textproduct: Indianapolis

KEY MESSAGES

- Small threat for showers and isolated thunderstorms late this afternoon and evening

- Cooler and less humid on Monday

- Hot and humid conditions returning Tuesday and Wednesday with highs reaching the 90s

DISCUSSION (This evening through Sunday)

Issued at 134 PM EDT Sun Sep 13 2026

Overview.

A weak frontal boundary moving across Central Indiana will bring a brief chance of showers late this afternoon and early evening before high pressure brings cooler and less humid air on Monday. Hot and humid conditions will return quickly on Tuesday and Wednesday as an upper-level ridge builds across the Ohio Valley, driving high temperatures into the lower 90s. Occasional showers and storms are expected late Tuesday through the weekend with best chances Tuesday night and again on Thursday.

Tonight Through Tuesday.

The aforementioned surface cold front will drift through Central Indiana this evening and will exit to the southeast during the overnight hours. HREF guidance and HRRR show isolated to scattered showers along the baroclinic zone through sunset, diminishing rapidly as surface heating is lost and mid-level PVA departs. Moderate CAA in the 925-850mb layer behind the front will push lower dewpoint air across the region, while surface high pressure expanding from the Upper Midwest builds into the Ohio Valley. Efficient radiational cooling under clearing skies will drop Sunday night lows into the 50s across most locations. There may be some fog/low stratus across the Upper Wabash Valley but with the much drier air, confidence is low.

Surface high pressure across the region on Monday will result in broad-scale subsidence, light northerly flow, and pleasantly cool conditions. Model sounding profiles show a dry tropospheric column with low-level thermal mixing supporting daytime highs ranging from the mid 70s northeast to the lower 80s in the lower Wabash Valley. As the high shifts toward New England Monday night, return low-level southerly flow will initiate rapid moisture advection aloft. Mid- level height rises driven by an expanding 500mb ridge over the Tennessee Valley will take hold by Tuesday. Strong thermal advection at 850mb will lift daytime maximum temperatures into the upper 80s to lower 90s on Tuesday, while dewpoints rise back into the upper 60s.

Wednesday Through Sunday.

The synoptic pattern on Wednesday features a strong mid-level ridging over the southeastern states and Tennessee Valley, holding low-level baroclinicity and main storm tracks north over the Great Lakes. Continued S/SW surface flow and low-level WAA will maintain hot and muggy conditions across Central Indiana, with highs in the lower to mid 90s. There are some signs that a frontal system Tuesday night may sag south into central Indiana, but with the strength of the aforementioned ridge, confidence is fairly low as to how far south the front and associated rain threats will go. While diurnal MUCAPE values may exceed 1500 J/kg, weak mid-level capping and background subsidence will keep convective risks isolated and unorganized through Wednesday afternoon.

The next best chance for rain arrives late Wednesday night into Thursday as models show an upper trough ejecting out of the Plains toward the Upper Mississippi Valley. An advancing surface cold front will interact with an intensifying 40-50 kt 850mb LLJ, driving a deep plume of moisture northward with PWAT values surpassing 1.6 to 1.8 inches. Scattered showers and the potential for more organized thunderstorm clusters are likely at times Thursday into Friday along and ahead of the frontal boundary. Diurnal timing and cloud cover will determine peak instability, though shear profiles remain sufficient to support convective organization. As the front slowly pushes south by Sunday, post-frontal CAA and building surface high pressure to the north will lower high temperatures into the upper 70s to low 80s.

AVIATION (18Z TAF Issuance)

Issued at 134 PM EDT Sun Sep 13 2026

Impacts:

- Small threat for showers late afternoon and early evening

Discussion:

Mostly clear to partly cloudy skies expected for much of the TAF period with VFR conditions outside of a low chance for showers during the late afternoon and evening hours. Coverage is expected to be fairly minimal, but the best window will be from 21Z to 00Z with a 15-20 minute window for rain. Can't rule out a rumble of thunder, but the threat is low enough to not warrant a mention in the TAF. Winds will remain westerly to northwesterly at 8-12kts through the evening with a low chance for gusts up to 20kts. Winds will then become more northeasterly at 5-10kts towards Monday morning.

IND WATCHES/WARNINGS/ADVISORIES

None.