Area Forecast Discussion
Issued by NWS Indianapolis, IN

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000
FXUS63 KIND 210151
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
951 PM EDT Sun Aug 20 2017

.UPDATE...
The NEAR TERM Section has been updated below.

&&

.SYNOPSIS...
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
Tuesday.

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 /Wednesday Night through Sunday/...
Issued at 201 PM EDT Sun Aug 20 2017

Splendid stretch of late summer weather for the extended period as
the Great Lakes and Ohio Valley reside under a broad upper trough
and strong high pressure at the surface for much of the period.
With dry low level flow from the north then east...expect days
full of plentiful sunshine and mainly clear nights with no
threats of rainfall. High temperatures will be pleasant...ranging
from the mid 70s to lower 80s Thursday through Saturday. Nights
will have a distinct autumn feel with temperatures dropping as low
as the lower and middle 50s.

Heights aloft will recover with the upper trough shifting off to
the east by Sunday. The region will remain under the influence of
a surface ridge...but low level flow will transition to southerly
on the back side of the high...resulting in temperatures warming
into the lower to possibly middle 80s on Sunday. Not entirely out
of the question that a potential for isolated convection would be
present by late weekend with moisture being advected north into
the Ohio Valley...but chances too remote at this point for
inclusion into the forecast.

&&

.AVIATION /Discussion for 210000Z TAFs/...

Issued at 646 PM EDT Sun Aug 20 2017

VFR with south to southwest winds below 10 knots through the
period.

Daytime heating cumulus will dissipate after sunset Sunday but
still patches of mid level and high level precipitation remnant
cloud debris across Illinois to move into Indiana overnight.

Another minor trough in the upper flow will enhance cumulus cloud
development again by 211800z with isolated thunderstorms perhaps a
potential during peak heating Monday.


&&

.IND WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SYNOPSIS...50
NEAR TERM...Tucek
SHORT TERM...50
LONG TERM....Ryan
AVIATION...Tucek



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