textproduct: Indianapolis

KEY MESSAGES

- Areas of fog late tonight, especially near river valleys.

- Scattered showers and a few thunderstorms are possible late Friday night into Saturday afternoon.

- Below normal temperatures are expected Sunday and Monday with highs mainly in the 70s and lows in the 50s.

- Patchy fog will also be possible late night and early mornings early next week.

- Another chance of showers and thunderstorms will return middle of next week.

DISCUSSION (This evening through Thursday)

Issued at 330 PM EDT Thu Aug 20 2026

Late this Afternoon...

Sufficient instability remains across central Indiana that a few pop up showers remains possible across far southern areas and also in the far north-northeast. Both areas reside closer to deeper low level convergence, with the area to the north associated with weak lake boundary convergence moving south. Have inserted 20 pops in the far north-northeast and kept the same pops in the south until around 00Z.

Tonight and Friday...

High Pressure will push southward from the Great Lakes region allowing for light and variable winds. Clearing skies this evening will lead to favorable radiational cooling conditions. With very moist ground, the set up looks favorable for at least patchy fog, and areas of fog, some potentially dense, are expected near river valleys where warmer water will support increased moisture flux into the shallow boundary layer. Fog conditions are expected to diminish quickly with daytime heating Friday AM. Residual surface ridging will lead to generally light winds. A few diurnal clouds are expected during the afternoon. An upstream shortwave will initiate TS along a cold front over the Upper MS valley/western Great Lakes during the afternoon hours.

Friday night into Saturday...

Thunderstorms in one to two lines extending from the northern portions of Illinois into eastern Iowa in a regime of moderate instability with MUCAPE from 2000-3000 J/KG will move SE 20-30 mph. With eastward extent the instability drops off dramatically with all model guidance suggesting less than 1000 J/KG of MUCAPE will exist on the western border of Indiana late Friday night. Therefore, TS activity is expected to weaken substantially with time as it develops closer to central Indiana. Have kept low chances of TS in far western zones towards daybreak Saturday to account for the slightest chance of a rogue storm.

The weak cold front associated with this progressive system will push across central Indiana Saturday morning. There is some indications that a secondary shear induced mid level vorticity max will move through the region on Saturday afternoon. With residual instability, a few lower topped showers/TS will be possible behind the main cold front but ahead of this mid level feature during the afternoon, mainly over eastern zones. Have bumped up pops to account for this pattern.

Saturday night through Sunday...

In the wake of the shortwave/cold front Saturday, surface high pressure will gradually build southward through the northern/central plains into the upper MS valley. Diabatic effects of warmer Great Lakes waters should intensify the shortwave trough and associated deepening of the longwave trough over the eastern Canada/CONUS. Broad N-NW low level flow will support some lake enhanced clouds during Sunday afternoon when daytime heating is maximized. However, stout mid level inversion noted on forecast soundings should limit convective cloud depth and only a few sprinkles will be possible. Have kept pops below 10 percent Sunday afternoon.

Sunday night through Tuesday...

Broad area of surface high pressure will push SE into the Ohio Valley through the period, providing for much drier conditions, with dewpts falling into the 50s. Temperatures will be slightly below normal thanks to several days of weak CAA. Most areas will have lows in the 50s, with highs in the 70s areawide on Monday. Areas of fog will be possible overnight, but confidence isn't overly high at the moment so will leave out of the forecast for now.

Wednesday/Thursday...

Another upper wave/surface cold front will move through during the period. Confidence in timing is very low considering the system is currently off the west coast, so for now will only have some low PoPs. Seasonable temperatures are expected with increasing humidity with increasing SW flow in the low levels.

AVIATION (00Z TAF Issuance)

Issued at 742 PM EDT Thu Aug 20 2026

Impacts:

Chance of fog at outlying terminals late tonight into early Friday

Discussion:

VFR conditions expected across central Indiana through Friday evening. Diurnal cumulus field to diminish through 01Z amid light winds veering to north-northeast. Mostly clear skies tonight will then promote MVFR/worse visibility in BR/FG after 06Z...especially at the outlying terminals.

MVFR ceilings are also possible at outlying terminals through pre- dawn hours. Expect more diurnal cumulus Friday by 16Z, at mainly low-VFR level decks...along with at times VRB winds that will most often be from the WNW/NW and usually sustained under 7KT.

IND WATCHES/WARNINGS/ADVISORIES

None.