Area Forecast Discussion
Issued by NWS Wilmington, OH

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FXUS61 KILN 180724

Area Forecast Discussion
National Weather Service Wilmington OH
324 AM EDT Fri Aug 18 2017

A weak cold front will exit the area this morning taking any
remaining showers and low clouds with it. A noticeable drop in
humidity is expected but temperatures will remain warm through
the weekend as high pressure builds across the Ohio Valley. A
weakening weather system will cross the area on Saturday with a
threat for showers and thunderstorms, particularly north of the
Ohio River. Dry weather is then expected Sunday and into early
next week as high pressure reasserts itself.


GOES 16 water vapor loops and RAP analyses this morning
indicated a shortwave trough over eastern Wisconsin with the
mid-Ohio Valley still in modest southwesterly mid level flow.
This has led to little airmass change despite pre-frontal
convection overturning the boundary layer to some degree.
Surface analysis indicated that the primary surface cold front
still lagged west of the area, but will be entering the forecast
area around sunrise as the shortwave over Wisconsin spreads
downstream into Michigan. The trough axis will cross the area
this morning, so residual shower activity mainly over the lower
Scioto Valley as of 07Z will slowly clear to the east, though it
may take until after sunrise for the last bit of precipitation
to exit due to the lack of boundary layer push through about 12Z.
Once this push occurs, expect a pretty decent drop in PWAT but
not much low level cold advection. Thus, expect highs to push
back toward the middle 80s, perhaps even a few upper 80s over
northern KY with central OH lagging back in the lower 80s.
Clouds will be somewhat problematic - but there is enough
consensus in forecast soundings that current low stratus which
has formed in the I-71 corridor will begin pushing east as it
mixes out with increasing westerly flow after sunrise. By
afternoon, there should be a healthy amount fair weather cumulus
forming due to residual boundary layer moisture /dewpoints
holding firm in the 60s/ and insolation. So while the day may
start rather cloudy in some areas, think sunshine will win out
by afternoon.


Attention quickly turns to a rather sharp/impressive mid/upper
level shortwave trough which amplifies/digs into the western
Great Lakes this evening, and continues to sharpen/amplify
/especially in 500mb-250mb layer/ as it crosses the area
Saturday morning into early afternoon. Kinematic forcing is
strong aloft, but due to short wavelength with departing wave
from today, and relative PWAT minimum in front of the wave with
little chance for low level trajectories to enhance a flow of
moisture, system seems moisture starved. To that end, despite a
rather impressive mid/upper level presentation to this wave,
lower level cyclogenesis/frontal development is slow to occur
which decreases convergence and necessary boundary layer lift
where instability is adequate.

Expect an area of showers and thunderstorms to be crossing
Indiana later tonight into Saturday morning, likely weakening to
some degree through Saturday morning as it moves through the
forecast area /higher chances along/north of I-70/. Given strong
forcing aloft, don/t expect this precipitation to die off
Saturday morning so this may hamper destabilization to some
degree west of I-71. There does appear to be a small window in
the afternoon in the Scioto Valley where destabilization may
overlap the best forcing for a threat of strong/severe storms.
Forecast soundings at ILN/CMH at midday Saturday certainly would
support organized/strong updrafts with MLCAPE > 1000 J/kg and
effective shears approaching 40kts with broad turning from swly
boundary layer winds to strong west-northwesterly upper
tropospheric flow with sharp upper jet. So there remains
uncertainty on timing and overlap of best diurnal
destabilization and ongoing clouds/rain chances on Saturday that
casts some doubt degree of severe threat. Kept rain chances in
the 40-55% range especially focusing on central Ohio closest to
the better dynamical lift with this system. In cloud/rain free
areas, it will be quite warm again with highs pushing toward the
middle/upper 80s.

Expect the wave to quickly transition east of the area Saturday
evening with rain chances ending from west to east as drier air
filters in, with cooler overnight lows from previous nights.


Guidance is in fairly good agreement for the start of the long term
period, showing surface high pressure parked right over the heart of
the Ohio Valley region in the wake of Saturday/s system. With ample
subsidence, expect dry conditions Sunday as mid level ridging begins
to nudge back northward into the area. With mostly sunny skies and a
strengthening upper level ridge over the southeastern U.S., trended
temps up a degree or two across the board -- with highs in the mid
to upper 80s expected.

By Monday, the high will have shifted east of the immediate Ohio
Valley region, with return flow once again becoming established in
the area. With this, and the midlevel ridging aloft, highs will top
out several degrees warmer than Sunday -- with the potential for
highs near 90 degrees for the southern half of the FA Monday

The main item of interest for Monday will be extent /or lackthereof/
of cloud cover for the impending solar eclipse. While the GFS is
currently showing some cirrus working into western parts of the FA
during the afternoon, the ECMWF shows all thick cirrus remaining
well to the north/west of the immediate area. Even with the
uncertainty regarding coverage or thickness of cirrus, it appears
likely that Monday will be pcpn-free for the entire area. Even with
some thin cirrus, diurnally-driven FEW to SCT Cu are possible during
the eclipse timeframe.

With increasing southerly flow and the corresponding increase in low
level moisture, expect a warm Monday night period, with lows
generally around 70 degrees.

By early Tuesday, increasing low level moisture and the slow
approach of a cold front will allow for the introduction of slight
chance PoPs, especially for northwestern portions of the FA. PoPs
will slowly increase during the day on Tuesday as low level
destabilization occurs in the pre-frontal environment. Even
with this, model solutions are in remarkable agreement (this far
out) showing the front (and corresponding highest PoPs) moving
through Tuesday night (NW to SE). Although it currently appears
that best forcing will hold off until early Tuesday night, any
shift in timing may act to increase or further decrease
alignment/overlap with peak diurnal instability Tuesday.
Nevertheless, any mid level disturbances ahead of the surface
frontal boundary may be enough to initiate storm activity during
the afternoon on Tuesday before the arrival of the front

Behind the front, a big change in airmass will occur as upper level
troughing becomes more amplified east of the Great Lakes region.
Unseasonably chilly temperatures aloft are forecast midweek and
beyond, suggesting a period of below normal surface temperatures for
the second part of the long term period. It appears probable that
the area will see at least one or two days where temperatures do not
make it out of the 70s Thursday and/or Friday.


In the wake of showers which have largely pushed away from the
TAF sites, an area of IFR/MVFR stratus has formed along I-71 and
is impacting DAY/ILN/CVG/LUK, and am expecting this to
spread/develop into the CMH/LCK terminals as well soon. This
area of stratus on the back edge of shield of mid/higher level
cloud but there is a back edge that will be progressing east
/albeit slowly/ given still high boundary layer moisture as weak
cold front works through the area. So through sunrise have hit
low clouds and minor reductions to visibility hard in the newest
TAF issuance. With sunrise, vertical mixing will increase and
winds will become gusty especially at wind prone sites like DAY.
This will serve to break up/lift any residual low clouds and
expect a scattered cumulus deck to form, which may be broken in
coverage for a time at some sites, but for now running
scattered. Expect VFR conditions past about 13 or 14Z. Next
weather system will spread mid/high level clouds toward the TAF
sites on Friday evening.

OUTLOOK...Thunderstorms are possible on Saturday, and again on




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