315
FXUS63 KIWX 170827
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
427 AM EDT Sun Aug 17 2025

.KEY MESSAGES...

- Scattered showers and thunderstorms across northwest and
  portions of north central Indiana will diminish later this
  morning. A few showers and storms early this morning may
  produce brief heavy rain and wind gusts to 40 mph.

- Mainly dry tonight and Monday, with the next notable chance of showers
  and thunderstorms (40-60%) late Monday night into Tuesday.

- Remaining hot and humid through the middle of the week.

&&

.DISCUSSION...
Issued at 423 AM EDT Sun Aug 17 2025

A weak to modest west-southwest low level jet feeding into northeast
Illinois is combining with increasingly convergent flow from a
southward dropping low level trough to aid in continued
regeneration of showers and thunderstorms early this morning
along an axis from KRFD to KRSL. This corridor is also along a
well-defined MUCAPE gradient that extends from north central
Illinois into west central Indiana. Some pockets of higher
downburst winds are being observed across northeast Illinois on
the more unstable side of this gradient. There is some low end
potential that some sub-severe gusts to 40 to 50 mph could
affect northwest Indiana in IWX forecast area early this morning
with the stronger precip loaded cores, but the potential for
severe remains quite low with the possible exception of far
northeast Illinois into portions of extreme NW Indiana.
Otherwise, brief heavy rainfall is expected that could lead to
some isolated ponding issues this morning as a narrow pooled
axis of 1.8 to 2.0 inch PWATs is sprawled across this favored
zone. These type of favored upwind propagating convective setups
are usually very conducive for some hydro issues, but the
overall mild nature to the strength the low level jet should
hopefully prevent a more substantial hydro event from evolving.
By mid to late morning, the pre-frontal low level jet should
diminish in strength that should lead to a resultant lessening
in convective coverage as the low level front drops across the
southern Great Lakes. Some uncertainty still exists in how
quickly this diminishment occurs, so PoPs will likely need
refinement.

For later this afternoon, some isolated convective potential may
exist south of US 24 where some respectable 1500-2000 J/kg MLCAPEs
should linger immediately ahead of the cool front. Forcing should be
lacking however as the convectively enhanced vort from early this
morning races across the eastern Great Lakes. Mid/upper level
ridging trying to build back from the west will also not provide a
favorable environment, and a much more limited shear profile in
comparison to earlier this morning.

Otherwise for today, hot and humid conditions persist, especially
just south of the cool front where an axis of sfc dew points in the
mid 70s remains possible. Some isolated heat indices near advisory
criteria are possible across the extreme southwest, but this
potential of 100+ heat indices appears much greater west and
southwest of the local area.

Hot and humid conditions persist tonight and Monday, with additional
precip chances during this period looking to be on the small
side. Mid level ridging should have some decent amplitude during
this period, with perhaps some low end chance of isolated
storms across northwest/west central Indiana on the edge of the
low level theta- e/instability gradient.

The next notable chance of showers and storms still appears to move
in for later Monday night into Tuesday. By this timeframe, the
upstream upper level ridge should exhibit some amplification across
the northern Rockies, making the local area susceptible to
weaker northwest flow waves. This evolution will lead to a very
strong west- east oriented instability gradient from northern
Illinois into northwest Indiana Monday afternoon. This should
tend to keep greatest severe risk west of the local area into
Monday night. Stronger low level theta-e advection builds east
again on Tuesday, but extent of convective coverage/organization
is somewhat in question as shear should be limited and forcing
with the aforementioned northwest flow waves will be weak.

Cooler and drier air builds in for Wed-Thu with indications that
more active short wave track will be closer to US/Canadian border
for the end of the work week. Some moderation in temps expected late
work week, but mainly dry conditions expected.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 142 AM EDT Sun Aug 17 2025

The combination of remnant convective outflows and increasing
confluent/convergent flow from northern Illinois to the southern
Great Lakes region will result in a continued favor zone of
shower/thunderstorm production through the remainder of the
overnight. Orientation of instability should favor areas west of
KSBN for the greatest thunder threat, but will continue to carry
TEMPO TSRA conditions at KSBN through 09Z. A sfc cool front will
drop across the southern Great Lakes this morning eventually
resulting in a tapering of showers and thunderstorms from north
to south. This front may be accompanied by at least some MVFR
cigs/vsbys for a time in the 12Z-15Z timeframe. Will continue
trend of previous forecast in keeping TAFs dry after 15Z with
instability axis getting suppressed southwest of the terminals.
Variable winds due to convective influence overnight will become
northwest during the day.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...None.
OH...None.
MI...None.
MARINE...None.

&&

$$

DISCUSSION...Marsili
AVIATION...Marsili