Area Forecast Discussion
Issued by NWS Indianapolis, IN

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174
FXUS63 KIND 221711
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
112 PM EDT Sat Jul 22 2017

.UPDATE...
Aviation section has been updated below.

&&

.SYNOPSIS...
Issued at 338 AM EDT Sat Jul 22 2017

Central Indiana will remain on the edge of a strong upper ridge
through tonight....maintaining hot and exceptionally humid
conditions. Chances for thunderstorms will continue until a frontal
boundary can push through the area overnight into early Sunday. A
secondary front may bring additional scattered storms on Sunday
evening before high pressure builds in with mainly dry weather and
seasonable temperatures for the early part of the week.

&&

.NEAR TERM /Rest of Today/...
Issued at 1005 AM EDT Sat Jul 22 2017

.Update...Radar indicated a cluster of thunderstorms across central
Illinois was moving into the northwest part of our forecast area.
Satellite indicated tops were warming and this area will weaken
as it moves eastward.  The storms will probably dissipate by early
afternoon and then more storms redeveloping late today as indicated
by the high res model.

Made a few minor tweaks to temperatures...but overall current
forecast temperatures are on track.

Portion of previous near term discussion follows...
Convection and heat remain the primary concerns for today once again
as the region resides on the periphery of the heat ridge.  The first
order of business will focus on the potential for severe convection
prior to daybreak with expectations of additional robust storm
development by late day over northern counties near the sagging
frontal boundary.

With moisture pooling and a surge of heating ahead of the
approaching frontal boundary...potential is there for dewpoints to
hang in the mid and upper 70s into the afternoon as temps surge into
the 90s.  This will promote a strongly unstable airmass by late
afternoon with MLCAPEs nearing 4000 J/KG being realized immediately
ahead of the front over northern counties. With a low level jet
likely to energize over the region by late day and a remnant outflow
boundary from the morning convection setting up over northern
portions of the forecast area...expect convection to erupt quickly
after 21Z and grow upscale rapidly into the evening aided by cooling
aloft. Damaging winds will be the primary severe weather threat but
all forms of severe weather will be possible.  Precip water values
above 2 inches will support a continuing heavy rain and flooding
threat as well.

Temps...as mentioned above...heat and humidity will again be factors
today. Expect highs ranging from the upper 80s over northeast
counties where convective impacts lend towards more uncertainty on
temps to mid 90s in the lower Wabash Valley.  Max heat indices again
will get above 100 degrees in most areas with the highest values
over southwest counties.  The heat advisory will continue in
its current state and location until 23Z.

&&

.SHORT TERM /Tonight through Monday Night/...
Issued at 338 AM EDT Sat Jul 22 2017

Forecast challenges focus on precip chances...especially tonight and
again late Sunday and Sunday evening.

Convection will likely be ongoing this evening as development takes
place over northern counties and progresses southeast through the
evening and overnight ahead of the cold front.  A surface wave along
the boundary...continued influence of a low level jet...BL shear
values around 30kts and a strongly unstable airmass all support a
threat for severe storms this evening gradually transitioning into
more of a heavy rain threat overnight. Highest pops will be employed
over the eastern half of the area where parameters will be just a
bit more favorable for more widespread convection. Precip water
values will remain above 2 inches and combined with freezing levels
near 15kft supporting warm rain processes...torrential rainfall and
flooding will continue to be concerns as the storms surge south
overnight.

Growing confidence that most if not all of the convection will be
out of the forecast area by 12Z Sunday with the front settling south
of the Ohio River.  Much of the day Sunday should be dry with an
edge being taken off the humidity levels. A secondary front
associated with a sharp upper wave diving out of the western Great
Lakes may serve as a focal point for renewed convective development
to the northwest of the region Sunday afternoon...possibly spreading
into the northern half of the forecast area during the evening.
Steep mid level lapse rates in response to cooling aloft suggestive
that strongest storms may carry a damaging wind and large hail
threat.  Will bring low chance pops back into the northern counties
Sunday evening as a result.

Convection will diminish overnight Sunday with high pressure
building south into the region on Monday with cooler and noticeably
less humid air. Likely to see diurnal cu develop on Monday before
skies become mostly clear Monday night.

Temps...it will remain very warm on Sunday with low level thermals
supporting highs from the mid 80s to around 90. Temps will fall back
into the lower and middle 80s Monday with cooler air aloft advecting
into the region. Lows will slide back into the 60s through the
period.

&&

.LONG TERM /Tuesday through Friday/...
Issued at 338 AM EDT Sat Jul 22 2017

High pressure will maintain the dry and pleasant airmass across
central Indiana and the Ohio Valley through midweek before a more
active pattern returns as a cold front drops through the area late
week.

The upper ridge that will be squashed back to the southwest by
Monday will begin a reexpansion back towards the area by the middle
of next week with southerly flow reestablishing.  This will lead to
a progressively warm and more humid airmass back into the forecast
area by Wednesday as highs rise into the upper 80s to near 90 over
much of the region. A cold front will track slowly south into the
area by Thursday...forcing the core of the ridge to retrograde back
into the southern Plains as mid level heights buckle once again.

The best chance for convective impacts over central Indiana will
exist on Thursday and Thursday night as the frontal boundary is
draped across the Ohio Valley.  The boundary should shift south of
the region by Friday with rain chances diminishing as weak high
pressure expands in from the north.

&&

.AVIATION /Discussion for the 22/18Z TAF Issuance/...
Issued at 112 PM EDT Sat Jul 22 2017

Mainly VFR outside of convection.  However a period of MVFR
ceilings possible most areas late tonight and early Sunday morning.

A cold front over northern Indiana will drift south across Indiana
tonight.   Only isolated showers and storms expected next few hours.
However they should become more numerous ahead of the cold front
this evening and will mention a period of tempo thunderstorms
KIND...KHUF and south tonight.  These will end from the north
overnight...but model soundings indicate MVFR ceilings from stratus
and light fog possible late tonight and Sunday morning with
improving conditions Sunday as drier air slowly moves in from the
northwest.

West to southwest winds up to 10 knots this afternoon will decrease
to 5 to 8 knots this evening and become northwest late tonight and
sunday.

&&

.IND WATCHES/WARNINGS/ADVISORIES...

Heat Advisory until 7 PM EDT this evening for INZ051>053-060>062-
067>072.

&&

$$

SYNOPSIS...Ryan
NEAR TERM...Ryan/JH
SHORT TERM...Ryan
LONG TERM...Ryan
AVIATION...JH



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