250 J/kg).
Forcing is the clear limiting factor in terms of convective
potential today. Warm frontal processes are failing to generate
much upstream activity early this morning, though a few showers
may still be possible in the northwestern CWA between 09Z-15Z.
With no discernible surface features in the area today, attention
will turn toward forcing slightly further aloft, though notably
focused to the north and west of the ILN forecast area. A
shortwave is forecast to move across southern Michigan during the
late afternoon, with convergence in an area of enhanced
925mb-850mb flow pushing across northern Indiana. This should
spark convection upstream of the region, which may eventually
propagate into the forecast area toward the greater instability.
With storms following this pattern ending up separated from the
stronger flow and the source of lift, it is not surprising that
high-resolution models have been fairly sparse in forecast
coverage this far south. Will keep PoPs in a similar range as the
previous forecast -- 20 percent in the south to 40 percent in the
north. In terms of hazards, hard to see more than perhaps a threat
for some gusty winds as the isolated/scattered storms propagate
forward, but the severe threat over the ILN CWA appears pretty low
without higher-end instability or a better-organized source of
forcing.

GFS/NAM projections indicate a 3C-4C rise in 925mb/850mb
temperatures from yesterday to today, and max temps have been
adjusted accordingly. This should allow for rises from yesterday of
a few degrees in the northern CWA (where clouds will be a little
more prevalent) and as much as 4-6 degrees in the south (where
temps are likely to top out in the upper 80s).

&&

.SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY/...
The thermodynamic environment over the Ohio Valley will continue
to modify from Wednesday into Thursday, with a steady stream of
theta-e advection to the north of the upper ridge. 850mb
temperatures are forecast to gain another 3C-4C by 18Z Thursday,
and forecasts for surface temperatures have responded in kind --
no doubt, also helped by the slightly drier forecast, as will be
discussed later. Interestingly, SREF plumes do not seem to be
totally picking up on the warmer forecasts, and the new max temp
grid for Thursday is way at the high end of the SREF spread (but
in line with raw GFS/NAM projections). This allows for lower 90s
in the southern third of the ILN CWA, with upper 80s elsewhere.

The zone of quick WSW winds aloft will remain in place just
northwest of the forecast area on Thursday, and will actually
become even better established. Still, the forcing along with this
feature (and a weak shortwave) will remain focused too far north
for a direct impact to the middle Ohio Valley, and if anything,
models have been a little more explicit in keeping drier
conditions in place for Thursday. Though this shift means a
lesser chance of convective debris affecting the area on Thursday
morning, it`s still a possibility. That would really be the only
limiting factor for instability, with SREF means (running on
boundary layer temps that may actually be too low) indicating
MLCAPE values of up to 2500 J/kg. Overall, the kinematic and
forcing pattern looks quite similar to Wednesday, but with greater
instability, and a little bit stronger shear as well. That will
mean that if storms are able to propagate this far south and east,
the threat for some strong storms will exist, with a primary
threat of wind damage. Coverage is really the big question mark,
and PoPs remain quite conservative. Until this comes into better
view, it seems premature to add the threat to the HWO.

As the dampening upper level wave moves east into Quebec, a very
weak surface front will make some progress into the ILN forecast
area on Friday, though it may never clear through the area before
stalling (or washing out). The weak forcing warrants nothing more
than a 20-percent chance during diurnal timing in the southern
third of the forecast area, with only the northern half of the
forecast area seeing a notable drop in temperatures from Thursday
to Friday (and even that will only be by about 5 degrees).

A slightly drier air mass, courtesy of a large area of high
pressure, will settle in over the Great Lakes region on Saturday.
Dry and mostly sunny conditions are expected, allowing for
temperatures to increase by a degree or two from Friday, despite
light wind flow and a fairly neutral advection pattern.

&&

.LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/...
For Sunday, moisture and energy spilling southeastward from Great
Lakes low pressure will provide a chance of thunderstorms.
Additional thunderstorms will be possible Monday when the low is
forecast to drag a cold front across Ohio. A few thunderstorms may
persist on Tuesday as the boundary sags gradually south to
Kentucky.

With a large and slow moving high in the upper atmosphere centered
from the Central Appalachians to the Mid-Atlantic coast, and with
mainly warm advection in the lower levels, a return to mid-summer
heat is indicated. High temperatures ranging from the mid 80s to the
low 90s will couple with dew points near 70 to make outdoor
activities uncomfortable at best.

&&

.AVIATION /10Z WEDNESDAY THROUGH SUNDAY/...
A mid level ridge over the lower Gulf Coast states will slowly
build northward through the TAF period. At the same time, embedded
disturbances will rotate eastward through the Great Lakes. Return
moist flow, along with the proximity of the disturbances, will
bring at least a chance of showers and thunderstorms to the
terminals today. Chances do not appear high at this time, but it
looks like all terminals will see enough of a threat for a VCTS/CB
to be employed. Southeast to south surface flow will become south
later this morning around 10 to 12 knots with some local gusts in
the 15 to 20 knot range by afternoon.

For tonight, the latest high resolution/convection allowing
models indicate that the threat for showers/storms overnight will
likely be closer to the lower Great Lakes, or mainly north or
northwest of the terminals. As a result, have just VFR conditions
at this time.

OUTLOOK...Thunderstorms possible from Thursday into Thursday
night, mainly near the northern terminals.

&&

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

&&

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



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