textproduct: Indianapolis

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.