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738
FXUS63 KIND 230829
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
429 AM EDT Sun Jul 23 2017

.UPDATE...
The AVIATION Section has been updated below.

&&

.SYNOPSIS...
Issued at 332 AM EDT Sun Jul 23 2017

One more day with very humid conditions is expected today before a
cold front sweeps through the area tonight. A chance of
thunderstorms will exist later today into this evening ahead of the
front. High pressure will build in Monday bringing dry...cooler and
less humid air to central Indiana through the first half of the
week. Rain and thunderstorms return Thursday as another cold front
passes through the area before dry weather returns to the Ohio
Valley for next weekend.

&&

.NEAR TERM /Today/...
Issued at 332 AM EDT Sun Jul 23 2017

Another active early morning across parts of the region as
convective clusters track along an old outflow boundary across
southern portions of the forecast area. 07Z temperatures remained in
the 70s across central Indiana.

Lower confidence forecast for today than it originally appeared just
24 hours ago as the tropical moisture lingers well north of the
current convection early this morning...extending all the way back
into the Great Lakes.  Frontal boundary extending across central
Indiana and Illinois appears to be losing its characteristics
rapidly. Growing sense that the oppressive conditions of recent days
will not be pushed out of the area until a secondary front located
over the upper Midwest this morning swings through the region
tonight.

Initially...focus is on the convective cluster drifting southeast
across southern portions of the forecast area. Most of the
thunderstorms fired off an old outflow boundary from the early
Saturday morning storms that roughly settled in an axis from near
KHUF southeast into northern Kentucky. These storms have been
subsevere...but have produced a few inches of rainfall in an arc
from southern Vigo into western Lawrence County. Convection is
steadily moving towards the more unstable airmass present over
central and northern Kentucky...and expect most of these storms to
move out of southern counties prior to 12Z. Will have to keep an eye
on yet another convective cluster quickly approaching KSTL early
this morning. These storms should continue to weaken as they move
into a more stable environment present over southern Illinois and
Indiana...but will likely need to keep a low chance pop in the far
south for a few hours beyond daybreak.

High uncertainty for the rest of today as upper wave approaches
later today with the secondary cold front. As both of these features
interact with the warm and unstable airmass over the region...
potential is there for isolated to scattered storms to develop.
Remnant outflow boundaries from storms last evening into this
morning may also aid in development. Soundings indicate a weakly
capped airmass initially but as temperatures warm...should see
MLCAPES peak near 2000j/kg. Storms will be reliant on instability
and moisture with the forcing associated with the upper wave as poor
lapse rates and weaker flow through the boundary layer should work
against more widespread development. Overall...think storms largely
remain subsevere but WBZERO values dropping below 10kft with mid
level heights falling would support more of a large hail threat in
stronger cells than in recent days. Damaging wind gusts will be
possible too. Will place highest pops over the northeast half of the
forecast area in closer proximity to the energy associated with the
upper wave.

Temps...Low level thermals supported a model blend with highs in the
upper 80s and lower 90s.  Dewpoints will remain in the 70s which
will make for one more humid day. Max heat indices should largely
stay below 100 degrees with the exception of the lower Wabash
Valley.

&&

.SHORT TERM /Tonight through Tuesday Night/...
Issued at 332 AM EDT Sun Jul 23 2017

Forecast challenges focus on precip chances tonight before high
pressure takes over with cooler and less humid air finally arriving.

Any scattered convection with the secondary front this evening will
likely be ongoing at 00Z. Storms should diminish in coverage and
intensity by late evening with loss of diurnal heating and as the
upper wave shifts east of the region. Will maintain low chance pops
through 06Z then go dry.

The frontal boundary will sag to near the Ohio River by early Monday
morning...continuing further south on Monday into the Tennessee
Valley. High pressure will expand south from the Great Lakes and
finally advect a more refreshing airmass into central Indiana. The
high will serve as the dominant influence on weather across the
region for the remainder of the short term as it tracks east across
the Great Lakes. Cu remain possible during the afternoons but
overall cloud coverage will be low as model soundings show capped
mid levels and plenty of dry air and subsidence present aloft.

Temps...more seasonable temperatures will return through the period
as highs fall back into the low and mid 80s with lows in the 60s.
Maybe most importantly...the very high dewpoint air will be forced
south with the front tonight and Monday and will not return through
Tuesday night as dry low level flow from the north and east
persists.

Overall forecast confidence is high after tonight.

&&

.LONG TERM /Wednesday through Saturday/...
Issued at 215 AM EDT Sun Jul 23 2017

This is a high confidence forecast. Temperature errors should mostly
be 2 degrees or less. POP errors should be mostly 10 percent or
less.

Wind in the upper atmosphere are going to remain unusually strong
for this time of year, producing well defined weather systems
that are relatively easy to forecast. The American and European
systems are in good agreement, suggesting they are on the right
track.

&&

.AVIATION (Discussion for 230900Z TAF update)...

Issued at 430 AM EDT Sun Jul 23 2017

Thunderstorms ended over the TAF sites by 09Z. There is a lot of
uncertainty about if and when they will return.  They will be left
out of the TAFs for the time being, but may be added later as more
data becomes available.

Otherwise, VFR will predominate, but areas of MVFR ceilings are
possible this morning. Winds should veer from southwest to
northwest today, with speeds remaining below 10 knots. &&

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

$$

SYNOPSIS...Ryan
NEAR TERM...Ryan
SHORT TERM...Ryan
LONG TERM....JK
AVIATION...JK