Area Forecast Discussion
Issued by NWS Wilmington, OH

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FXUS61 KILN 281054
AFDILN

Area Forecast Discussion
National Weather Service Wilmington OH
654 AM EDT Wed Sep 28 2016

.SYNOPSIS...
A large area of low pressure will move into the Ohio Valley today,
moving very little throughout the rest of the week. This will result
in cloudy and cool conditions, with occasional chances for rain
through Saturday. By later in the weekend, warmer and drier
conditions will return, as the area of low pressure moves away to
the northeast.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
The weather through the next several days will be dominated by a
large upper low, currently moving southward into the Ohio Valley
from the Great Lakes. Mid-level clouds associated with this system
are already encroaching on the ILN forecast area, with chances for
precipitation expected to increase over the next few hours.

Though it is easy to describe this system as a stacked low, in
reality, the surface low is fairly poorly organized. This will
result in an muddled boundary layer wind field. It also means
that forcing for precipitation will largely be driven by features
above the surface, especially during the morning. 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. This is expected to allow for
the rapid development of precipitation by 12Z, expanding in
coverage over the northwestern corner of the forecast area. As
positive vorticity advection forcing increases going into the
afternoon, and a weak surface boundary does manage to work its way
in from the west, convective coverage is expected to expand
considerably by 18Z. This appears to warrant categorical PoPs for
nearly the entire forecast area, with the expectation of one or
several curved bands of broken convection developing and moving
east. As the center of the upper low moves into southeast Indiana
this evening, precipitation chances appear to lessen considerably
after 21Z.

Trying to forecast mesoscale details in a complex low pressure
scenario like this one is often difficult (if not impossible) at
distance in time. Now that this event is in the first period of
the forecast, and is being resolved by an array of
higher-resolution models, the specifics are coming better into
view -- and they are somewhat interesting. Steep lapse rates (both
low-level and mid-level) have always been expected, with cold air
associated with the upper low moving into the Ohio Valley. 850mb
temps are expected to range from 7C-12C from northwest to
southeast by afternoon, with freezing level heights of around
8000-9000 feet. A significant tropopause fold exists, reaching
down to about 450mb. One interesting way to visualize this is to
compare a 300mb RH plot -- which is in the stratosphere, and thus
is bone dry -- to a saturated 500mb RH plot near the center of the
low. All of these factors work to decrease atmospheric stability,
even without an expectation for significant surface heating.
However, with some southerly low-level flow remaining in place
ahead of the surface boundary, some heating may indeed occur
(especially in the southern sections of the forecast area). A
conservative instability forecast would suggest MLCAPE values of
around 300-600 J/kg, but if temperatures can get into the lower
70s, values could perhaps approach 800 J/kg. These numbers may
sound small, but this is nowhere near a typical summertime
convective situation -- with steep lapse rates, this CAPE will be
favorably located at temperatures cold enough to support the
development of both lightning and hail. Thus, coverage of thunder
in the grids has been expanded significantly.

Though steep lapse rates and marginal instability were always
expected, the presence of favorable wind shear was much less
clear, as it had a lot to do with the juxtaposition of the surface
boundary and the larger-scale flow around the low. It now appears
that shear will be rather favorable across the ILN forecast area,
with 45-55 knots of deep layer southwesterly shear expected
during the afternoon (though values are a bit lower north and west
of Dayton). Recent HRRR/NCAR Ensemble/NAM runs are also suggesting
a brief period of time in which low-level shear may become
favorable. As was mentioned earlier, the low-level wind flow is
somewhat disorganized. However, if some modest southerly flow can
establish itself ahead of the surface boundary, there will be a
decent bit of turning in the lowest 1-2 kilometers. This is one
mesoscale aspect to the forecast that remains uncertain, and will
need to be examined in real time, as it could enhance the severe
threat if it occurs.

Ultimately, these parameters support the development of a few
low-topped severe storms, possibly with supercellular
characteristics. Hail appears to be the greatest threat, with
organized updrafts in a cold environment. While some small hail
may be possible with the morning activity, large hail appears more
likely with storms during the afternoon. A chance for gusty winds
will probably be limited to storms near the surface boundary.
While a tornado threat is not completely out of the question, it
would really require all the shear/instability to come together
just right, especially with LCLs looking rather VFR-ish and not
altogether favorable.

