textproduct: Indianapolis

KEY MESSAGES

- Scattered thunderstorms Sunday afternoon and evening - Heat returning Tuesday and Wednesday...highs in the low 90s

FORECAST UPDATE

Issued at 958 PM EDT Sat Sep 12 2026

Central Indiana has returned into the warm and humid pattern with very weak mid-summer style gradients, with this trend to promote increasing coverage of fog tonight. Outside of KIND nearly all reporting stations are observing winds of 3KT/less as of 930PM EDT, with widespread upper 60s/lower 70s dwpts. Given dwpt-depressions outside of the Indy heat island currently only 0-4 degrees, with patchy BR already bringing modest visibility reductions as low as 7SM at Anderson...potential for locally dense fog is on the table through pre-dawn into early morning hours Sunday. Temperatures will display a subtle downward trend through dawn, with lows early Sunday expected to range from 65F in rural Clinton/Tipton County to 69F in downtown Indianapolis.

DISCUSSION (This evening through Saturday)

Updated at 1000 PM EDT Sat Sep 12 2026

Overview...

A cool front passing through Central Indiana tonight with rain chances decreasing through the evening. This front won't make much of a difference in temperatures with highs climbing into the low to mid 80s for Sunday. High pressure will keep conditions calm and mild through early next week. Rain and thunderstorm chances return Wednesday night into Thursday as another storm system approaches from the west, followed by cooler air on Friday.

Tonight Through Sunday Night...

A broad upper-level trough over the Upper Midwest will pivot eastward into the Great Lakes overnight, driving an attendant surface cold front southeastward across Central Indiana between 03Z and 10Z. HREF guidance shows a narrow plume of moisture transport preceding the boundary, pushing PWAT values near 1.3 to 1.5 inches. Despite limited elevated instability, moderate PVA aloft combined with 30-35 kt of 850mb jet dynamics will support a organized band of rain showers and embedded thunderstorms through the evening hours with rain chances gradually dropping ahead of the frontal passage. HREF ensemble members maintain peak MLCAPE values below 500 J/kg, limiting severe convective potential, though brief heavy downpours and wind gusts up to 30 kt remain possible with the strongest cells.

Post-frontal cold advection aloft will increase through Sunday morning in the 925-850mb layer as the surface trough exits. Deepening boundary layer mixing under steep low-level lapse rates will allow momentum transfer down to the surface, resulting in sustained northwest winds of 10 to 15 mph with gusts up to 15 to 20 mph. Lingering boundary layer moisture trapped beneath a 850mb subsidence inversion will maintain a broken stratocumulus deck through midday Sunday before large-scale anticyclonic flow and dry air advection scour out remaining cloud cover. As surface high pressure builds eastward into the middle Mississippi Valley Sunday night, clearing skies and decoupling surface winds will foster efficient radiational cooling, dropping low temperatures into the upper 40s to lower 50s across the forecast area.

Monday Through Saturday...

Surface high pressure centered over the Ohio Valley on Monday will shift off the Mid-Atlantic coast by Tuesday. Mid-level height rises across the Midwest under building ridge flow will maintain strong broad-scale subsidence across Central Indiana. The combination of dry atmospheric profiles and gentle southerly return flow on the backside of the surface high will allow high temperatures to moderate from the upper 70s to low 80s on Monday with much warmer temperatures arriving Tuesday into Wednesday.

The synoptic pattern transitions to a more active configuration by mid to late week. Both operational GFS and ECMWF, supported by their respective GEFS and EPS ensemble means, depict a deep mid-level trough ejecting out of the Southern Rockies into the Central Plains on Wednesday. Strong warm advection and an intensifying 45 to 55 kt 850mb low-level jet will direct a surge of Gulf moisture northward into the region, pushing PWAT values above 1.60 inches by Wednesday night. Enhanced upper-level jet dynamics and increasing PVA ahead of the advancing cold front will support widespread rain showers and embedded thunder late Wednesday night into Thursday. While shear profiles appear sufficient for organized convection, the overall magnitude of instability will depend heavily on cloud cover and diurnal timing. The cold front completes its passage by Friday morning, returning the region to northwest flow aloft, post-frontal CAA, and drier, cooler conditions heading into the weekend.

AVIATION (06Z TAF Issuance)

Issued at 100 AM EDT Sun Sep 13 2026

Impacts:

- Fog possible through daybreak Sunday - Brief period with wind gusts to around 20kts late this afternoon - Small threat for showers late afternoon and early evening

Discussion:

Weak high pressure has settled across the area with near calm winds and clear skies early this morning. With plentiful moisture in the near surface layer...have already seen fog form at the outlying terminals and expect that to increase through the predawn hours. Fog will burn off quickly shortly after sunrise.

Mid level clouds and scattered cu will increase into the area in the afternoon with gusty northwest winds in advance of high pressure expanding from the upper Midwest. Cannot rule out a few light showers developing during the late afternoon and early evening but this will have no impact on the terminals. Winds will veer to N/NE tonight and diminish to around 10kts.

IND WATCHES/WARNINGS/ADVISORIES

None.