textproduct: Indianapolis

KEY MESSAGES

- Hot temperatures will continue at times into Saturday, especially south. Heat indices in the low 100s again Thursday

- Narrow bands of 2 to 4 inches of rain will be possible tonight through Thursday morning within persistent rounds of thunderstorms over NE central Indiana. A Flood Watch has been issued from 12 AM to 2PM EDT

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

FORECAST UPDATE

Issued at 1000 PM EDT Wed Sep 16 2026

Another unseasonably warm and humid overnight is on tap for central Indiana as the region continues to rest under the northern periphery of a broad late summer subtropical ridge. Fetch of monsoon moisture from Four Corners region through central Plains...will maintain 2.00 inch or greater precipitable water values over at least the northern half of the CWA. Potential exists for associated areas of heavy rain tonight...although decreasing confidence in flood concerns through 12Z Thursday with CAMs reflecting lack of forcing and if anything slight northern trend with axis of any scattered to perhaps steadier rain...with better confidence for R+ into the Thursday morning timeframe. Elevated dewpoints to again guide low temperatures, with low 70s common across the region...and patchy fog possible, especially along/just south of I-70 corridor where late day showers dropped up to 2.00 inches Wednesday.

DISCUSSION (This evening through Wednesday)

Issued at 329 PM EDT Wed Sep 16 2026

Today...

Much of weather over the last 36 hours has been dominated by a large, broad area of high pressure over the SE CONUS. A combination of a strong subgeostrophic upper jet and weak low pressure over the Atlantic has subdued the amplitude and strength of ridge, with the upper jet now dipping into portions of the Ohio Valley.

In the lower levels, a closed low and associated SW jet is exiting to the NE. This jet pushed large amounts of moisture into the region, of which has created a broad area of instability over central Indiana. Latest soundings indicate SBCAPE values of around 1500 J/kg, with 2500 J/kg expected by late this afternoon. With convective temperatures around 90 degrees, a broad cirrus shield, and a modest low level warm layer, convection is likely going to be confined to weakly forced areas along residual outflow boundaries.

The severe threat today will be rather limited with bulk shear values less than 20kt, but lapse rates in the mid to upper levels are steep enough for a few isolated strong cores to develop. In these cases, a brief area of enhanced winds (likely 40-50MPH) would be possible.

Tonight...

As we head into the overnight period, a weak positively tilted trough will approach from the NW. As this interacts with the high pressure to the south, confluence will promote the development of a nocturnal LLJ. This in combination with a modest low level boundary and a saturated thermal profile will likely lead to a narrow (50- 100mi) corridor of persistent thunderstorms. Model guidance is still widely varied on placement of these thunderstorms, with the latest HREF LPMM highlighting a corridor of 3-5" over E/NE Indiana. Confidence is not high on if NE central will be within this narrow corridor, but the margin of error is low enough, and the impact potential is high enough that a Flood Watch has been issued for far N/NE central Indiana from midnight tonight until 2PM EDT tomorrow encompassing the greatest timing concern of 4AM to 11AM EDT.

Severe weather a distant secondary concern overnight, but the mid level lapse rates due remain steep enough for a few stronger cores to be embedded within the persistent thunderstorm threat. For this reason, isolated gusts of 40-50 MPH will be possible overnight.

The LLJ will weaken with diurnal heating on Thursday, of which should dampen convective initiation late Thursday morning into the early afternoon. This should lead to a lull in thunderstorms for a portion of tomorrow before diurnal convection returns along remnant boundaries in the late afternoon and evening.

Friday and beyond...

The upper high will continue to gradually retreat southwest. The surface front will sink far enough south initially to keep PoPs low on Friday, but it will return north again on Saturday as an upper wave interacts with the front. This will bring more rain chances, mainly north.

After a brief reprieve from heat for many areas Friday, hot conditions return on Saturday as the front moves back north.

Sunday into next week, larger upper waves moving through will keep chances for rain around. Cooler air will filter back into the area, with highs by Tuesday in the upper 70s to middle 80s.

AVIATION (00Z TAF Issuance)

Issued at 807 PM EDT Wed Sep 16 2026

Impacts:

- Thunderstorms are possible at KLAF late tonight and potentially again at KIND/KHUF Thu morning.

- MVFR ceilings expected to occur at KLAF towards 12Z Thu.

Discussion:

Convection has largely come to an end across central Indiana this evening.

Overnight, the primary frontal boundary will have shifted northward towards KLAF and northern portions of central IN. With an approaching weak shortwave, thunderstorm activity is expected to be more numerous around KLAF after 06Z as the low level jet increases into the boundary enhancing convergence. There is some chance that this greater coverage of TS will move towards KIND/KHUF by 12Z Thursday and have introduced PROB30 for TSRA for this scenario through 18Z.

There is decent agreement that ceilings will fall overnight with low level cooling and saturation associated with both convective outflow and rainfall further N-NW of central IN. Ceilings are expected to fall into the MVFR to IFR range for KLAF towards 12Z.

IND WATCHES/WARNINGS/ADVISORIES

Flood Watch through Thursday afternoon for INZ031-038-040>042- 049.