982
FXUS63 KIND 210121
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
921 PM EDT Sun Sep 20 2026

.KEY MESSAGES...

- Scattered to numerous showers and storms late today through tonight

- An isolated severe storm with damaging winds possible but
  locally heavy rain remains the primary threat.

- Much cooler temperatures for the first half of the week, gradually
  warming back to near normal levels by next weekend

&&

.FORECAST UPDATE...
Issued at 920 PM EDT Sun Sep 20 2026

Surface analysis this evening shows a surface trough stretching
across southern IL and southern Indiana before reaching Central IL.
Convection had developed along the boundary, with strongest storms
over OH. Airmass across much of central Indiana remains very moist
with dew points near 70. Plume of tropical moisture was streaming
across eastern TX to MO and then NE toward IL and Indiana. Radar
shows a thunderstorm near IND, showers across northern Central
Indiana and a pair of thunderstorms over SE IL, progressing SE.

Overall, ongoing forecast remains in good shape. Thunderstorm
chances will need to remain in place as the frontal boundary slowly
sags southward along with the few scattered showers found across
northern Central Indiana. Forecast soundings overnight show a
saturated column in place overnight with pwats still near 1.85
inches. Thus pops will still be needed through the night. Cloudy
skies will be expected. Expect low temperatures in the upper 50s
north to the mid 60s south.

&&

.DISCUSSION (This Evening through Sunday)...
Issued at 233 PM EDT Sun Sep 20 2026

A slow moving cold front will remain the focal point for periodic
convection tonight into early Monday. The front will move south of
the Ohio River on Monday but isolated showers will remain possible
into early Tuesday prior to the passage of an upper level wave. Much
cooler air will overspread the region in the wake of the front. An
upper low will cutoff over the mid Atlantic during the second half
of the week...with much drier air being pulled south into the Ohio
Valley. This will result in pleasant early Fall conditions late this
week into the upcoming weekend.

Rest of Today through Tuesday Night

Mid and high level clouds continue to overspread the region this
afternoon with a frontal boundary draped across central Indiana.
Temperatures varied across the boundary...ranging from the mid 70s
over north central Indiana to near 90 along our southern border at
18Z.

The frontal boundary will serve as a focal point for renewed
convective development late today into tonight as a surface wave
rides along it. Showers over northwest Indiana will drift east with
additional development back to the west across Illinois. While
initial movement should keep the bulk of the showers and storms
largely to the north of the forecast area...expect a gradual
southern drift into the northeast half of the forecast area by early
evening with additional showers and storms riding along the boundary
overnight.

There remains a non-zero threat for a few severe storms but overall
the thermodynamics remain unimpressive. Peak CAPE values are only at
about 1000 j/kg with limited BL shear. There is a narrow window
between now and 22-23Z where MLCAPE values may nudge up slightly in
an axis along and just north of the front across the northern half
of the forecast area where any stronger cell that can develop may
carry an isolated damaging wind risk. All in all though...the
severe risk appears very low into this evening.

The greater threat from convection will be locally heavy rainfall as
an axis of PWATs in excess of 1.75 inches pivots south across the
area tonight. Model soundings continue to advertise a deep warm layer
up to near 500mb and the presence of a modest westerly low level jet
tonight all support potential for efficient rainfall rates and some
backbuilding of showers and storms in the vicinity of the boundary.
Briefly considered a Flood Watch over far northeast portions of the
forecast area for tonight but the bulk of the heavier rainfall over
the last 72 hours has bypassed us to the north and east. 1- and 3-
hour flash flood guidance numbers are manageable and also argue
against a Flood Watch. Will certainly monitor convective trends into
the evening but will not carry any flood headlines at this time.

Showers will linger into Monday but coverage will steadily decrease
throughout the day as the front shifts into the Tennessee Valley.
Abundant moisture will persist through the boundary layer however
trapped beneath a stout inversion. This will keep skies mostly
cloudy through Monday night and likely even into Tuesday as an upper
wave approaches from the northwest. Cannot rule out a few showers
Monday night and Tuesday but will be isolated at best with moisture
largely confined to the boundary layer. A decrease in clouds will
commence Tuesday night as drier air attempts to advect south on the
back side of the departing wave aloft. The development of an upper
low over the eastern Great Lakes Tuesday night may serve however to
delay more substantial clearing from the west.

Temps...today will be the last very warm day as the passage of the
front by early Monday will advect much cooler air into the region
from the north. Low level thermals support highs in the upper 60s
north to near 80 in the lower Wabash Valley Monday with upper 60s to
lower 70s Tuesday. The current forecast high of 69 degrees for KIND
for Tuesday would be the first sub-70 degree day since May 23. Lows
will transition down into the lower and middle 50s by MOnday/Tuesday
nights as well.

Wednesday through Sunday

Strong signals in the extended model suite suggest the
aforementioned upper low will amplify and become cutoff from the
mean flow aloft over the Mid Atlantic region during the second half
of the week. Cool E/NE flow and at least some lingering cloud cover
on the back side of the developing cutoff low will maintain below
normal temperatures Wednesday and for parts of the area Thursday as
well with highs ranging from the upper 60s to the mid 70s from
northeast to southwest. A few light showers cannot be entirely ruled
out over far eastern portions of the forecast area Wednesday but
confidence is far too low for a mention in the forecast as better
forcing aloft will be east of the region.

As the upper low becomes better organized over the mid Atlantic
Friday into the weekend...an axis of deepening subsidence will be
advected south from Canada and across the Ohio Valley. A narrow
ridge aloft will expand into the area allowing temperatures to
recover back to late September normals with abundant sunshine and
pleasant early Autumn conditions for the forecast area. Highs will
be in the 70s this weekend with low 80s possible across the
southwest half of the area by Sunday. Comfortable lows in the 50s
are expected throughout the period...although a few locations over
northeast counties may slip into the upper 40s mainly Thursday
through Saturday nights.

&&

.AVIATION (00Z TAF Issuance)...
Issued at 614 PM EDT Sun Sep 20 2026

Impacts:

- Isolated convection possible early this evening. Showers likely
overnight.
- Deteriorating ceilings this evening with IFR conditions developing
  overnight into Monday morning
- Winds becoming northeast tonight and Monday

Discussion:

A lingering frontal boundary across Central Indiana will remain the
focus for shower and isolated storm development this evening. More
widespread showers and isolated storms will be possible after
midnight as the TAF sites get deeper in to the new air mass. Trapped
cold air advection SC is expected in the wake of the front,
resulting in MVFR to IFR CIGs arriving overnight.

Forecast soundings remain saturated within the lower levels for much
of Monday, suggesting continue MVFR cigs through much of the day.


&&

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

$$

UPDATE...Puma
AVIATION...Puma
DISCUSSION...Ryan