Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 211044

Area Forecast Discussion
National Weather Service Indianapolis IN
644 AM EDT Fri Jul 21 2017


The Aviation section has been updated below.


Issued at 352 AM EDT Fri Jul 21 2017

Central Indiana will remain on the edge of a strong upper ridge into
Saturday.  This will maintain hot and exceptionally humid
conditions...along with threat for clusters of thunderstorms to
impact the region.  A frontal boundary will shift south of the area
by early Sunday with drier and slightly cooler air spreading into
the region for the second half of the weekend. High pressure will
bring mainly dry weather with seasonable temperatures beginning
early next week.


.NEAR TERM /Today/...
Issued at 352 AM EDT Fri Jul 21 2017

Steamy early morning across the region as 07Z temperatures were in
the upper 70s to near 80. Combined with dewpoints in the mid and
upper 70s...heat indices remained in the 80s. Isolated convection
continued to fire in the vicinity of a frontal boundary draped over
our extreme northern forecast area. More extensive storms were
located across northeast Illinois.

Focus will once again be on heat and convection today as both impact
parts of central Indiana. Forecast leaning heavily on persistence
and mesoanalysis with the model suite leaving much to be desired.

First issue is the convective potential from now through mid
morning. Storms continue to track slowly E/SE across northern and
northeast Illinois in the vicinity of the frontal boundary. Despite
an abundance of instability over the region...presence of a weak cap
near 700mb has kept storms from taking off over northern portions of
the forecast area so far. The cap should weaken some as a small low
level jet shifts east into the area and helps to maintain current
convection over Illinois into our northern counties.

Storms should continue to ride the instability gradient and theta-e
ridge...both which point towards the northern half of the forecast
area. Current mesoanalysis indicating an axis of precip water values
in excess of 2.1 inches with torrential rainfall as a primary threat
along with gusty winds and frequent lightning. Will focus highest
chance pops essentially along and northeast of Interstate 74 through
about 13-14Z as storms slowly weaken.

Expect a break during the midday and early afternoon hours...perhaps
and very likely with a period of sun and rapid warming similar to
what happened Thursday...before new convective development occurs
for the second half of the afternoon and continuing into the
evening. Current thinking is that a boundary will get laid down
oriented northwest to southeast from the morning convection and
serve as a focal point for the afternoon storms to fire off.
Additionally...the potential is there for a thermal boundary to set
up nearby as well with southwest counties likely not to be impacted
by the morning convection to any great degree. Model soundings
indicating high levels of instability developing along with very
steep low level lapse rates...both supporting the idea that the
strongest cells will carry a damaging wind threat and possibly a
potential for large hail. Precip water values will remain in the
vicinity of 2 inches with heavy rainfall and localized flooding
potential. Will bring pops back up from mid afternoon on...once
again focusing the highest pops near to just north of the I-74

Temps...the other big issue today is the heat and humidity. Kept
highs nearer to the cooler MAVMOS considering potential impacts to
part of the forecast area from debris clouds and convection. With
that being said...even a few hours with sunshine will enable temps
to rocket similar to Thursday afternoon in the wake of the storms
over the northern half of the forecast area. Will carry a range on
highs of upper 80s over the northeast to near 95 in the lower Wabash
Valley. Dewpoints will likely hold in the mid to upper 70s for large
chunks of the day and any rainfall will just enable dewpoints to
rise even higher.  Max heat indices will make it to 100 in most
areas with 105 or better across the southwest.  Will be adding in
four more counties to the heat advisory for today where max heat
indices should reach 105 or better...Vigo...Clay...Owen and Monroe

Confidence is medium to high on the heat...low on timing and
location of convective impacts.


.SHORT TERM /Tonight through Sunday Night/...
Issued at 352 AM EDT Fri Jul 21 2017

Forecast challenges focus on precip chances through Sunday before a
cold front shifts the heat...excessive humidity and storms all to
the south by the end of the weekend.

Convection will likely be ongoing along the remnant boundary this
evening before diminishing in coverage after dark. Large
thunderstorm cluster expected to develop over the Upper Midwest
ahead of a surface low late today into this evening...then turn
southeast into the Great Lakes along the upper ridge periphery
tonight.  Will have to monitor the evolution of this system as parts
of this complex could spread back into the forecast area overnight.
Storms would once again carry a severe weather and flooding threat
with them.

Capping will likely reestablish on Saturday with any convection
likely to remain isolated at best until peak heating during the
afternoon when more robust development can take place along any
leftover boundaries.  The approach of the frontal boundary from the
north by late day along with cooling aloft should amplify the
convective threat for the late afternoon and evening with storms
becoming more numerous as a low level jet develops as well. These
storms Saturday afternoon and evening will again carry a potential
for severe weather with damaging winds as the primary threat. While
the storms should be moving...torrential rain and flooding will be
concerns with precip water values being progged in excess of 2.25
inches with a freezing level at 15-16kft supporting warm rain

Most rain and storms will shift south of the forecast area Sunday as
the front settles into the Tennessee Valley.  Weak high pressure
will build in and bring a drier end to the weekend along with
slightly cooler and less humid air through Sunday night. last hot and humid day for Saturday as most areas should
warm into the lower and possibly mid 90s. Heat indices will again
rise into the low 100s. Low level thermals support highs mainly in
the mid and upper 80s for Sunday with dewpoints slipping back to a
slightly more comfortable level in the upper 60s to near 70.


.LONG TERM /Monday through Friday/...
Issued at 204 AM EDT Fri Jul 21 2017

The long term portion of the forecast will present an opportunity
for temperatures to cool down nearer to seasonal normals and at
least a window of dry weather.

A remnant boundary near the Ohio River may produce a few showers
and storms on Monday, but from Monday night through early
Thursday, surface high pressure moving through the area will keep
conditions dry and far more comfortable, with dewpoints likely to
be in the low to mid 60s rather than the current unpleasant low to
mid 70s.

Late in the week, northwest flow reestablishes and a frontal
system appears likely to begin to influence the area. This will
necessitate a return to daily chances for thunderstorms.

Blended initialization handled things relatively well and few
major changes were required.


.AVIATION /Discussion for the 21/12Z TAFs/...
Issued at 644 AM EDT Fri Jul 21 2017

Predominantly VFR through the period.

Main concern will be convective impacts to the sites, and these
are not being particularly well handled by model guidance. Will
carry a VCTS mention early at IND/LAF where convection has been
ongoing this morning, along and north of a frontal zone.

Will carry VCTS again later this afternoon at all sites as an
upstream MCV will move into the area and may spark more widespread
activity yet again.

Cannot rule out drops into MVFR and perhaps brief IFR with the
more intense thunderstorms, but again, uncertainty precludes
mentioning this.

Winds through the period will be less than 10KT, occasionally



Heat Advisory from noon today to 7 PM EDT Saturday for

Heat Advisory until 7 PM EDT Saturday for INZ060-067>069.



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