Area Forecast Discussion
Issued by NWS Wilmington, OH

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FXUS61 KILN 161914

Area Forecast Discussion
National Weather Service Wilmington OH
314 PM EDT Wed Aug 16 2017

As surface low pressure moves into the Great Lakes, increasing
southerly flow will bring warmth and moisture into the Ohio
Valley tonight into Thursday. A cold front will move into the
area later on Thursday, bringing a chance of showers and storms
during the afternoon. Drier and cooler conditions are expected
on Friday, as high pressure moves into the area.


Surface low pressure is currently located in the central plains,
and will be moving gradually northeast into the northern Great
Lakes by tomorrow. The Ohio Valley is still sufficiently ahead
of this system, such that stronger southerly flow has yet to
envelop the region. Modest instability has allowed for some
showers to develop this afternoon, though absent any notable
low-level forcing, there should not be significant coverage of
precipitation. Given the diurnal nature of this activity, and
signals from recent high-res model runs, PoPs through the
overnight hours have been decreased (would not be surprised if
much/all of the CWA was dry through most of the 04Z-12Z time
frame). With surface dewpoints remaining high -- upper 60s /
lower 70s -- min temps will likely be close to the same.


The low pressure system moving into the northern Great Lakes
is forecast to more or less hold its intensity, rather than
deepening or weakening, as it progresses forward. Mean WSW flow
aloft will align over the Ohio Valley as this low moves
northwest of the area, with increasing southerly flow building
into the region. This will lead most notably to a further
increase in moisture -- with surface dewpoints climbing into the
lower to middle 70s, and precipitable water values increasing to
near or slightly above 2". Conditions to start the morning will
likely be more dry than wet (despite some solutions that appear
overdone from the 12Z GFS/NAM) but convection should begin to
develop relatively freely by late morning and into the early

It would be easy to say that the incoming cold front will drive
the convective forecast for the day, but it is not that simple.
Since the surface low is not deepening, the orientation of the
front actually becomes slightly less favorable with time, and
there is no hard gradient in wind direction or theta-e to pin a
timing forecast on. Rather, a pre-frontal trough or general
convergence will help to organize (or semi-organize) storms
before and during peak heating, before downstream propagation
takes over later in the day. With the actual cold front not
moving through until late, a few showers may persist into the
late night, though waning (or used-up) instability will limit
the strength of this convection.

In terms of potential hazards during the afternoon storms, both
shear and instability look decent but not spectacular. While
there is some turning in the low levels, overall bulk shear
values are around 30-35 knots at 0-6km, closer to the higher end
of that range in the northwestern ILN CWA. MLCAPE values should
be able to reach 1500 J/kg, at least in pockets, but the
increasing moisture aloft raises cloud concerns that could limit
stronger heating. While this scenario could support a few strong
to severe storms, the threat appears marginal overall. The going
HWO mention appears sufficient, so no changes will be made.
The amount of moisture in the atmosphere will support heavy
rain, but the generally-progressive flow should keep flash
flood issues localized rather than systemic.

PoPs were increased to 70 percent for the entire CWA, timed
across the CWA from 18Z to 00Z, with somewhat-generous gradients
of lesser PoP values on either side of the peak. No big changes
to the temperature forecasts for tomorrow or tomorrow night,
allowing for mid 80s during the afternoon and upper 60s to
around 70 by early Friday morning.


Models continue to adjust their timing of cold frontal passage and
convection Thursday night into Friday morning. Latest runs have come
into good agreement on a slightly quicker solution, which now leaves
the fa dry at 12Z Friday as the pcpn is shunted off just e of the
area. Wly flow behind the front should bring lower dewpoints and
more stable air, so dried out Friday.

H5 s/w is still advertised to swing through the Great Lakes on
Saturday, but the amount of pcpn it brings is not certain. Went with
chance PoPs across the north, tapering down to slight chance in the

As the s/w pulls away Saturday night, high pressure builds in at the
surface. It will keep the region dry both days. By Tuesday, models
drop energy into the region from the Great Lakes, bringing scattered
thunderstorms. The GFS is quicker with the return of the pcpn than
the ECMWF or the Canadian hemispheric as it brings QPF in by 12Z
Tuesday. Decided to go with a blend of the extended, so brought 20
PoPs into the west. Convection should develop ahead of the front on
Tuesday into Tuesday evening. PoPs should be decreasing from the nw
to se on Wednesday.


Isolated/scattered and disorganized showers are developing over
the region today, with a slight chance of a thunderstorm in the
Cincinnati area this afternoon. It still appears this will be
too isolated to include specifically in any of the TAFs, outside
of running a VCSH at KCVG/KLUK.

With mid-level clouds moving into the region overnight, it is
unclear how much fog will be able to form. However, given the
past few nights and the abundant low-level moisture, some MVFR
visibilities appear possible by morning.

Storms will become more widespread tomorrow afternoon, though
the most likely timing will be after the end of the TAF period.
Nonetheless, some rain or storms may be possible in the late
morning and early afternoon. Widespread MVFR or lower conditions
are not expected as of now.

Wind gusts of 15-20 knots will be possible tomorrow afternoon.

OUTLOOK...Thunderstorms are possible Thursday afternoon and




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