textproduct: Indianapolis

KEY MESSAGES

- Heat Advisory in effect for all of central Indiana through late Friday with peak afternoon heat index values up to 108 degrees

- Isolated to scattered thunderstorms will be possible at times Friday and Saturday

FORECAST UPDATE

Issued at 957 PM EDT Thu Sep 3 2026

Another anomalously warm late-summer overnight is in progress across central Indiana as Indianapolis cools off from its first 3- consecutive September days reaching 95F+ since 2011. Late day/early evening RWs and stray t-storms that graced the far northern corners of the CWA have mainly dissipated with the setting sun, with a final shower exiting Randolph County into Ohio as of 950 PM EDT. Cannot rule out a sprinkle or brief shower near the I-70 late tonight along a subtle boundary...with no impacts expected. Otherwise light SSW winds will allow readings to slowly fall through the low 80s and upper 70s amid another night of low to mid-70s dewpoints. Low temperatures by early Friday to range from 70F in Winchester to 76F at Monroe Lake.

DISCUSSION (This evening through Thursday)

Issued at 252 PM EDT Thu Sep 3 2026

Ridging will continue today across central Indiana, but a shortwave will develop on the periphery and push just to the north and east. A weak outflow boundary is currently draped across NE IL and NW IN, but latest observations are showing this boundary becoming more diffuse as it progresses eastward. This trend is expected to continue with minimal impacts over central Indiana outside of a slight increase in diurnal cumulus.

Most CAMs depict additional convective development later this afternoon over northern IN where the shortwave is located, with a general progression E/NE during the evening. While parameter spacing does not support convective development over central Indiana, cold pool development may allow for decaying thunderstorms or light showers to reach northern central Indiana between 7-10PM. Temperatures and heat index values will be high once again today. There is some signs of mixing this afternoon which may drop dew points a few degrees. A few occasional gusts to 18kt will also be possible as the PBL becomes mixed this afternoon.

Conditions are expected to be very similar tomorrow, with a near identical synoptic setup. The main difference is the position of a mid level boundary and additional moisture convergence through the 850-700mb layer. This may allow for enough saturation in this layer for isolated convective showers and thunderstorms in the late afternoon and evening. While severe weather is not expected, DCAPE of 900-1000 J/KG may support a few downbursts within this "popcorn" convection to approach severe criteria. Where this mid level boundary situates tomorrow will also be the primary location for additional MCS development tomorrow afternoon through Saturday morning. This is currently expected to remain NE of central Indiana, but the area of concern will be close enough to where a brief glancing from organized thunderstorms will be possible over far NE portions of the area.

The mid level boundary will have more synoptic support Saturday, with continental polar air attempting to move into the area from the NE late Saturday into Saturday night. This should generally increase convective activity across the area though moisture convergence and increased wind shear. Confidence on where greatest thunderstorm coverage will be located is low at this time.

A brief cooldown is likely Sunday through Monday as a deepening trough over the East Coast, pushes the ridge westward. High pressure is expected to build beneath upper level confluence leading to a drying trend with highs in the mid 80s with dew points in the upper 50s to mid 60s. Towards the middle of next week, the ridge is expected to build back over the region, with an additional stretch of well above normal temperatures and humidity.

AVIATION (06Z TAF Issuance)

Issued at 149 AM EDT Fri Sep 4 2026

Impacts:

- Isolated convection possible Friday PM, mainly near KIND/KLAF but low confidence

Discussion:

VFR conditions are expected through the TAF period. Hot and humid subtropical ridge will shift its center from the SE CONUS to the Plains which will allow a small northern disturbance to possibly drag isolated convection across the region late in the TAF period. So far confidence too low in TSRA for any mention in TAFs past VCSH.

Minor VIS reductions from patchy fog are possible late tonight, but certainty in MVFR too low to include in TAFs. Overall light winds tonight may maintain subtle organized southerly flow to at least 06Z. Winds Friday afternoon outside of any convection will be from 240-280 degrees, nearing 7KT/10KT thresholds, with most gusts weaker than prior days.

IND WATCHES/WARNINGS/ADVISORIES

Heat Advisory until 8 PM EDT this evening for INZ021-028>031- 035>049-051>057-060>065-067>072.