textproduct: Indianapolis

KEY MESSAGES

- Heat returns today and Wednesday with peak heat indices in the upper 90s and low 100s Wednesday afternoon

- Strong thunderstorms possible late Wednesday; rain and storms continue Wednesday night into Thursday with heavy rain and localized flooding as the primary concern

- Cooler weather returns late week, warming up during the weekend.

FORECAST UPDATE

Issued at 919 PM EDT Tue Sep 8 2026

No major changes needed to the forecast with conditions generally evolving as expected. Temperatures have begun to fall with the loss of daytime heat, but not quite as fast as previous days with the much higher surface dewpoints in place.

There may be a few areas of fog towards daybreak in more favored fog formation zones, but coverage should be fairly low with winds remaining in the 5-7 mph range for much of the night.

DISCUSSION (This evening through Tuesday)

Issued at 303 PM EDT Tue Sep 8 2026

Upper-level ridging over the Plains is flattening and moving eastward as a potent shortwave digs southward into the Dakotas. This has pushed the ridge axis over Indiana allowing temperatures to once again warm up. Readings in the upper 80s to low 90s are expected today and Wednesday. The shortwave's attendant cold front arrives late Wednesday, slowing in its forward speed and likely stalling over southern Indiana. Upper-level flow becomes parallel to the boundary allowing for repeated rounds of precipitation Wednesday evening into Thursday. The vort max departs later on Thursday with broad ridging taking hold again over the Plains as we head into the weekend.

WEDNESDAY

Southwesterly flow increases ahead of the approaching front late Wednesday morning into the afternoon. This will promote strong low- level moisture advection with dew points rising into the 70s. Combined modest cold air advection aloft / height falls, strong instability is expected by late Wednesday afternoon. Guidance shows between 2000-3000 J/Kg MLCAPE across southern Indiana decreasing with northward extent. Shear increases as well due to the approaching shortwave, easterly at around 30kt. The front itself will provide the forcing necessary for convection.

Given a shear vector largely parallel to the front, a linear convective mode appears likely. Marginal to poor lapse rates should limit severe potential. Mid-level dryness could lead to a downburst or two, given about 1000 J/Kg DCAPE. Our primary hazard appears to be flash flooding, with storms potentially training due to the slow- moving front and boundary-parellel flow. Convection may continue into the night as additional rounds develop due to embedded disturbances aloft. The best chance for both severe and hydro threats is over our southern CWA where the front is most likely to stall.

A potential failure mode for convection on Wednesday is related to convection upstream tonight. Should an outflow boundary surge southward out of Illinois tomorrow morning, a low-level stable layer and cap may remain in place most of the day. This, as shown by the 12Z HRRR may substantially limit convective development.

THURSDAY

Guidance is in good agreement showing showers and thunderstorms persisting into the day Thursday, with a few differences. These differences are mainly regarding the northern extent of precipitation chances. Global models such as the ECMWF and GFS tend to bring a slug of heavier precip right through central Indiana, where finer-scale models like the RAP and NAM are further south. For now, we will nudge the forecast towards the southern side of guidance as there has been an overall trend in that direction today. This all depends on where the front ends up stalling, however, which may be influenced by Wednesday's convective evolution. Needless to say, there is a bit of uncertainty in terms of precipitation placement on Thursday.

FRIDAY ONWARD

Cooler weather arrives Thursday and continues into Friday before ridging returns on Saturday. Highs near 80 are expected to end the week with temperatures rising back towards 90 over the weekend. Temps in the mid 80s to near 90 then persist into early next week. A weak front may arrive from the north on Sunday but model consensus is poor. PoPs will be limited to chance (40 percent) Sunday and Monday. Most locations should remain dry through the weekend.

AVIATION (06Z TAF Issuance)

Issued at 100 AM EDT Wed Sep 9 2026

Impacts:

- Peak wind gusts around 20kts this afternoon - Scattered convection possible by late morning at KLAF and into the afternoon at the other terminals

Discussion:

Cirrus blowoff from convection over Iowa will expand into the region during the predawn hours with mid and high level cloud cover becoming increasingly dense later this morning. There remains high uncertainty with regards to whether the ongoing convection can make it into central Indiana late morning into the afternoon but model soundings show enough instability present to maintain PROB30s at all of the terminals for expected scattered convection. Coverage will drop off with the loss of heating this evening and it now appears they may be a lull that lasts into the first half of tonight before more widespread showers expand into the region towards daybreak Thursday.

Southwest winds will become gusty through the afternoon as the cold front to the north approaches the region. The front will slide into the area this evening with winds becoming light and variable.

IND WATCHES/WARNINGS/ADVISORIES

None.