Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 210550

Area Forecast Discussion
National Weather Service Indianapolis IN
150 AM EDT Mon Aug 21 2017

The LONG TERM Section has been updated below.


Issued at 253 PM EDT Sun Aug 20 2017

A few upper waves will bring chances for thunderstorms tonight into
Monday, before a cold front brings better chances for rain Monday
night into Tuesday evening. Dry conditions will then prevail mid
week into next weekend. Hot and humid conditions to start the week
will give way to below normal temperatures mid week on.


.NEAR TERM /Tonight/...

Issued at 932 PM EDT Sun Aug 20 2017

No significant changes made and will still carry chance pops
across north half of area overnight but with low confidence.

Early evening southern IL/IN thunderstorms have died. Scattered
showers NE IL/NW IN tied with upper shortwave over southern Lake
Michigan continue eastward track with no strength/coverage growth.

Operational HRRR has developed this shower and storm area over
multiple runs and carry the convection SE across central Indiana
overnight.  Observed mesodata shows axis of decent CAPE pointing
towards this shower area while low level lapse rates remain fairly
steep and moisture convergence aligns with CAPE axis. Thus all
parameters point toward upscale growth potential. Problem has been
this was supposed to have started by 00Z and 01Z and upscale
growth has yet to occur.

So potential is there, models suggest it could occur, we have a
mention for storms in the forecast, so will keep it there, but
again not excited that it will come to fruition given the upper
support is starting to pull away and stabilization has begun.


.SHORT TERM.../Monday through Wednesday/
Issued at 253 PM EDT Sun Aug 20 2017

Models are close enough that the model blend initialization
was accepted for most items.

There could be some lingering convection across the far north at the
start of the Monday period so will keep some slight chance PoPs
there early. After a brief lull, more scattered convection is
possible as yet another upper disturbance moves in. Looks like the
best forcing remains north, so will keep PoPs across the northwest
2/3 of the area or so and keep them in chance category or lower,
with the highest PoPs in the afternoon when instability is highest.

Next concern is then sky cover for the eclipse. Have real concerns
that mid and high clouds will impact the event and reduce the
chances of having a good view. Models have been trending up sky
cover at 18Z with each run. However, confidence is low in sky cover
for the eclipse window as timing of any convection and exact
location of thicker high clouds is hard to pinpoint. Will keep skies
partly cloudy (50-70% coverage of opaque clouds) for now.

A cold front will then approach the area Monday night and move
through Tuesday. Forcing will be decent with upper jet support and
decent 850mb flow. However, timing for convection isn`t that great
as front moves in when instability is lower due to time of day. Rain
could be in the weakening phase as it moves in Tuesday morning then
get stronger again south of the area when instability is maximized
later Tuesday afternoon. For now though timing looks good enough for
the threat of strong to severe storms across mainly the southern
half of the area Tuesday afternoon.

Still feel there will be enough rain around to measure at least 0.01
inch most areas so will keep the blend`s likely category PoPs

Will keep some low PoPs Tuesday evening to account for any lingering
rain behind the front. High pressure will then build in and bring
dry conditions later Tuesday night through Wednesday.

Model blend temperatures look good. Heat indices could reach around
100 Monday afternoon if cloud cover is thin enough.


.LONG TERM (Thursday through Sunday)...

Issued at 150 AM EDT Mon Aug 21 2017

Confidence is high the period will be dry with below seasonal
temperatures. How far below seasonal is somewhat uncertain.

The main models all have high pressure providing dry weather and
relatively cool Canadian air to central Indiana.  There is some
uncertainty about temperatures because the American models are more
aggressive than the European about delivering cool air. Today`s
forecast will be based on consensus, with possible errors of 2-4


.AVIATION /Discussion for 210600z TAFs/...

Issued at 1143 PM EDT Sun Aug 20 2017

Brief visibility restrictions possible in thunderstorms predawn
hours...otherwise VFR conditions expected. Winds will be below 10
knots through the period and mainly from the south or southwest.

Scattered thunderstorms over eastern Illinois will track east into
the north half of Indiana through predawn hours. These may briefly
drop visibility into IFR range if in the core of a storm
otherwise expect VFR conditions to prevail. More storms may be
possible during peak heating Monday afternoon as weak trough which
is expected to develop convection west of the Mississippi river
early this morning will be moving across central Indiana. Presence
of the upper wave nearby and sufficient moisture at low levels
will be enough to generate some cumulus by late morning. One
negating factor in cumulus development may be abundant cirrus
debris from Iowa early morning convection which would then delay
cumulus development here until early afternoon Monday.




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