textproduct: Indianapolis

KEY MESSAGES

- Patchy fog possible overnight

- Low rain chances return tonight, scattered to numerous showers and storms are expected on Wednesday

- Localized flash flooding is possible on Wednesday

- Turning generally milder and drier late week into the weekend, chance for showers and storms on Saturday

DISCUSSION (This evening through Tuesday)

Issued at 258 PM EDT Tue Aug 18 2026

High pressure situated over southwestern Indiana continues to provide quiet weather. Increasing cumulus driven by boundary layer processes have expanded in coverage across most of the region. Aloft, cirrus advances from the west ahead of a shortwave trough located over the northern Plains.

Winds are light and variable owing to the surface high. Winds are expected to become southerly on Wednesday as the high exits eastward. Fog is possible again tonight, with cloud cover being the mitigating factor compared to previous nights. Cumulus will dissipate after sunset, but cirrus will continue to expand aloft. These may remain thin enough to allow for fog development. We will cap the mention of fog to patchy for now, given the uncertainty with cirrus thickness overnight.

Wednesday

The aforementioned shortwave arrives on Wednesday with the risk for scattered showers and thunderstorms. Guidance does not suggest a surface low will develop in association with the shortwave, but there are signs of a weak cold front or boundary drifting southward in tandem with it. This boundary could act as a focus for convective development, though convection is possible at any possible at any point during the day Wednesday.

There are two potential rounds of convection that guidance is beginning to hone in on. The first, early in the day associated with a weakening upstream MCS from the night before. The second, redevelopment during the afternoon along the slow-moving cold front.

Model soundings show weak to modest instability along with poor lapse rates. However, fast upper-level flow leading to around 40- 50kt of shear should allow for some storm-scale organization. Storms may briefly show supercell characteristics before growing into clusters, due to the shear vector being largely parallel to the front. Though the potential is low due to poor lapse rates and instability, some sporadic damaging wind gusts cannot be ruled out.

The primary threat may end up being flash flooding, since the ground across the region is very saturated from recent rains. Guidance shows high PWATS around or over 2 inches, along with a deep warm cloud layer to promote efficient rainfall production. Storm clusters may organize into bands that train over the same areas, allowing for concentrated pockets of locally very heavy rainfall (1 to 3 inches).

Some uncertainty remains on convective coverage, which may range from isolated to widespread. This depends on how potent the parent shortwave is, with a more compact/intense system allowing for more robust lift and therefore higher convective coverage and vice versa.

Thursday through early next week...

The weak frontal boundary meandering southward over Indiana on Thursday will keep low precipitation chances in the forecast. Further north, surface high pressure begins to build in providing mostly quiet weather conditions. Look for the surface high to build further south Thursday night into Friday resulting in quiet weather conditions. Fog is possible during the overnight period both Thursday and Friday. Another embedded disturbance moves in on Saturday supporting at least low rain chances with sufficient moisture advecting into central Indiana. Long range guidance generally depicts surface high pressure building in once again late weekend into early next week. Large scale subsidence supports quiet weather conditions.

AVIATION (18Z TAF Issuance)

Issued at 145 PM EDT Tue Aug 18 2026

Impacts:

- Patchy fog possible overnight at LAF, HUF, BMG.

- Scattered showers and thunderstorms possible after 15z Wednesday.

Discussion:

Diurnal cumulus has developed this afternoon oscillating between SCT and BKN. Generally VFR ceilings have been observed, around 4000-5000 ft agl. Cirrus likewise continues to increase overhead.

Cumulus diminishes tonight, leaving just the cirrus which will continue to increase. The cirrus may be thin enough to allow for another night of patchy fog, mainly in the outlying TAF sites.

A shortwave trough approaching from the northwest is expected to bring showers and thunderstorms to central Indiana on Wednesday. These are expected to be scattered in nature, and thus a Prob30 group was included to cover this potential.

Winds remain light and variable this afternoon and may go calm overnight. Winds should increase from the WSW as the shortwave approaches on Wednesday with speeds remaining under 10kt.

IND WATCHES/WARNINGS/ADVISORIES

None.