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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

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