Area Forecast Discussion
Issued by NWS Wilmington, OH

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FXUS61 KILN 261027
AFDILN

Area Forecast Discussion
National Weather Service Wilmington OH
627 AM EDT Fri May 26 2017

.SYNOPSIS...
Low pressure will move east of the region this morning, allowing
for an area of high pressure to settle over the Ohio Valley
today. However, an unsettled weather pattern is expected for the
weekend, with several upper level disturbances helping to bring
occasional chances of showers and storms to the area. Behind a
cold front on Sunday night, drier conditions are expected on
Monday.

&&

.NEAR TERM /THROUGH TONIGHT/...
While the middle Ohio Valley remains on the back side of low
pressure over the Mid-Atlantic region, heights are beginning to
rise, and the air mass is gradually drying. Some persistent
but patchy stratocumulus remains in place across the eastern
two-thirds of Ohio, with some lower clouds (and possibly some
fog) also advecting through NE Indiana early this morning. Aside
from these items of interest (which should cease to be an issue
by mid to late morning) a fairly tranquil day is expected. A
flattening of the upper level pattern will allow for more
westerly winds, eventually advecting in some high clouds,
without allowing any sustained ridging to build in. At the same
time, a weak surface ridge will extent into the area from the
southeastern states, but glancing more than encompassing the
Ohio Valley. 5-10 knot westerly flow will thus be in place at
the surface, with max temps in the upper 70s to near 80
(southwestern CWA) to lower 70s (northeastern CWA).

&&

.SHORT TERM /SATURDAY/...
An active weather pattern is setting up for the weekend, with
several chances for convection that will be discussed here and
in the Long Term AFD section below.

As of 3AM EDT on Friday morning, radar and satellite data
indicate an area of convection over northeastern Kansas,
associated with a shortwave trough that is tailing a fairly
sharp mid-level ridge. This shortwave is expected to drive
convection eastward over the next 24 hours, and there are
indications (for reasons discussed below) that the convection
may be strong as it eventually reaches these longitudes late
tonight. It is the latitude of this shortwave that will provide
the most challenging part of the forecast. The key will be in
how quickly (and to what degree) the ridge ahead of the
shortwave is able to dampen. The GFS/GEFS are an interesting
example to look at for this case, with (for a short-range
forecast) an unusual amount of spread in 24HR GEFS 500mb height
contours as the shortwave is expected to be moving into the
region. Using QPF swaths as a first-guess, the operational runs
show a great deal of spread in the location of the axis of
convection, with a range of possibilities that is not even
clustered around specific model types. The operational GFS is
at the far northern end of the solutions (mainly outside the ILN
CWA), joined closely by the NCAR WRF and WRF-ARW. Slightly
further south (affecting the northern half of the ILN CWA) are
the WRF-NMM and ECMWF. The NAM/WRF-NMM/NSSL WRF are further
south, focusing storms along the Ohio River. These specifics are
presented as an example -- obviously, using any single one of
these model runs verbatim would not be a wise choice. However,
given the tendency for convection to propagate along instability
gradients, the northernmost solutions can probably be
discounted.

As mentioned, there is reason for some concern with this
system, particularly for a threat of heavy rainfall. Along with
the shortwave, the convection will be fueled by a low-level
jet, providing a source of convergence and moisture transport
-- in addition to an axis of precipitable water values of over
an inch and a half. This will allow for deep convection to
develop, with the potential for heavy rainfall rates. If there
was forecast confidence in the waterlogged southwestern quarter
of the ILN CWA being affected, there would already be
justification for a flash flood watch. As it is, one might
eventually be needed, but not for an event 24 hours out with
this kind of placement uncertainty. In addition, do not want to
totally negate a potential (though conditional) severe threat,
with sufficient deep-layer westerly shear to support storm
organization. Instability will likely need to be analyzed by way
of MUCAPE (elevated instability) rather than SBCAPE, owing to
the pattern and the time of day. Thus, while storms may not be
surface based, a chance for large hail could exist.

After the shortwave has moved through the region, it is somewhat
unclear on whether or not additional storms will be able to
develop during the day on Saturday. There will definitely be
increasing low-level moisture and steepening mid-level lapse
rates, but capping and subsidence behind the shortwave may limit
this potential, at least through the afternoon. Will keep
lower-end PoPs in place, and mainly focused in the southern
sections of the CWA.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
A frontal boundary will be laying near the Ohio River Saturday
night, and then it is forecast to pivot northeast as a warm front on
Sunday ahead of a surface wave of low pressure. Uncertainty still
exists at this juncture on the potential timing and coverage of an
MCS to eject out of the mid Mississippi Valley into the Ohio Valley
Saturday night into Sunday morning. Have broad brushed PoPs, with
likely showers and thunderstorms currently favoring locations along
and south of the Ohio River where the axis of instability will be
favored. Damaging winds, large hail, and locally heavy rainfall will
all be a concern during this window of convection. Later Sunday
afternoon and night, surface wave of low pressure will move
northeast to the southern Great Lakes, which will eventually allow a
cold front to push eastward through our region. Again, will be
concerned with severe weather potential ahead of the cold front
along with locally heavy rainfall. Will mentioned hazardous weather
threats in the HWO.

A large upper level low will slowly track across Ontario for the
first half of the week and then weaken and lift Thursday. This will
result in generally cool conditions across the region through the
rest of the period. A couple of embedded disturbances rotating
around the upper low may bring some showers to northern zones Monday
afternoon, and again on Tuesday.  Models then diverge at the end of
the week as to how fast a cold front will push through the region.
As such, have kept a low threat for showers/storms on Thursday.

&&

.AVIATION /12Z FRIDAY THROUGH TUESDAY/...
MVFR clouds have now almost completely cleared out from the TAF
sites, and some stratus / mist over northwestern Ohio is
unlikely to make it to any of the airports. Thus, aside from
perhaps a brief MVFR ceiling or visibility in the first hour of
the valid TAFs, VFR conditions are expected through the day
today, with WSW winds generally at or below 10 knots.

Clouds will begin to lower and thicken tonight. It is appearing
likely that thunderstorms will move into the area some time
after 00Z, though it is not yet clear exactly where they will
move. This means that thunder (and lowered conditions) will not
yet be included in the TAFS, but at some point it will likely
need to be, once it becomes clear which sites will be impacted.
Where storms do occur, MVFR/IFR conditions are expected to
occur.

After the storms move out, some showers may persist through the
overnight and going into the day on Saturday. Ceilings are
likely to drop to MVFR, and IFR ceilings are a possibility.

OUTLOOK...MVFR ceilings and visibilities are possible Saturday,
along with a chance of thunderstorms. Intermittent chances of
thunderstorms will remain possible from Saturday through Monday
morning.

&&

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

&&

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


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