381
FXUS63 KIND 131317
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
917 AM EDT Sun Sep 13 2026

.KEY MESSAGES...

- A few showers or storms possible late this afternoon and early
  evening

- Briefly cooler on Monday

- Heat returning Tuesday through Thursday with highs into the lower
  and mid 90s in many locations

- Thunderstorm chances increase mid week into next weekend

&&

.FORECAST UPDATE...
Issued at 916 AM EDT Sun Sep 13 2026

No major changes needed to the ongoing forecast with conditions
generally evolving as expected. The last of the fog has dissipated
across central Indiana with visibilities now at or above 5 miles
across the state. Skies are mostly clear to partly cloudy with only
a few spots where low stratus hasn`t quite burned off.

Highs will generally range from the low 80s in the north to the
upper 80s to the south with plenty of sunshine expected through the
day. The primary focus going into the afternoon will be tracking
rain chances associated with a weak secondary front on the backend
of the exiting system. Overall chances look low with the forcing
minimal and a weak cap at the top of the boundary layer. Best
chances for precipitation will be in the late afternoon to early
evening hours when the vort max associated with the system moves
through.

&&

.DISCUSSION (Today through Saturday)...
Issued at 249 AM EDT Sun Sep 13 2026

A subtle frontal boundary will slip through the area today with a
small threat for showers as cold advection strengthens in the
immediate postfrontal airmass. Strong high pressure will expand
across the Ohio Valley tonight and Monday with a brief shot of
cooler and drier air.

Upper ridging will expand back north into the region by Tuesday with
a resumption of hot and humid conditions through midweek as a
frontal boundary becomes quasi-stationary over the lower Great
Lakes. Rain and thunderstorm chances will largely be confined to the
northern half of Indiana late Tuesday and Wednesday before expanding
across the area Thursday as the ridge retrogrades and enables the
boundary to come south. Cooler temperatures are expected by late
next weekend with continued chances for showers and storms.

Today through Monday Night

Muggy...stagnant airmass over the region early this morning under
clear skies and light winds. Patchy fog has developed with abundant
moisture in the near surface layer. 06Z temperatures ranged from the
upper 60s to the mid 70s.

Fog will be the primary focus through daybreak with further
expansion of current fog likely through daybreak under ideal
radiational cooling conditions. Any fog will burn off quickly after
sunrise with a mainly sunny start to the day.

A subtle frontal boundary will drift quietly through the forecast
area during the first half of the day highlighted primarily by a
wind shift and an increase in clouds into the early afternoon. The
approach of a strong Canadian high pressure diving out of the
northern Plains will create a sharpening baroclinic zone in the
immediate postfrontal environment which will strengthen cold
advection across the forecast area by late day and bring a 3-5 hour
period with breezy northwest winds. Scattered convection may break
out by late afternoon in the vicinity of the boundary but this will
be focused primarily to the east of the area across Ohio where model
soundings show steeper lapse rates through about 600mb and modest
instability. Any rain for the forecast area likely comes in the form
of showers or possibly a rumble of thunder associated with the
baroclinic zone late afternoon and early evening before diminishing
near sunset.

The aforementioned high will swing through the Great Lakes tonight
and Monday with a notable area of deep subsidence pivoting across
the northeast half of the forecast area. This will aid in a
spectacular day Monday highlighted by sunny skies...much lower
humidity levels and cooler temperatures. Diurnal cu will be limited
to the lower Wabash Valley primarily where afternoon humidity levels
will be subtly higher. As the high moves into New England and takes
the subsidence east of the region Monday night...return flow will
develop quickly with gradual moisture advection aloft back across
the Ohio Valley by early Tuesday.

Temps...highs in the mid and upper 80s will be common by the
afternoon but the increase in cold advection and a drop in dewpoints
will bring a refreshing feel to the air over the northern half of
the forecast area by late day. Low level thermals on Monday support
highs ranging from the mid 70s northeast to the lower 80s in the
lower Wabash Valley. Lows tonight will fall into the 50s over much
of the area then recovering back into the lower and mid 60s Monday
night.

Tuesday through Sunday

The expansion of the upper ridge back into the Tennessee Valley
combined with strengthening S/SW flow will draw higher dewpoint air
back into the Ohio Valley with hot and humid conditions returning
Tuesday and Wednesday. A cold front will shift across the upper
Midwest and into the Great Lakes on Tuesday as deep low pressure
moves to near James Bay but any further southward progression to the
boundary  will be stopped there with the strong ridge in place to
the south and the western part of the front becoming nearly parallel
to the mean flow aloft. There remains potential for the boundary to
get close enough to the area Tuesday night and Wednesday morning for
more organized convection to impact mainly the northern half of the
forecast area. This will be temporary as the retrogression of the
upper ridge leads to height rises back into the central Plains and
subsequently pushes the boundary back to the north on Wednesday.

Beyond Wednesday...extended guidance begins to deviate in the
handling of just how quickly heights aloft buckle in response to the
center of the upper ridge drifting west into the Southern Plains.
This will be aided by a sharp upper wave traversing across southern
Canada but uncertainty remains with regards to how far south into or
through the forecast area that the frontal boundary can make it.
Convective threats will increase for Thursday and perhaps Friday as
well before a brief period of drier conditions return will the
passage of another stronger surface high to our northeast.
INcreasing signs of the front lingering in the area through next
weekend as the upper ridge remains confined to the southwest. A
stronger surface wave tracking along the boundary may be able to
finally shift the front south of the area by the end of next weekend
but overall confidence is low in this solution at this time.

Temperatures in the lower and mid 90s Tuesday through Thursday
particularly over the southern half of the forecast area will fall
back steadily into the 70s by next Sunday. Max afternoon heat
indices in the upper 90s and low 100s are possible.

&&

.AVIATION (12Z TAF Issuance)...
Issued at 617 AM EDT Sun Sep 13 2026

Impacts:

- Fog diminishing shortly after 12Z
- Brief period with wind gusts to around 20kts late this afternoon
- Small threat for showers late afternoon and early evening

Discussion:

Weak high pressure has settled across the area with near calm winds
and clear skies early this morning. Patchy fog has producing
oscillating visibilities at the outlying terminals for much of the
last several hours. Fog will burn off quickly shortly after sunrise.

Mid level clouds and scattered cu will increase into the area later
this morning and in the afternoon. A brief period of gusty northwest
winds will develop late day before decreasing towards sunset. Cannot
rule out a few light showers developing during the late afternoon
and early evening but this will have no impact on the terminals.
Winds will veer to northeast tonight and diminish to 5-10kts early
Monday.


&&

.IND WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

UPDATE...White
AVIATION...Ryan
DISCUSSION...Ryan