Marco, Indiana Weather Forecast Discussion

899
FXUS63 KIND 220245
AFDIND
Area Forecast Discussion
National Weather Service Indianapolis IN
1045 PM EDT Sun Sep 21 2025
.KEY MESSAGES...
- Isolated showers and thunderstorms possible this afternoon
and evening, becoming numerous overnight.
- Additional rainfall expected tomorrow, with highest chances of
moderate to heavy rainfall rates over southern Indiana.
- Milder with daily chances for showers and a few thunderstorms
through much of the work week.
&&
.FORECAST UPDATE...
Issued at 1044 PM EDT Sun Sep 21 2025
Made a few minor adjustments to the short term forecast, mainly
adjusting PoPs to fine tune timing and track of the next wave of
showers and storms to move through Central Indiana.
Latest satellite and radar imagery show the cluster of showers and
storms developing in Illinois and Missouri in response to a wave
rounding the base of the broad upper trough over the Midwest. While
CAMs all differ in the exact evolution and progression of this
developing area of convection, confidence is increasing in more
widespread showers and storms to enter Central Indiana from the west
during the overnight hours and into Monday morning. The local
environment is conducive for convective development as ACARs
soundings show enough elevated CAPE above 1000 j/kg, a moistening
profile, and 20-30 kt unidirectional southwest flow through the
column. The low level jet increases somewhat overnight, but it
should be enough to continue moist air advection into the region and
support convective development overnight. While widespread severe
weather is not anticipated over the next 12 hours, a few storms may
produce gusty winds in excess of 40-50+ mph. The main threats with
any storms tonight and tomorrow will be heavy rain, lightning, and
gusty winds. As for timing, expect showers and storms to push into
Central Indiana from the west and southwest after 2am, with the
greatest threat for storms in the 4am-10am timeframe. Will fine tune
the timing and placement of storms as needed.
Showers and storms may persist into the afternoon hours before
diminishing from west to east during the evening. Keeping  at least
chance PoPs in the forecast for the rest of the day Monday and into
Monday night as this pattern is supportive of at least scattered
showers to persist after the main round ends.
&&
.SHORT TERM (This evening through Monday)...
Issued at 221 PM EDT Sun Sep 21 2025
Strong ridging over the Western Conus has led to broad but weakly
forced troughing central CONUS, of which in now reaching central
Indiana. Although the longwave pattern is weakly forced, shortwaves
are relatively strong inducing moderate lift across the region. This
will likely lead to periodic rain and storm chances of which began
yesterday and will continue through the short term. Initially, the
thought was there would be some diurnally induced convection this
afternoon; however, an increase in mid level subsidence ahead of the
next wave, partially observed on the 12Z ILX sounding and better
observed on IND ACARs soundings has led to a decrease in coverage of
precipitation across a majority of Indiana and is expected to
continue through the afternoon. Temperatures will remain elevated
today, but curtailed a bit by widespread cloud cover. The
current expectation is for highs in the mid 80s
Forcing will ramp up overnight as an upper level jet moves to the
northwest of the area, and upper heights fall as the upper trough
moves closer. Moisture will be plentiful, with southwest winds
providing and uninhibited flow of moisture into the area through the
night. Minimal, but still sufficient enough, diurnal cooling will
help stabilize the PBL tonight, but lapse rates above 3kft should
still be high enough for some embedded elevated convection within
numerous showers. Total rainfall amounts will likely vary greatly
depending on mesoscale features unresolvable at this time, but
generally rainfall amounts are expected to be between 0.25-1.00
inches tonight into tomorrow morning.
Additional rain is expected later tomorrow through tomorrow night
ahead of another aforementioned shortwave, there is still some model
ambiguity in specific placement of greatest rainfall, but slightly
amplified PWATs and more organized forcing may lead to an additional
0.5-1.5 inches over portions of central Indiana. That said, the
greatest probability of occurrence for moderate to heavy rainfall
late Monday is over southern central Indiana.
&&
.LONG TERM (Monday night through Sunday)...
Issued at 221 PM EDT Sun Sep 21 2025
Tuesday through Sunday...
Compared to the dry days of weeks past, large changes are expected
in our weather this week as daily chances for rain will persist.
Tuesday through Friday Morning...
Models suggest broad troughing in place aloft over the upper midwest
and into the Ohio Valley on Tuesday, with the development of a cut
off upper level low by Wednesday and Thursday. This upper low will
slowly pass across Illinois and Indiana through Friday morning.
Within the lower levels during this time a surface low will push out
of the Middle Mississippi Valley with a cold front and push toward
Michigan by Friday morning. This will keep Central Indiana within
the warm sector for a few days with the upper low in place aloft.
Forecast soundings suggest saturation within the column front time
to time during this period with pwats over 1 inch. Thus given all
these ingredients, chances for rain each day, with best chances on
Wednesday and Thursday seem reasonable. Given the expected clouds
and rain, highs will only reach the middle and upper 70s with lows
in the 60s.
Friday through Sunday...
On Friday in the wake of the cold front and slowly departing upper
low, models suggest weak ridging and subsidence building within the
pattern aloft along with the departure of the surface low and cold
front. Should this signal hold, this would suggest rain ending on
Friday morning and dry weather arriving for Friday afternoon and
into Saturday.
However, this is then followed on Saturday Night and Sunday to the
re-development of the upper low over TN and KY, with wrap around
easterly flow in place across Central and southern Indiana. Light
rain chances will persist at that time due to this feature, although
surface high pressure appears to remain in place over the Middle
Mississippi Valley. Confidence in this solution is very low at this
time due the overall evolution and progression of the previous upper
low. Temperatures through this time should be at or below normals as
cooler northeasterly flow would be expected.
&&
.AVIATION (00Z TAF Issuance)...
Issued at 749 PM EDT Sun Sep 21 2025
Impacts:
- Showers and thunderstorms increasing after 08z
- MVFR cigs and vis through midday Monday
Discussion:
Lower than normal confidence in the aviation forecast for the next
24 hours as guidance struggles resolving timing and placement of
mesoscale features and associated convective activity.
As of 00z, satellite and radar imagery shows Central Indiana clear
of any convection with the majority of showers and storms located
along a line from Chicago to St Louis and points southwest. Most
convection has weakened or dissipated as it approaches the IL/IN
border today, however increasing coverage of showers and storms is
expected through Central Indiana overnight tonight after 08z. While
cigs and vis may start off VFR through much of the night, by
daybreak cigs and vis should begin deteriorating as showers and
thunderstorms saturated the near surface layer and lower cigs to at
least MVFR levels. At the moment, best timing for more widespread
thunderstorms and MVFR or worse conditions is within the 09z to 16z
timeframe.
Convection becomes more scattered in nature Monday afternoon and
evening as the main area of showers and storms from the morning
pushes off to the northeast. Expect cigs and vis to improve to VFR
levels by the late afternoon and evening.
Winds are expected to remain southerly through the period at or
below 10kts outside of showers and thunderstorms; however erratic
wind directions and speeds possible in any storm.
&&
.IND WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
UPDATE...CM
SHORT TERM...Updike
LONG TERM...Puma
AVIATION...CM

NWS IND Office Area Forecast Discussion