textproduct: Indianapolis

KEY MESSAGES

- Severe storms likely today, best chances during the afternoon and evening

- Flood Watch today across much of central Indiana, Heat Advisory across the southwest

- Additional storms daily through the rest of the week, flooding will be the main threat

FORECAST UPDATE

Issued at 857 AM EDT Tue Aug 11 2026

GOES19 this morning shows a convective complex over NE Indiana progressing SE within the upper flow. Radar trends show this system clipping our NE forecast area before exiting within the next 90 minutes. Another convective cluster was found over NE IA, drifting SE within the flow aloft. The current convective cluster was laying out an outflow boundary across Central Indiana that was stretching from southern Madison county, across Fishers and Carmel to Lebanon and Lafayette. This area could be the focus for additional development or storm propagation later today.

A weak surface trough was found stretching IA across Nrn IL to northern Indiana. This was the area where current convection continues. Water vapor shows NW flow in place from MT and WY, flowing across the northern plains to Illinois and Indiana. embedded forcing remains within this flow helping to generate our rounds of thunderstorms. ACARS soundings at IND shows deep CAPE available with a lower level inversion present. Dew points remain quite high, in the low to mid 70s. Thus plenty of moisture and potential instability remain in place across Central Indiana. HRRR quickly moves the ongoing system SE to OH this morning, but redevelopment is shown to appear over over northern Indiana by mid afternoon. Potential for this to be further south, along the previously defined boundary is present. Thus will drop pops low late this morning, but return to higher pops in the northern parts of the forecast area given the expected daytime heating, forcing and moisture in our area. Cloud cover across the area may limit highs acrs the NE parts of the forecast area. Otherwise, the highs in the low 90s, within the heat advisory area, appear on the mark.

DISCUSSION (Today through Monday)

Issued at 139 AM EDT Tue Aug 11 2026

Overview.

Severe weather is likely today with a complex of storms moving in from the northwest. Repeated rounds of thunderstorms are expected to continue through Wednesday night with lower chances through the remainder of the week. Damaging winds and flooding will be the main threats today and tomorrow. Outside of storms, heat will be a concern today with heat index values up to 105 in southwest Indiana.

Today Through Tonight.

Central Indiana remains positioned along the northeastern periphery of a strong subtropical ridge centered over the southern Plains, maintaining an active northwest flow regime aloft. Low-level WAA and moisture transport continue to stream across the region, yielding surface dewpoints in the mid-70s and PWAT values approaching 2.0 to 2.2 inches. Strong diurnal heating will destabilize the boundary layer today, yielding SBCAPE values exceeding 3500-4500 J/kg by afternoon. High temperatures rising into the upper 80s to low 90s combined with elevated humidity will create heat index values near 105 degrees, particularly across southern and central counties. A Heat Advisory is in effect for southwestern Indiana from 12PM to 9PM.

Convective initiation and timing remain tied to subtle shortwaves riding the ridge edge and residual boundaries from previous storms.

00Z model guidance actually is in fairly good agreement on the broader timing for convection through daybreak and into today, but differ on the exact details. Broadly, there is good agreement in initiation continuing over the next couple of hours over northern Illinois/northern Indiana, likely along the remnant outflow boundary from the earlier convection. Some models have this quickly growing upscale into an MCS that dives south, but with the fairly stable near surface, expect these storms to remain elevated and generally sub-severe. The second round would then initiate across eastern Iowa and northern Illinois towards mid-morning with quick upscale growth into an MCS and an arrival near the I-74 corridor towards the late afternoon to early evening. By the time storms arrive SB CAPE values will be in excess of 3500 J/kg across central Indiana with sufficient shear to maintain an organized MCS with the potential for widespread damaging winds. If storms through daybreak do end up growing upscale, this could delay the second round by a few hours, but with the very humid airmass, it looks unlikely that any morning convection would significantly lower any risk later in the day.

Regardless of timing, heavy rainfall and flash flooding will be another significant concern. High freezing levels above 14,000 feet and deep warm-rain processes combined with the potential for training convective cores along the western periphery of the complex raise flash flood potential. HREF probability matched mean guidance indicates localized rainfall totals of 2 to 4 inches where storm training occurs. With that in mind, a Flood Watch is in effect from 12PM through 10PM with the potential need for an extension or additional watch for Wednesday.

Wednesday Through Monday.

The active synoptic configuration persists into Wednesday as central Indiana stays situated in the baroclinic gradient on the eastern flank of the southern ridge. Global guidance, including the GFS and ECMWF, depicts a prominent mid-level trough digging from the Upper Midwest into the Great Lakes on Wednesday. This wave will help to push the aforementioned surface cold front southward through the CWA late Wednesday through early Thursday morning. Deep-layer moisture remains unseasonably high across the southern half of the forecast area with PWATs lingering near 2.0 inches ahead of the boundary. The combination of frontal forcing, upper-level jet divergence, and moderate instability will generate another round of showers and thunderstorms Wednesday afternoon and night. The GEFS and EPS ensemble means show strong agreement on severe weather and heavy rainfall potential continuing along and south of I-70 until frontal passage occurs early Thursday.

By Thursday night into Friday, model guidance agrees on the southward progression of the surface cold front into the Ohio Valley, bringing at least some minor relief from the hot and humid conditions. High temperatures Thursday and Friday will moderate into the low to mid- 80s with dewpoints dropping into the upper 60s to low 70s for all but the southern portions of the state. Low-end chance PoPs are maintained across southern counties beginning Friday morning to account for residual elevated moisture and minor shortwave energy overriding the frontal zone, but the overall threat for severe weather and flash flooding will wane significantly.

For the weekend, the overall synoptic evolution diverges slightly between deterministic operational runs. The ECMWF maintains quasi- zonal flow aloft with surface high pressure shifting toward the East Coast, keeping central Indiana mostly dry with temperatures near seasonal normals. The GFS suggests a faster return of low-level warm air advection and ridge building northward into the Ohio Valley by Sunday, introducing isolated diurnal thunderstorm chances. EPS and GEFS ensemble cluster analyses favor a drier consensus for Saturday before gradually increasing PoPs across western counties late Sunday into Sunday night.

AVIATION (12Z TAF Issuance)

Issued at 600 AM EDT Tue Aug 11 2026

Impacts:

- TSRA possible at times through the TAF period, confidence highest at LAF and IND - Best chances currently look to be from 17Z to 00Z - Southwesterly wind gusts up to 20kts 15Z to 00Z - Potential for fog tonight, mainly at LAF

Discussion:

VFR conditions expected outside of convection through the TAF period. The first chance for precipitation will be during the pre- dawn hours towards LAF but confidence remains low that the ongoing convection to the north will impact the site. Better chances arrive towards the afternoon and evening hours with a bowing line of storms which could bring damaging winds to LAF/IND. With confidence low on timing, will limit the TSRA mention to just a PROB30 between 18Z and 00Z. As confidence on timing increases, will narrow down the windows in later TAF issuances. Southwesterly winds will occasionally gust up to 20kts through the afternoon before dropping closer to calm after 00Z. Fog will be possible tonight depending on the extent of rainfall today with confidence highest at LAF.

IND WATCHES/WARNINGS/ADVISORIES

Flood Watch from noon EDT today through this evening for INZ021- 028>031-035>049-054>057-063>065-072.

Heat Advisory from noon today to 9 PM EDT this evening for INZ060-061-067>070.