Area Forecast Discussion
Issued by NWS Wilmington, OH

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FXUS61 KILN 282358
AFDILN

Area Forecast Discussion
National Weather Service Wilmington OH
758 PM EDT Wed Sep 28 2016

.SYNOPSIS...
A large low pressure system over northern Indiana will drift very
slowly south into Kentucky...before reversing course and drifting
back to the northwest toward Chicago...before very slowly
weakening and losing grip over the Ohio Valley by the end of the
weekend. Moist southerly to southeasterly flow around this low
will keep rain and scattered thunderstorms in the picture all way
into Sunday...with the better chances of rain focused on Thursday
and Friday. Temperatures will remain below normal underneath
frequent clouds and showers...before warmer and drier weather
spreads through the Ohio Valley early next week.

&&

.NEAR TERM /THROUGH THURSDAY/...
Lots of weather to look at today thanks to anomalous mid/upper
level trough and closed low which is now directly impacting CWA.

At 18Z...center of mid-level circulation was almost stationary in
NW Indiana...perhaps very slowly drifting SSE. Core of moderate to
strong mid/upper level flow existed around the periphery of this
system...into and through the basal portion of the longwave trough
into southern IND and western KY...and across much of the WFO ILN
CWA. Attendant to this mid-level circulation...a discombobulated
and weak surface flow existed with several weak surface lows able
to be identified in surface observations to the southeast of the
cold core...with the primary /weakening/ 1009mb surface cyclone
just south of Indianapolis moving east into sern IND. A large
shield of rain/embedded thunder was moving across much/all of the
ILN CWA due to very strong ascent via PVA ahead of the upper
trough and on the immediate poleward/cyclonic shear side if the
mid/upper level jet. Back to west where a few breaks existed in
the cloud cover /allowing sfc temps into the lower-middle 60s/
discrete cellular convection recently developed as expected where
steep mid level lapse rates and the diabatic heating has combined
to allow SBCAPEs to reach 500 J/Kg. These storms have been
producing many reports of small hail.

Through tonight...expect current band of rain/storms over ILN CWA
to move east slowly while more scattered convection will continue
to develop and move in from the west. Current rain will have
tempered destabilization to some degree...but an hour or two of
sunshine in southeast Indiana to the Ohio border...will allow
storms to remain intact at least to the Ohio border thru 6 PM if
not a little later. As with out west...expect sub-severe hail to
be the primary threat...with lesser threat of wind. As eluded to
in previous discussion...brief/weak tornado threat is non-zero.
Despite boundary layer flow over the area being weak/messy...there
is a decided backed/southeast component to the flow thanks to the
weakening surface low approaching. Thus...given ample/robust
mid/upper level flow and respectable instability...effective
shears will support storm organization /brief supercellular
structures/ and if the right juxtaposition of backed low level
flow near/northeast of the surface low and an organized
updraft...could see the threat for a rotating storm. An unlikely
threat...again given that 0-1km shear values on the whole are <
20kts...but one that bears mentioning/monitoring. There have been
many CAMS today with simulated updraft helicity /25m2/s2/ streaks
across western OH/ern IND so will be watching discrete storms
closely in the 4PM to 7PM window.

Another concern...and always a concern on periphery of
compact/closed lows will be pivoting rain/convective bands for
potential flooding. Widespread rains today have dropped mostly <1"
but some areas in ecntl IND/wcntl OH have seen up to 1.25 to
1.5"...and this area in particular could see pivoting band of
convection late this afternoon/evening which could cause some
runoff issues. Ambient PWAT is not impressive /near to even just
below seasonal norms...so environment isn`t primed...but
stationary/slowly moving convective elements will still cause
concern.

Expect weakening of all activity after 7 PM...but rain will linger
through the evening in the Scioto Valley into CMH area much of the
evening...wrapping back westward into northwest Ohio...but dry
slot wrapping into the area from south to north should effectively
shut most precipitation down save for a few showers from later
this evening through the overnight.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY NIGHT/...
Pesky closed low...with 500mb standardized anomalies to 2 or 2.5
sigma below climo...will drop south into southern KY by Thursday
evening...before stalling. As it fills...this system will begin
drifting aimlessly back into northern Indian by Saturday morning
and there is strong agreement in the suite of NWP on this
solution. This will guarantee a continued threat of episodic
showers and storms Thursday...Friday...and Saturday...though with
time the large scale forcing and cold-core of the system will
weaken to the point that precipitation will become less
organized and lighter...especially Saturday.

Still-strong flow around this system will advect deep moisture off
the Atlantic and into/across the Appalachian mountains and into
the Ohio Valley through the end of the week. A particular area of
enhanced rain threat will be across the northern/eastern CWA in
closest proximity to deeper moisture and enhanced isentropic
ascent. Given cold pool aloft...and slowly enhanced moisture in
the boundary layer on both days...instability is sufficient enough
that storms are possible...and will be drifting from SE to NW - a
decidedly uncharacteristic storm motion. Both days will have a
threat for an anchored or slowly moving arc of rain/storms...so
the heavy rain/minor flood threat is not completely out of the
question. But as with all precipitation around closed upper
lows...there is a wide disagreement in locations/timing of the
individual vort lobes rotating around the central
circulation...thus rain chances are not yet ramped up as high as
they otherwise will likely be in coming forecasts. 60s by day and
50s by night in this reduced diurnal temperature range thanks to
plenty of clouds.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
This period will be characterized by a much warmer drier/period vs
the short term. On Sunday morning...the closed low will be
drifting northeast...weakening...and opening up into a more
progressive shortwave trough over New England by Monday.
Thus...there is still a small threat of showers into Sunday but
right now the timing/placement of this threat is very tough but
seems focused north of I-70 closer to mid level cold pool. Large
scale ridging ahead of the next longwave trough over the west will
allow for dry weather Monday thru Wed as high pressure at the
surface moves across the eastern U.S. Temperatures will be warming
steadily and by the middle of next week should be well into 70s if
not the lower 80s with dewpoints hanging tough in the 50s.

&&

.AVIATION /00Z THURSDAY THROUGH MONDAY/...
Shower activity will decrease in coverage for the overnight hours
and then pick up during the daytime hours on Thursday, especially
during the afternoon to early evening hours. A few thunderstorms
will also be possible around KCMH and KLCK later in the day on
Thursday.

Cigs and Vsbys will lower during the overnight hours and slowly
improve during the day on Thursday. IFT to LIFR conditions will
be possible at times overnight especially later in the overnight
time frame Wednesday night into Thursday morning.

OUTLOOK... MVFR ceilings and visibilities possible with showers
Thursday night through Saturday. IFR/LIFR ceilings and
visibilities possible Friday morning, with IFR ceilings and
visibilities again possible on Saturday morning.

&&

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

&&

$$
SYNOPSIS...Binau
NEAR TERM...Binau
SHORT TERM...Binau
LONG TERM...Binau
AVIATION...Novak


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