Area Forecast Discussion
Issued by NWS Wilmington, OH

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043
FXUS61 KILN 250752
AFDILN

Area Forecast Discussion
National Weather Service Wilmington OH
352 AM EDT Sun Sep 25 2016

.SYNOPSIS...
High pressure centered northeast of the area will provide one more
dry and warm day for the region. A cold front will move into the
area early on Monday, bringing a chance of precipitation. Behind
this front, temperatures will become cooler for the rest of the
week, and will be slightly below normal for late September.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Surface high pressure is currently centered somewhere near Ottawa,
with enough influence to keep relatively dry and quiet conditions
all the way southeast into the Ohio Valley. A frontal boundary
previously in the vicinity has stalled out well south of the ILN
forecast area, leaving a pattern that will support weak easterly
flow over the area through the day. Skies are very clear at this
early hour, and cumulus development will likely be somewhat
limited by the dry air mass over the area right now -- with
dewpoints generally in the 50s. Slightly greater moisture exists
near and southwest of Cincinnati, where afternoon cloud
development may be a little more robust. A few high-res models
even suggest that some weak showers could develop in south-central
Indiana, possibly close to the ILN CWA borders. With instability
as limited as it is, and no signs of forcing, this forecast will
be kept dry.

With the front out of the area, the temperature gradient across
the forecast area today should be a little more relaxed.
Essentially still in the warm sector of the next large-scale
weather system, as discussed below, temperatures will remain well
above normal for late September.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY/...
A developing mid-level cyclone is currently located over North
Dakota. Along with a surface low, this feature will move ESE over
the next few days. Within the regime of positive vorticity
advection, a cold front is forecast to clear its way through the
Ohio Valley.

Model agreement on frontal timing is quite strong, leading to high
confidence in the expected frontal passage -- entering the
forecast area in the 10Z-12Z time frame on Monday morning, and
clearing the CWA by 18Z-20Z during the afternoon. However,
confidence in precipitation coverage along and ahead of the front
is not as certain. Though convergence is fair and upper support is
not really lacking, the front is a long distance from its parent
low, and the quality of the deep-layer moisture is suspect (a
narrow ribbon of around 1.50" PW). Now at the tail end of the
range of a few high-res model runs, there are reasonable
expectations that shower activity may be scattered rather than
solid. As a final note, instability is also very marginal (only a
few hundred J/kg of MLCAPE) owing to the passage during the
diurnal minimum. Will keep PoPs only 60 percent at most in the
north and northwest, limited to 40-50 percent further south and
southeast. There is certainly an expectation that coverage (and
quality of convection) may be increasing as the front exits the
ILN CWA. SPC has drawn in a marginal risk for Day 2 over WV /
western PA / far eastern OH. The ILN CWA looks like it will be in
an ideal spot for a minimum in convection, warranting only a
slight chance of thunder with this system.

With frontal passage occurring during the upswing in the diurnal
cycle, the temperature forecast for Monday found its way to being
a little bit complicated. Areas in the far southeastern CWA
(Lewis KY / Scioto OH) will likely climb back into the 80s one
more time, before conditions begin to cool during the early to mid
afternoon. Elsewhere, cold advection will already be ongoing by
late morning or early afternoon, limiting highs to the 70s. In
fact, the far northwestern CWA may struggle just to get to 70.
With the tightening pressure gradient and cold advection behind
the front, some gusts in the 20-25 knot range will be possible
north of I-70.

With a new air mass in place, low temperatures on Tuesday and
Wednesday morning will be in the middle to upper 40s -- the first
time in the 40s since June. Highs will actually come in below
normal for the first time in a while, with max temps for Tuesday
and Wednesday expected to be in the upper 60s to around 70. With
continued subsidence and cold advection, winds are expected to be
breezy again on Tuesday, and wind gust grids were increased from
the previous forecast.

After nearly stalling over the Great Lakes, the mid-level low will
gradually lumber its way toward the Ohio Valley on Wednesday.
There is still a great deal of uncertainty with this system, but
there is enough of a signal to include some 20-30 percent
precipitation chances in the northern sections of the CWA on
Wednesday -- in addition to dropping temperatures there by a
couple degrees.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
A closed upper level low will move across the Great Lakes through
mid week. Energy rotating around the low will eventually allow for
the trough/low to dig down into the Ohio Valley region. The 12z
GFS is more progressive with this feature, keeping it as a trough
and shifting it off the East Coast through the end of the week.
However, the 12Z ECMWF closes off the low and actually retrogrades
slowly west across the Upper Ohio Valley through the end of the
week. This results in some uncertainty in the forecast as we head
through next week. For now, will allow for a few showers across
mainly our north on Wednesday as the better energy transfers down
across our area. Will otherwise maintain a dry forecast through
the long term, although if the retrograding ECMWF ends up being
right, some lower end pops would also be necessary Thursday into
Friday but given the uncertainty, think the best option at this
point is to trend toward a blend. This will lead to below normal
temperatures through mid week with highs mainly in mid to upper
60s. Will then allow for a gradual warming trend through the end
of the week with highs into the mid 70s by Saturday.

&&

.AVIATION /07Z SUNDAY THROUGH THURSDAY/...
Surface high pressure centered over southeast Canada/Eastern Great
Lakes this morning will gradually move east today. Surface
ridging will extend back into the Ohio Valley, while a mid level
ridge traverses west to east overhead. Clear skies this morning
can be expected. The exception will be at KLUK where some IFR
conditions are forecast due to river valley fog. This should burn
off by 13Z. For this afternoon, some scattered diurnal cumulus
clouds will develop over the western terminals.

For this evening, mid level ridge will move east. Diurnal cumulus
will likely dissipate by sunset, leaving mostly clear skies.

For the overnight period, large scale mid level trough will dig
southeast into the western Great Lakes and northern/middle
Mississippi River Valley. An attendent cold front will accompany
this system at the surface. Showers and embedded thunderstorms are
expected along and ahead of the front, which should make in roads
toward the western terminals between 09Z and 12Z.

OUTLOOK...Showers and embedded thunderstorms will bring MVFR
conditions Monday morning and early afternoon. Breezy west winds
expected on Tuesday with gusts between 25 and 30 knots.

&&

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

&&

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



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