015
FXUS63 KIWX 011048
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
648 AM EDT Tue Sep 1 2026

.KEY MESSAGES...

- Heat indices from the upper 90s to around 105 are expected
  today and Wednesday. A Heat Advisory is in effect. Heat
  indices around 100 may persist south of US Route 30 into
  Thursday.

- An isolated thunderstorm is possible today into this evening
  with greatest chances this afternoon and early evening.
  Widespread severe weather is not expected, but if an isolated
  storm can develop, it would have the potential to produce
  isolated damaging wind gusts mainly north of US Route 30. Some
  isolated hail is also possible but of very low confidence.

- An isolated strong to severe thunderstorm is also possible
  late Wednesday afternoon into Wednesday evening mainly across
  south central Lower Michigan and northwest Ohio. Isolated
  damaging wind gusts and isolated large hail would be the
  primary severe threats, but confidence in isolated severe
  weather is low at this time.

&&

.DISCUSSION...
Issued at 335 AM EDT Tue Sep 1 2026

Showers have largely diminished across the area early this morning.
Regional radar mosaic does depict some low coverage showers across
portions of north central Illinois in what has been a persistent
zone of westerly flow isentropic upglide from north central Illinois
into northwest Ohio. One characteristic that has changed over past 6
to 12 hours is the northeast advection of warmer mid level profiles
as evident with substantial low level CIN noted per KDVN and KILX
00Z RAOB from last evening. This west-east oriented zone of
isentropic forcing will persist through daybreak although in
terms of mid/upper level forcing, this does appear to coincide
with a lull. Did maintain some 15-20 PoPs across the northern
third into mid morning, but weak mid/upper level forcing and
somewhat more unfavorable mid level thermal profiles should keep
coverage very low if any showers are able to develop.

For this afternoon, some sheared vorticity associated with stronger
northern latitude westerlies will top the upper ridge and begin to
shift east across the western/southern Great Lakes late this
afternoon into early this evening.  Primary surface boundary will be
displaced well to the northwest but strong buoyancy is expected to
develop in pre-frontal environment from northeast IL eastward
into southern Lower MI/far northern IN (MLCAPEs around 3000 J/kg
possible). The more robust shear profiles should be displaced
closer to main sfc boundary across, but deep layer shear
magnitudes on the order of 25 to 35 knots will likely have a
narrow spatial overlay with this stronger buoyancy north of the
US Route 30 corridor. The very warm mid level profiles in place
raise some question as to the potential of surface based CIN
lingering in the afternoon, although with the very warm/moist
near sfc airmass in place, would suspect this CIN will be
minimal during peak hating. What appears to be a large component
missing today from an appreciable severe threat is any
pronounced stronger pre-frontal convergence or trigger
mechanism. Aforementioned shear vorticity max will likely be
migrating northward across east central WI/central Lower MI
later this afternoon/evening, with not a great juxtaposition of
forcing and better shear/instability. A Marginal Risk of severe
weather has been maintained today for areas roughly north of US
Route 30 corridor which seems reasonable given
shear/instability combination. Some persistent isentropic
upglide in 310-315k layer persists today which could aid in
initiation, but overall weak nature to mid/upper level forcing
continues to suggest very low confidence in severe weather
occurrence today.

For most, the main weather concern today will be high heat and
humidity, with warm/moist airmass now firmly in place across the
region. Given aforementioned warmer mid level profiles and lack of
clear upstream wave early in the day, better insolation should be
realized today in comparison to past few days with most areas
reaching around 90 or lower 90s for highs. Dew points should be
fairly uniform in low to possibly mid 70s across the area which
will produce heat indices in the upper 90s to around 105. Given
continued low confidence in coverage of showers/storms even across
the north today, derived heat indices from this morning`s forecast
grids show rather minimal differences across the area with peak
afternoon heat indices. Given Wednesday should feature even greater
confidence in reaching advisory criteria area-wide, did go ahead and
extend the pre-existing Heat Advisory across the remainder of
southern Lower Michigan in effect from 18Z today to 00Z Wednesday
evening for all areas. Have not expanded this advisory into
Thursday yet, but the possibility exists that areas roughly
along and south of US 30 may need the Heat Advisory extended
into early Thursday evening.

In addition to the ongoing heat concerns for Wednesday,
convective potential will once again need to be watched for
afternoon/evening. The larger scale primary upper trough will be
tracking across western Quebec Wed afternoon but another short
wave will be exiting out of the Dakotas with a secondary
northward surge of a low level theta-e ridge across Upper
Midwest/Corn Belt. Synoptic pattern would tend to favor frontal
boundary position north of the local area into early Wednesday
evening, but if a more organized area of storms can form across
Upper Midwest/Corn Belt Wednesday at nose of this secondary
theta-e max, it is possible associated outflow could modulate
position of this boundary to the southern Lower Michigan by
evening. In any event, stronger mid/upper level forcing still
looks to bypass the area to the north, and isolated severe
threat may require this southward modulation of effective
frontal boundary by upstream outflow which is of course of low
confidence at this time. Day 2 Marginal Risk still remains in
place given potential of moderate to strong buoyancy and
persistence of sufficient deep vertical shear profiles. Chances
of storms will likely persist through later night period
Wednesday night as some increase in low level southwesterly flow
is expected later in the night which could generate some more
convection near this southern Great Lakes boundary.

For Thursday, low level front will likely remained stalled
across the Great Lakes region given some mid level height
recovery across Mid MS Valley/western Great Lakes Thu afternoon.
Additional smaller scale short waves progressing through upper
ridge could interact with this boundary for additional
thunderstorms Thursday afternoon and evening, and perhaps
another day of isolated strong/severe potential.

Medium range guidance remains consistent with transition in longwave
pattern to more amplitude Fri-Sat with maintenance of central CONUS
longwave ridge. This could keep local area positioned along
instability gradient for good portion of the late work
week/weekend period with some additional shower/storm chances as
additional NW waves dive down the ridge. This ridge positioning
should end concern for heat headlines later this week with
temps likely settling back near normal toward the end of this
forecast valid period.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 645 AM EDT Tue Sep 1 2026

A west-east band of showers is ongoing this morning from
northeast Illinois to just south of the KSBN vicinity. These
showers appear to be developing at the crest of a mid level
thermal ridge where a very subtle mid trough is approaching
southern Lake Michigan. Persistent isentropic ascent along this
west-east corridor (albeit weaker than last evening) should
sustain these showers eastward across northeast Indiana,
possibly taking a slight southeast turn mid morning. Isolated
showers should affect KSBN vicinity early this morning and may
clip KFWA during the mid morning hours. It does appear by midday
that this isentropic ascent should largely weaken and attention
will turn to any isolated shower/storm development this
afternoon and early evening across far northern Indiana.
Confidence remains low in this redevelopment this afternoon and
will keep TAFs dry after 18Z at this time. Better chances of
storms tonight should focus back across northern Iowa/southern
Minnesota at the nose of a low level jet. This may keep focus of
showers and storms focus northwest of the local area tonight
into early Wednesday morning.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...Heat Advisory from 2 PM EDT /1 PM CDT/ this afternoon to 8 PM
     EDT /7 PM CDT/ Wednesday for INZ005>009-012>015-017-018-
     020-022>027-032>034-103-104-116-203-204-216.
OH...Heat Advisory from 2 PM this afternoon to 8 PM EDT Wednesday
     for OHZ001-002-004-005-015-016-024-025.
MI...Heat Advisory from 2 PM this afternoon to 8 PM EDT Wednesday
     for MIZ078>081-177-277.
MARINE...None.

&&

$$

DISCUSSION...Marsili
AVIATION...Marsili