Area Forecast Discussion
Issued by NWS Wilmington, OH

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FXUS61 KILN 190738

Area Forecast Discussion
National Weather Service Wilmington OH
338 AM EDT Sat Aug 19 2017

An upper level disturbance will move through the region on
Saturday bringing showers and thunderstorms to the region. High
pressure will then be in place for Sunday and Monday. A cold
front will approach the region on Tuesday bringing a return of
precipitation to the area.


Spent a great deal of time on today`s forecast given potential
rain/thunderstorm chances and less-than-desired confidence on
how things will evolve.

At mid/upper level shortwave trough was crossing
Lake Michigan with seasonably strong southwesterly mid level
flow downstream over the Ohio Valley. 50-60m height falls
/500mb/ were accompanying the trough and an area of
thunderstorms had recently developed immediately downstream of
the strongest PVA with the wave across northern Indiana with a
trajectory towards northwest Ohio. Slowly thickening cirrus was
spreading across the ILN forecast area, with mid cloud back
along the mid-level trough axis.

Using an amalgam of the most recent high resolution and
convective allowing guidance and expected path of strongest
height falls and synotpic forcing with the shortwave trough,
expect area of storms over northern Indiana to track mainly east
and perhaps brush far northern counties through sunrise hours as
it weakens. Further south, modest low-mid level warm advection
and moisture transport will continue to support a few showers
from southern/central Indiana...and these should move across
southeast Indiana/southwest Ohio in the hours just after sunrise
with weakening toward mid-late morning. Thunder will be
possible with this activity owing to weak elevated instability,
but expect it to be isolated.

By early afternoon, main shortwave energy will be pushing
through northeast Ohio and pulling away from area. With lack of
a respectable surface reflection of this system, boundary layer
convergence will be hard to come by, but what semblance of a
cold frontal structure that does eventually organize will focus
its boundary layer convergence over central Ohio around
17Z...sinking slowly south/east through the afternoon. The
result of this forcing should be a slowly organizing band of
sct/bkn storms feasting on peak diurnal destabilization /MUCAPE
around 1250 J/kg/ amidst modest effective shear /25-30kt/ which
support some brief storm organization and severity. Expect
highest threat area to be Scioto River Valley of central/south-
central Ohio where a few gusty storms are likely before
instability quickly is shunted south/east as the deeper
northwesterly flow spreads in as the wave departs. Will continue
to mention a few strong/severe storm possibilities in central
Ohio mainly between noon and 4 PM which lines up well with new
SWODY1 with likely a better focus of marginal severe threat east
of the ILN area once storms exit and become a little better
organized. There is considerable uncertainty how far west any
afternoon development will occur. It`s possible some isolated
updrafts may form as far west as ILN/CVG/DAY immediately
along/ahead of the mid level trough axis ~18z, but convective
allowing guidance is underwhelming and erratic on the likelihood
of meaningful updrafts that far west. Overall, highest rain
chances in central Ohio during the early-mid afternoon with much
less chance further south/west.


This will be a period of relatively quiet weather as the wave
from this afternoon shifts rapidly east bringing an end to any
early evening shower and thunderstorm threat. Should see clouds
dissipate quickly as good subsidence moves in.

Perhaps a little fog later tonight especially in the valleys as
good radiational cooling signature develops as 1020mb surface
high moves across Ohio with light flow and residual 60s

Sunday to feature plenty of sun per forecast soundings from
19.00Z NAM/GFS. Most recent NAM runs however have been a little
more robust on low level moisture suggesting some deeper cumulus
is possible in the afternoon. But either way there should be
ample sunshine with highs well into the 80s on light winds.

Weak shortwave energy deamplifying as it crosses the Great Lakes
Sunday night shouldn`t have much of an impact on an expected
clear/mostly clear night.

During this entire period, a bias-corrected MOS blend with minor
adjustments should perform quite well given rather
quiet/unchanging airmass, and this yields highs slightly above
normal and lows around normal.


