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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KILN 271044
AFDILN

Area Forecast Discussion
National Weather Service Wilmington OH
644 AM EDT Tue Sep 27 2016

.SYNOPSIS...
A large area of low pressure will gradually move south from the
Great Lakes over the next day or so, eventually remaining in place
over the Ohio Valley through the end of the week. After a dry and
breezy day today, cooler and rainier conditions are expected from
Wednesday through Friday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
The Ohio Valley is currently between a cold front (which passed
through the area yesterday morning) and a sprawling area of
stacked low pressure (which is currently centered just north of
Lake Superior). In between the two systems, a fairly dry air mass
is in place, with the 00Z KILN sounding recording a precipitable
water value of just 0.35 inches. This dry air mass will present
little in the way of weather concerns for today -- perhaps just a
few cumulus, but even that is not a certainty.

Winds may actually be the most interesting part of the weather
story for the next 12-18 hours, as the pressure gradient is still
strengthening, as the Great Lakes low gradually moves south. The
west-southwesterly flow is fairly deep, and with sunny conditions
in a dry air mass, deep mixing is expected to occur. This will
fairly easily result in wind gusts of around 25 knots (perhaps up
near 30 knots) in the northwestern corner of the forecast area.
There should actually be a fairly quick drop-off in wind speeds
with southeastward extent, as the pressure gradient (especially in
the 925mb-850mb layer) weakens considerably closer to Kentucky /
SE Ohio / West Virginia.

Boundary layer RH values are probably not quite low enough today
to run the risk of fire weather concerns, but with the gusty winds
in place, it is something that should at least be kept in mind.
Thankfully, much of the CWA saw at least some light rain during
the past 24 hours, with the heaviest rain occurring in the
northwest -- where winds today are going to be the strongest.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH FRIDAY/...
As model agreement strengthens in the overall scenario for
Wednesday through Friday, the forecast will continue to be
adjusted as has been done tonight -- slightly cooler temperatures,
slightly greater cloud cover, and slightly higher chances of
precipitation. For the most part, it still looks well too early to
try to time out the vorticity maxima rotating around the huge
upper low, especially as model agreement in the exact position of
the system is still only marginal at best.

With that said, attention will focus to the first such area of
upper support, which will begin to develop directly over the
forecast area early on Wednesday. This will occur as the overall
envelope of the low is just getting into the area from the north.
NAM/GFS projections show a jet streak rotating around the southern
periphery of the low, with its left exit region tracking directly
through the ILN CWA between 12Z-18Z. Combined with a weak surface
boundary (and a boundary layer wind shift to the west) there is
enough evidence to allow for an increase to likely PoPs in the
eastern half of the forecast area for Wednesday (late morning
through mid-afternoon). Owing more to the cooling temperatures
aloft than anything else, some marginal surface-based instability
is expected to develop, and a few low-topped thunderstorms will
likely result from this -- especially if any breaks in the clouds
end up allowing for some slight surface heating.

Beyond Wednesday, it is much less certain where precipitation
chances will be favored -- or even which quadrant of the upper
low the ILN CWA will fall under. PoPs will be kept in the 20-40
percent range all the way into Saturday, favoring the diurnal peak
periods slightly. Though a little more questionable than
Wednesday, a slight chance of thunder was included for Thursday as
well.

Max temps will be held in the middle 60s through the period, owing
to considerable uncertainty in potential insolation, and the
expectation of occasional rain. This forecast is about 4-8 degrees
below normal for max temps. The air mass would support values
near 70 if the upswing of the diurnal cycle could coincide with
dry and partly sunny conditions, but confidence in a window like
that occurring is fairly low.

&&

.LONG TERM /FRIDAY NIGHT THROUGH MONDAY/...
The GFS has continued to trend more in line with the ECMWF,
lingering the upper level low across the region through at least
the end of the week. As a result, have continued to nudge up pops
each day through Saturday and tweaked down temperatures a bit.
Beyond that, both the GFS and ECMWF are trying to lift the low
northeastward and out of the area, although the ECMWF is still
slower in that process. At this point, will keep Sunday and Monday
dry but again, if things continue to trend toward the slower
ECMWF, we may need to introduce some low end pops into early next
week and lower temperatures a bit more toward the end of the long
term period.

&&

.AVIATION /12Z TUESDAY THROUGH SATURDAY/...
-- Changed Discussion --
Skies will remain clear with generally light winds from the
south/southwest through mid morning or 15Z.

Otherwise, our region will find itself between a departing cold
front to our east and a large scale upper level low dropping south
into the western Great Lakes today. An increasing pressure
gradient, combined with fairly decent diurnal mixing, will allow
winds to increase and become gusty from the west/southwest
between 15Z and 17Z. Wind gusts will range between 20 and 28
knots with the highest gusts expected at KDAY. At the same time,
models indicate a subtle moisture channel in the low levels, which
should result in a few high based diurnal cumulus clouds by 17Z.

For tonight, cumulus will flatten to some low altocumulus and will
be mixed with some cirrus through early morning. Upper level low
pressure will continue to move toward the southeast. Models
indicate that a cold front will begin developing to our west. The
combination of this approaching front and dynamic lift associated
with energy rotating around the upper level low will cause clouds
to increase/thicken between 09Z and 12Z with a few light
showers/sprinkles possible by 12Z.

OUTLOOK...Local MVFR ceilings and visibilities possible with
showers on Wednesday. Widespread MVFR ceilings with local MVFR
visibilities possible with showers Thursday through Saturday.
-- End Changed Discussion --

&&

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

&&

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