textproduct: Indianapolis

KEY MESSAGES

- Extreme Heat Warning in effect through 8 PM Friday, Heat Advisory afterwards through 9PM Saturday

- Thunderstorm chances will increase after 7PM today with downbursts possible

- Greater storm coverage for Saturday with a lower threat for severe weather

DISCUSSION (Today through Thursday)

Issued at 131 AM EDT Fri Jul 3 2026

Overview.

Patchy fog early this morning will give way to another day of high heat and humidity, with heat indices above 105 degrees. Thunderstorms are expected to develop during the late afternoon and evening hours both today and tomorrow, presenting a severe localized downburst and damaging wind threat. A slow cooling trend begins Sunday, though daily chances for scattered showers and storms will persist through mid-week.

Today through Saturday.

Near-surface saturation from residual boundary layer moisture and light to calm winds beneath a lingering low-level inversion will support patchy radiation fog development early this morning. This will primarily impact river valleys and low-lying areas before rapidly dissipating by 13Z with the onset of diurnal mixing. Thereafter, focus shifts to both the heat and storm chances today. While the anomalous sub-tropical ridge across the Ohio Valley begins to subtly deamplify and suppress southward, 850 mb temperatures around +22C to +24C and intense diabatic heating will act on a high theta-e airmass. With surface dew points forecast in the mid to upper 70s, convective temperatures will quickly be breached in the low 90s, driving peak afternoon heat indices between 105 and 110 degrees both Friday and Saturday.

Weakening large-scale subsidence will allow the capping inversion near 700 mb to erode, clearing the way for convective initiation during the peak diurnal heating windows. For Friday, timing for isolated to scattered convective development is centered between 23Z and 04Z, focusing along subtle mesoscale convergence zones or differential heating boundaries. On Saturday, a weak, low-amplitude shortwave traversing the flattening ridge periphery will provide slightly stronger synoptic forcing, shifting the initiation window slightly earlier to 19Z to 03Z and yielding higher storm coverage.

Thermodynamics both days are exceptionally potent, with MLCAPE values progged to exceed 3500-4000 J/kg. While weak deep-layer vertical wind shear of less than 20kts will limit organized, long- lived updrafts or supercellular structures, the environment is classic for severe pulse convection and microbursts. Model soundings display an ideal profile for severe downburst generation: PWAT values near 2.1 inches maximizing precipitation loading aloft, combined with steep low-level lapse rates with 0-3 km lapse rates exceeding 7.5 C/km and a dry mid-level layer creating DCAPE values over 1300 J/kg. Updrafts will rapidly accelerate, load heavy water cores, and subsequently collapse as intense negative buoyancy and evaporative cooling accelerate air downward. This may result in localized, significant straight-line downburst winds capable of exceeding 60-70 mph. The severe weather threat for today looks higher than Saturday with storm coverage on Saturday looking greater compared to today.

With the heat indices expected to top out in the low 100s again today, opted to issue a Heat Advisory with the known uncertainty as to storms bringing earlier relief than the current 9 PM endtime. With the extended stretch of heat already combined with the holiday weekend, felt the headline was warranted even as we remain just under typical criteria.

Sunday through Thursday.

The synoptic pattern undergoes a more noticeable transition by Sunday as the sub-tropical ridge is forced further into the Gulf Coast states, positioning central Indiana on the southern periphery of an active, progressive northwest flow regime aloft. This will open the door for a series of low-amplitude shortwave troughs and embedded convective impulses to traverse the Great Lakes and Ohio Valley through the mid-term period. While the extreme, warning-level heat will subside as 850mb temperatures drop back toward +18C, a warm, moist, and moderately unstable airmass will linger, maintaining daily opportunities for scattered showers and thunderstorms.

Convection during the Sunday through Monday timeframe will remain largely diurnal and mesoscale-dependent, focusing along residual outflow boundaries. Pulse severe hazards, specifically localized downbursts, will remain a secondary threat but with lower overall coverage as thermodynamics become less extreme.

By Tuesday into Wednesday, long-range ensemble guidance indicates a deeper, more cohesive northern stream shortwave digging out of south- central Canada. This feature will propel a distinct surface cold front southward through the state, serving as a robust linear forcing mechanism for more organized shower and thunderstorm development. In the wake of this frontal passage, strong high- pressure of Canadian origin will overspread the Midwest, inducing robust cold air advection and dropping a much drier, cooler airmass into the region. This will effectively suppress temperatures and humidity back to slightly below seasonal averages by next Thursday.

AVIATION (06Z TAF Issuance)

Issued at 131 AM EDT Fri Jul 3 2026

Impacts:

- Scattered thunderstorms possible late in the day today into tonight

Discussion:

VFR conditions through the remainder of the overnight except for patchy ground fog towards daybreak similar to the last few days. Diurnal cu will become more prevalent after 16Z with storm chances increasing as early as 22Z in western Indiana and coverage being greatest from 00Z to 03Z. Storms look to be widely scattered, but could produce downbursts capable of briefly strong wind gusts. Winds are expected to maintain a generally southwesterly component through the forecast period at 8-12kts outside of a period of near calm winds through daybreak.

IND WATCHES/WARNINGS/ADVISORIES

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

Heat Advisory from 8 PM this evening to 9 PM EDT Saturday for INZ021-028>031-035>049-051>057-060>065-067>072.