Models remain in good agreement showing a broad area of surface
high pressure centered over the Mid-Atlantic region with its
influence stretching westward into the eastern Ohio Valley on
Monday. With the displacement of the high from the immediate
area and increasing low level warmth and moisture, a very
isolated afternoon shower/storm cannot be completely ruled out
Monday -- particularly across the northwestern third or so of
the FA. This being said, even with sufficient instability, there
is not any notable forcing mechanism to initiate such activity
(either at the surface or aloft), so maintained a dry fcst for

The main item of interest for Monday will be the sky condition for
eclipse viewing. In addition to development of an afternoon Cu
field, do think there will be some thin cirrus streaming into
western parts of the area by Monday afternoon. There continues to be
some model variability on both the extent and thickness of such
cirrus, but anticipate it to be fairly thin in nature given current

Monday will likely be the warmest day of the week with lack of
widespread dense cloud cover and the nudging of midlevel ridging
into the Ohio Valley (particularly evident on ECMWF solutions).
Highs in the upper 80s to around 90 degrees are expected area-wide.

Past Monday afternoon, attention will turn to the next system that
will impact the area on Tuesday. Model solutions continue to be in
remarkable agreement with the broad-scale features of Tuesday/s
system, including the general timing. 00Z solutions came in a bit
faster than previous runs, showing precipitation entering
northwestern parts of the FA as early as late Tuesday morning. This
initial activity would be isolated to scattered in nature, but
expect fairly substantial destabilization in the pre-frontal
environment Tuesday afternoon and evening. Although wind profiles
will be nearly unidirectional (WSW at sfc to W aloft) in nature,
wind field magnitudes should be sufficient to support scattered to
widespread thunderstorm development ahead of and/or along the
advancing cold front. With this in mind, did maintain likely PoPs
Tuesday afternoon and evening, but shifted time frame ahead by
several hours. Pcpn should end from NNW to SSE very early Wednesday

The front will bring a big change in airmass to the region, with
amplified upper-level troughing situated across the eastern third of
the country by Wednesday. The coolest day of the week may end up
being Thursday as a disturbance rotates around the broader-scale
trough and unseasonably cool temperatures aloft filter into the
region. Would not be entirely surprised to see near-record cool
temperatures at some point late this week. However, some airmass
modification is anticipated for Friday into next weekend.


Sharp mid/upper level shortwave trough to pass quickly across
the TAF sites this morning into early afternoon. Little surface
reflection of this system. Currently /06Z/ conditions are VFR
across the sites with increasing cirrus shield over Indiana and
some mid clouds over Illinois where a few showers/storms exist.
Expect this to move across the sites this morning so have VFR
ceilings above 5kft passing across most sites. Kept a VCSH
mention at DAY/ILN/CMH/LCK for a passing sprinkle/shower after
sunrise but overall trend in data over the past 12 hours is for
less activity this morning. Early in the afternoon as the cold
front begins to sharpen up and advance east across central Ohio
expect a band of showers/storms to form likely impacting the
CMH/LCK terminals with a brief potential for MVFR conditions.
Will continue to mention a tempo of VFR showers to indicate the
likelihood of activity, but not go so pessimistic yet until
coverage/intensity is a little more certain. Sites to the
southwest /CVG...LUK...DAY...ILN/ are likely to remain
precipitation free in the afternoon so nothing in the TAFs right
now, though there is some uncertainty on if development can get
far enough west to impact these sites. Right now thinking is
that activity will remain to the east and any broken VFR cigs
will dissipate through the afternoon as flow becomes westerly
and a little gusty, then northwesterly and later toward evening
behind the front. Overnight expect clear skies, and there will
be some potential for fog late in the night so that will need to
be evaluated for the next issuance

OUTLOOK...Thunderstorms are likely on Tuesday into Tuesday




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