782
FXUS63 KIND 171834
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
234 PM EDT Thu Sep 17 2026

.KEY MESSAGES...

- Hot temperatures will continue at times into Saturday, especially
  south. Heat indices around 100 again today southwest.

- Persistent showers and storms overnight could lead to narrow
  swaths of 2+ inches of rain. A Flood Watch has been issued
  beginning at 10AM and ending at 10PM EDT

- Shower and storm chances persist for much of the time into
  next week, with temperatures turning cooler next week

&&

.DISCUSSION (This evening through Thursday)...
Issued at 223 PM EDT Thu Sep 17 2026

Today and Tonight...

A combination of a strong subgeostrophic upper
jet and weak low pressure over the Atlantic has subdued the
amplitude and strength of a ridge over the SE CONUS, with the upper
jet now dipping into portions of the Ohio Valley. This has been the
focus for mutliple round of convection over the last 24 hours, the
last of which as produced a outflow boundary currently pushing
through central Indiana. Locations south of the boundary will
continue to see typical diurnal curves with temperatures in the low
to mid 90s. North of the boundary, temperatures have dropped 5 to 10
degrees. As the boudnary mixes this afternoon, temperatures will
attempt to rebound, but likely will still remain at or below 90.

This strong outflow boundary has produced a multidue of caveats to
the forecast leading to fairly high uncertainty in the QPF forecast
just 12 to 24 hours out. Model guidance has had a difficult time
assimilating this boundary, of which will likely play a large factor
on where convection is focused late this evening through tonight.
There is alos a broader synoptic front that will near from the NE
tonight. This will likely play out in one of two ways:

    1. The mesoscale boundary and synoptic front are misaligned
leading to a more diffuse and irregular area of showers and
thunderstorms. Within this scenario, centtral Indiana would likely
get a broad area of 0.5-1.0" of rain with isolated pockets of higher
amounts

    2. The mesoscale boundary and synoptic front align prior to
initiation. This would likely result in a more prolific, but narrow
swath of heavy rain with amounts in the 2-3" range where the boudary
settles. Due to this potential scenario, a Flood Watch has been
issued from 10PM until 10AM EDT for a majority of East Central
Indiana.


Friday through Monday...

As mentioned, a synoptic boundary will work its way into central
Indiana tonight into tomorrow, with a large theta-e gradient across
the state tomorrow. This gradient will generally be aligned just
south of the I-74 corridor with temperatures in the low 80s to the
N/E and upper 80s to the S/W. Synotpic forcing will not be abundant
tomorrow, and the 800-700mb lapse rates look to be rather weak. That
said, a few isolated showers and storms cannot be ruled out
along the boundary in the afternoon.

The remainder of the weekend will be a constant push/pull of this
lingering stationary boundary over the region creating both a swing
in temperatures and persistenant rain chances. The best chances for
rain look to be Friday night into Saturday as the front shifts back
to the east. With additional chances late Saturday through Monday
morning as weak waves propegate along the boundary. Temperatures are
expected to remain warm, with some uncertainty depending on boundary
placement and cloud cover extent; general expectations are for highs
in the upper 80s each afternoon for Indianapolis.

&&

.AVIATION (18Z TAF Issuance)...
Issued at 132 PM EDT Thu Sep 17 2026

Impacts:

- Gusty winds at KIND/KHUF and KBMG ending by 20Z.

- Scattered convection may move across KBMG this afternoon and
  overnight into tomorrow morning for all terminals.

- MVFR ceilings are expected to develop overnight into Friday
  morning. Possibly IFR ceilings at KLAF.

Discussion:

A strong wind shift with gusts over 20 kts, caused by a special
kind of outflow boundary called an undular bore/gravity wave,
was generated from convection up north. This feature has pushed
through all but KBMG over the last few hours. A mesohigh pressure
in the wake of this feature has already pushed south and winds
have become variable at KLAF. Expect similar conditions at
KIND/KHUF through 18Z. The wind shift will reach KBMG in the next
hour before weakening across southern Indiana. There is some
potential that scattered TS will develop along the boundary
shortly after it passes KBMG, therefore have added PROB30 for a
period from 18-22Z for this potential.

Attention turns to tonight, when the boundary is expected to lift
back north while the primary front currently over the southern Great
Lakes pushes south and meets renewed convergence from this boundary
over northern IN. A convective complex is expected to develop
somewhere between central and northern Indiana overnight as a
result. Confident is fairly high and therefore have added VCTS/TEMPO
TS for all terminals, earlier at KLAF and latest at KBMG. Also, cold
air advection combined with saturation from the convection should
lead to development of MVFR ceilings at all terminals from north to
south between 09 and 15Z. IFR ceilings are possible for KLAF, but
have been left out as confidence is low. MVFR conditions are
expected to persist at KIND until Fri mid-afternoon.


&&

.IND WATCHES/WARNINGS/ADVISORIES...
Flood Watch from 10 PM EDT this evening through Friday morning
for INZ030-036>042-045>049-054>057-063>065-072.

&&

$$

AVIATION...Crosbie
DISCUSSION...Updike