Area Forecast Discussion
Issued by NWS Wilmington, OH

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FXUS61 KILN 201045

Area Forecast Discussion
National Weather Service Wilmington OH
645 AM EDT Mon Aug 20 2018

Low pressure over the mid-Mississippi Valley will move northeast
into the Great Lakes while weakening slightly. A return flow of
more humid air will arrive this afternoon and spark scattered
showers and storms, which will continue into tonight and Tuesday
as the low pressure pressure shifts to a position over southern
Ontario by the end of the day Tuesday. In the wake of the cold
front attendant to this system, a much cooler and more
comfortable airmass will move into the area, bringing dry
weather through the remainder of the work week.


GOES 16 water vapor loops clearly show deep/closed low pressure
over northwest Missouri this morning, with high level cirrus
spreading downstream across the Ohio Valley. Surface high
pressure over New England was maintaining a dry easterly low
level flow across the forecast area early this morning, though
radar VWPs were clearly showing low level flow veering to the
southeast as the system to the west moves slowly east. An arc of
showers and storms was aligned along the Mississippi River,
advancing steadily eastward. A warm front was developing
southeast of the low into portions of western Kentucky.

Today...the arc of convection along the MS River will spread
eastward with time, likely weakening with the loss /mixing out/
of a weak low level jet after sunrise. Along the leading edge of
this surface differential heating zone, in tandem with
moderate/strong 925-850mb moisture advection, convection should
redevelop in a NW-SE arc from Indiana into the Tri-State around
18Z and spread northeast deeper into the ILN CWA. Expect the
greatest coverage in the 18Z-22Z timeframe across southeast
Indiana into southwest Ohio and portions of northeast Kentucky,
before lifting slowly northeast and diminishing in
coverage/intensity by 00Z from west-central Ohio down into
south-central Ohio. Threat assessment for this active convection
will be a deep-layer shear profile that is quite marginal for
storm organization, with effective shear likely remaining <
30kts and even weaker shear in the 0-3km layer. However, the
return of anomalous tropospheric moisture /150-175% of late
August normals/ and a strong convergence signature atop and on
the cool side of a developing warm front lifting northeast
across the area, means thunderstorms will be common across the
southwest 1/2 of the ILN CWA during the afternoon and early
evening. The severe weather threat in this timeframe is not
impressive...but also not completely zero...given the presence
of the weak warm frontal structure, marginal deep layer shear,
and MLCAPE ~1500 J/kg at peak heating. However, low level flow
during this timeframe is very weak with only broad/weak
directional shear over a deep layer, and lapse rates aloft that
will not be favorable for much hail. Expect the bulk of this to
be general late-summer thunderstorm activity, lifting slowly
northeast on the nose of more anomalous low level moisture

There will likely be a sizable lull in organized storm activity
as this initial batch of showers/storms weakens early in the
evening as it pulls further away from thermodynamic/kinematic
forcing that will focus back to the west during the evening.
Can`t rule out a small storm cluster or two during the evening
back over eastern Indiana, but it seems we will need to wait
until after 06Z for stronger low level forcing to develop to
reinitiate thunderstorm clusters over Indiana. To that end,
21.00Z HREF shows a redeveloping low level jet ahead of the
weakening surface cyclone which will be shifting/redeveloping
northeast quickly as the trough aloft undergoes
weakening/opening up into a positive tilt shortwave trough by
Tuesday morning. In the left exit-nose of this 35kt low level
jet, expect an organized cluster or two of thunderstorms to move
in/develop over eastern Indiana into western Ohio. Here again -
the severe threat is nonzero given increasing low level shear
amidst a marginal-yet-overall-unimpressive thermodynamic
perspective. Still, 0-1km shear of ~30kts amidst MLCAPE around
500 j/kg may prove to be just enough for a spatiotemporally
small threat of damaging wind with any thunderstorm/cluster
which can organize and forward propagate, though the overall
setup appears quite unfavorable for any organized wind swaths,
and instability far too weak to support hail. Expect the better
coverage of storms after 06Z to be across west central into
central sections of the ILN CWA, but high rain chances are
warranted area-wide due to the arrival of the low level jet and
moisture transport/forcing across the area in the 06Z-12Z


Further deamplification of the shortwave trough and surrounding
mechanisms for forced ascent is expected on Tuesday as the
better destabilization and mass response to the wave begins to
focus east/northeast of the area. However, given low level
moisture will remain high with the arrival of the cold front in
the afternoon, redevelopment of showers and storms is expected
especially in the eastern side of the forecast area. There is
again a non-zero risk of a damaging wind event or two from
thunderstorms as they begin to deepen in the early afternoon on
the east side of the forecast area, but with unimpressive lapse
rates and plenty of cloud cover, instability should remain in
check compared to areas further downstream where deeper
convection is likely. With the surface low pulling away so
quickly to the northeast away from the ILN CWA in the afternoon,
this will induce a pretty rapid veering of the low level wind
profile which will also help mitigate convergence and low level
shear as the effective cold front shifts through.


As the extended period begins, the models have just pushed a cold
front through the fa, but a secondary front/sfc trof remains to the
nw. Until the second feature works through during the evening hours
there will be a chance of a lingering storm, especially in the east.
Drier air will begin to filter into the region during the second
half of the night. Temperatures will cool down to around 60 in
the northwest counties and the mid 60w in the far east.

A large high pressure area will then build in for the rest of the
work week. Much below normal temperatures are expected for the mid
point of the week as highs will range from the lower 70s in the
north to the upper 70s in parts of nrn KY. Lows Thursday morning
will be in the lower to mid 50s with highs in the mid to upper 70s
for Thursday. Temperatures on Friday will be a few degrees warmer
with highs around 80.

Models are digging a H5 low through the northern plains on Friday.
Associated with this an area of precipitation which is forecast to
move in Friday night. Models keep scattered pcpn across the region
over the weekend as the region is it on the srn edge of zonal
H5 flow. Will leave PoPs at 20 percent as to not flip-flop with
the previous forecast.


Cirrus will continue to spill across the TAF sites this morning
as any residual valley fog at KLUK quickly burns off. Weak
easterly/southeasterly flow will slowly strengthen through the
morning as a warm front over Kentucky approaches. By 18Z showers
and thunderstorms will move/develop into the KCVG/KLUK areas,
with potential for brief reductions to IFR conditions. This band
of scattered storms will shift north/east into the DAY/ILN
corridor a few hours later /around 21Z/ and will likely begin to
weaken thereafter, so only a mention of VCSH by the time the
remnants of this arc of showers and storms makes it into central
Ohio. There will be a lull in the activity to some degree until
a stronger low level jet shifts northeast into the Ohio Valley
after 06Z and this will focus a little more convection
especially across DAY/CMH/LCK late in the night through early
Tuesday morning. Enough signal for showers and a few storms
further south at CVG/LUK/ILN as well, but cigs/vsbys may not be
as low as further north. With enhanced convergence as the low
pressure and cold front approach that ceilings may go MVFR for a
time later tonight into Tuesday morning as winds turn
southwesterly and then westerly with the approach of the cold

OUTLOOK...Thunderstorms likely into Tuesday.




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