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.

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 (18Z TAF Issuance)

Issued at 139 PM EDT Tue Sep 8 2026

Impacts:

- Gusty SW winds around 20 kts Wednesday afternoon. - Showers with scattered TS possible by 18Z north terminals, more likely after 22Z at KIND.

Discussion:

Quiet aviation weather is expected for the remainder of today into tonight as S-SW low level winds remain generally under 10 kts under SCT-BKN cirrus clouds. Around sunset winds will decrease and become light southerly ahead of weak low pressure along a slowly moving cold front to the N-NW of central Indiana. Scattered convection is expected overnight well north of the area across Iowa into northern IL. Ahead of this sagging cold front, SW winds will increase by Wednesday afternoon with gusts around 20 kts expected at all terminals except KLAF where a few showers and increasing mid level cloud cover will arrive by early afternoon.

Still a good deal of uncertainty with regards to the amount of convection expected across central Indiana tomorrow afternoon and evening. This is primarily due to the amount of mid level cloud cover expected by mid afternoon, possible scattered showers and the combined negative effects on low level instability. For now have added PROB30 for TS at KIND starting after 18Z with SHRA/VCTS after 22Z, when the main frontal boundary approach will occur along with possibly slightly higher instability amounts.

IND WATCHES/WARNINGS/ADVISORIES

None.