549
FXUS63 KIND 182245
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
645 PM EDT Tue Aug 18 2026

.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 (00Z TAF Issuance)...
Issued at 645 PM EDT Tue Aug 18 2026

Impacts:

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

- Scattered showers and thunderstorms possible, mostly after 18Z.

Discussion:

Cumulus will diminish this evening, 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.
&&

$$

AVIATION...Updike
DISCUSSION...Melo/Eckhoff