438
FXUS63 KIWX 151035
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
635 AM EDT Tue Jul 15 2025

.KEY MESSAGES...

- Hot conditions again today with humidity levels gradually
  creeping up. Heat indices in the upper 80s to lower 90s
  today, and in the lower to middle 90s for Wednesday.

- Low chances of showers and thunderstorms later this afternoon
  south of US 24, with increasing chances of showers and a few
  storms later tonight into Wednesday AM, mainly along and east
  of I-69.

- Another round of thunderstorms some time Wednesday night into
  Thursday timeframe. There is a Marginal Risk of severe storms
  mainly Wednesday night for wind.

- Brief period of less humid conditions for Friday.

&&

.DISCUSSION...
Issued at 355 AM EDT Tue Jul 15 2025

Increasing humidity levels and additional thunderstorm chances
will be the primary short term forecast items of interest.

A strong low-mid level theta-e gradient persists across the region
this morning as a broad low amplitude upper level ridge remains
anchored from the southern Great Lakes into the Ohio Valley. 00Z
RAOBs depict this moisture gradient nicely with 0.75 inch PWATS at
Detroit and 2.05 inches at Nashville. Water vapor imagery early this
morning indicates some ongoing convection across Arkansas in
vicinity of an old cut off upper level vort max. As mean upper
ridge axis shifts east today, conditions will become conducive
for this vort max to get ingested in southwest upper flow into
the Ohio River Valley. Low level flow fields will not be
particularly strong, but the strength of this moisture gradient
will allow for moderate to strong moisture transport axis to
work across northeast Indiana/northwest Ohio into tonight.

In terms of precipitation chances, some isolated showers and perhaps
a few thunderstorms may develop later this afternoon particularly
south of US Route 24 where low level moisture recovery could be
enough to overcome weak low level CIN induced by subsidence
inversion. Primary vort max will be highly sheared in nature as it
lifts northeast, with stronger forcing and moisture transport
reaching far NE Indiana/NW Ohio later this evening into early
Wednesday morning. Collaboration with surrounding offices did
add some slight chance PoPs farther back to the north/west, as
cannot rule out isolated shower or storm along the northward
advancing strong moisture gradient. The greater precip coverage
is expected generally east of I-69 tonight into early Wed. High
temperatures today should reach into the mid 80s to around 90,
with warmest temps probably central/northern locations where
insolation is expected to be more pronounced. Heat indices
should top out from the upper 80s to mid 90s (highest across
southwest/south where some better moisture recovery is
anticipated).

After scattered showers and isolated storms, especially
along/east of I-69 corridor Wednesday morning, it is possible a
lull in higher precip chances could develop as the initially
sheared vort max shifts east of the area. Thus, PoPs have been
lowered for the 18Z-00Z timeframe Wednesday. A moderate to
strong instability axis should develop later in the day
Wednesday from west central Illinois to the southwest Great
Lakes region. Best chances of stronger convection late
Wednesday afternoon should be focused back across the Mid MS
Valley in vicinity of this instability axis and where large
scale forcing will likely be on the increase from progressive
eastern Pacific short wave (possibly convectively enhanced).
Associated synoptic frontal forcing and renewed deep moisture
profiles should shift across the area overnight Wednesday into
early Thursday morning with a possibility of some strong storms
and some needed rainfall across the region. SPC Day 2 Outlook
places much of northern Illinois in Slight Risk for severe
storms with a Marginal Risk across southern Lower MI and
northwest/north central Indiana. The primary concern would be
for some strong winds given potential of a more consolidated MCS
across the southern Great Lakes. Otherwise, Wednesday will
feature hot and humid conditions but weak low level dry air
advection in the PM hours in wake of the departing initial
sheared vort max should keep heat indices below advisory
criteria.

A temporary influx of less humid air follows the Thursday fropa for
Thursday night into Friday, but the continuation of a low amplitude
upper ridging pattern will allow heat/humidity to build again for
the weekend. This ridge should be suppressed some by the weekend
as active series of Pacific waves allow for several chances of
storms in the Saturday-Monday period. The potential exists for
convectively enhanced disturbances to provide some temporarily
better westerly shear at times across the region for a few bouts
of better convective organization.

Looking ahead, medium range deterministic/ensemble guidance
does support some amplification of longwave ridging heading
into the Tue- Thu timeframe which could lead to a multi-day heat
concern. Convective chances become more muddled early next week
as confidence lowers in more significant synoptic waves
dampening the upper ridge enough. Given low predictability in
this pattern, just maintained blended guidance low chance PoPs
early next week.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 619 AM EDT Tue Jul 15 2025

Light fog was in the vicinity of the SBN terminal, although
patchy fog with lower visibilities was over northern Indiana.
Have kept fog out for now, although there could be a brief
intrusion of fog before 15Z. Winds were calm or very light early
this morning. The pressure gradient will continue to be very
weak over the area, so have calm winds. Isolated storms should
develop by early this evening, especially near FWA. Felt
coverage was still to low to include in the TAF at this time.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...Air Quality Alert until midnight CDT tonight for INZ103-203.
OH...None.
MI...None.
MARINE...None.

&&

$$

DISCUSSION...Marsili
AVIATION...Skipper