222
FXUS63 KIND 201930
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
330 PM EDT Thu Aug 20 2026

.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 (18Z TAF Issuance)...
Issued at 1235 PM EDT Thu Aug 20 2026

Impacts:

- MVFR ceilings remain possible through the afternoon but clouds
  scattering
- Chance of fog at outlying terminals late tonight and early Friday

Discussion:

Drier air continues to filter into the region from the north slowly
but MVFR stratocu has been stubborn to mix out to this point with
moisture lingering in the boundary layer. Expect stratocu to further
mix out into a VFR cu field through the afternoon with light
northerly winds. Mainly clear skies initially tonight with near calm
winds over the saturated soils will support fog development late
tonight into Friday morning...especially at the outlying terminals.
MVFR ceilings may also briefly impact the sites. Expect clouds to
mix out to a cu field quickly on Friday with variable winds at 5kts
or less.


&&

.IND WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

AVIATION...Ryan
DISCUSSION...Crosbie