For temperatures, 3-hourly values were drawn in through the day,
in order to capture the slightly non-diurnal trends expected. In
the northwestern CWA, where rain will develop earliest,
temperatures will not rise significantly -- only into the lower to
middle 60s. However, in the far southeastern part of the forecast
area, temperatures may climb significantly through about 18Z --
possibly as high as the mid 70s.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY/...
Through the rest of the short term forecast, the center of the upper
low will gradually move southward (Thursday) before beginning a
northward progression (Friday) on its way out of the Ohio Valley
(Saturday). The short version of the forecast is that continued
cool, cloudy, and occasionally rainy conditions can be expected.
through the end of the week. As is unsurprising, the mesoscale
features from Thursday through Saturday are a little tougher to
resolve with certainty, as small differences in the model runs can
result in changes in the placement and timing of shortwaves. In
addition, when the center of the upper low is close to the ILN
CWA, shortwave/PVA forcing will be next-to-zero -- resulting in
occasional periods where widespread or heavier rainfall will be
very unlikely to occur.

As such, PoPs through the rest of the week generally follow a
model compromise in forcing timing and placement, with
expectations that this will need to be refined in the coming days.
Likely PoPs were used on Thursday afternoon for the northeastern
two-thirds of the ILN CWA, as there is reasonable confidence in
this being a favored area for ascent, with the low center about
200 miles to the southwest over central Kentucky. Instability
appears marginal, but nonetheless, a slight chance of Thunder
will be re-introduced for Thursday -- as lapse rates will remain
steep enough to allow for CAPE to reach high enough to allow for
electrification of at least a few of the deeper cells. Some small
hail could be possible also, but marginal instability and much
poorer shear than Wednesday will make a severe threat unlikely.
Owing to the aforementioned uncertainty, PoPs through the rest of
the week have been kept in the 20-40 percent range.

&&

.LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/...
When the low lifts farther to the northeast on Sunday, decreasing
moisture and forcing will be evident. There will be a slight
chance of showers in shallow residual moisture. Dry weather
appears in the forecast for Monday and Tuesday under high pressure
at the surface and aloft.

Expect below normal highs in the 60s Friday and Saturday under
clouds, precip, and low geopotential heights. A rebound back into
the 70s should be noted by Tuesday in a regime of warm advection
and insolation.

&&

.AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
A large scale upper level low will dig southeast into the Great
Lakes and Ohio Valley through the TAF period.

Jet energy rotating around the digging upper level low will
combine with some low level convergence above the surface to
generate showers and thunderstorms. Given the current dry
conditions across the region, and the elevated nature of the
thunderstorms, ceilings should stay VFR for the most part.
However, moderate to perhaps local heavy showers/storms will
lower visibilities at least into the MVFR category with some
pockets of IFR visibilities possible where heavier showers/storms
occur. Best chance for thunderstorms this morning will occur at
the KDAY, KCMH, and KLCK terminals. This is also where a brief
period of IFR visibilities may occur. KCVG, KLUK, and KILN will
also see some showers/MVFR visibilities, but the threat for
thunder may be a little less. Will make a last minute decision if
VCTS/CB will be need at these locations through the morning.

Operational models and high resolution models indicate that there
will be at least two waves of precipitation today. The first wave
will rotate east/northeast through early afternoon. Thereafter,
surface low pressure and a cold front is forecast to push east
through the region this afternoon and evening. It now looks like
the second round of precipitation will be associated with another
piece of energy aloft along with a prefrontal trof axis.
This feature will bring another round of showers and
thunderstorms. Again, ceilings should remain mostly VFR but
visibilities will fluctuate between MVFR and IFR in heavier
showers/storms.

The actually frontal boundary will work its way into the forecast
area late this afternoon and evening as surface low pressure
begins to translate toward the southeast. There could be a few
shower/storms along this boundary before precipitation shows a
decreasing trend in coverage.

For tonight, center of the upper level low is expected to drift
southward toward KSDF while the surface low rotates east/southeast
toward south central Ohio/eastern Kentucky. With winds becoming
light to calm, and with cooling, low clouds and fog will develop
especially between 06Z and 12Z time frame. Have gone with IFR/LIFR
conditions.

OUTLOOK...IFR/LIFR ceilings and visibilities possible Thursday
morning. MVFR ceilings and visibilities possible with showers
Thursday afternoon through Saturday.

&&

.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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