textproduct: Indianapolis

KEY MESSAGES

- Strong to severe storms with damaging winds, heavy rainfall, and flash flooding remain a significant threat through the weekend

- Heat index values in excess of 100 possible today and Sunday in southern portions of central Indiana

- Dry and milder weather early in the work week

- Additional storms possible Wednesday

MESOSCALE UPDATE

Issued at 459 AM EDT Sat Aug 15 2026

A large cluster of thunderstorms has developed over Illinois, slowly moving eastward into Indiana as of 5am. Overall, the cluster is fairly disorganized owing to little in the way of effective shear. Wind shear vectors are only around 20kt to the east. Nevertheless, recent radar trends show signs of cold pool development along with a weak rear inflow jet signature.

Wind shear is strongest north of I-70, with even less effective shear to the south. There is also little in the way of a low-level jet, less than 15kt per IND ACARS soundings. As such, storms that propagate too far south will likely become outflow dominant and weaken. Eastward propagation is therefore favored since the only real mechanism for MCS maintenance is achieving cold pool shear balance, with the easterly shear vector dictating system motion.

Poor lapse rates and minimal instability will limit severe potential. Sporadic strong but sub-severe gusts are possible, however, especially within any bowing segments. IND ACARS soundings show abundant moisture, with high freezing levels, so efficient rainfall production is anticipated. Given very saturated ground over the majority of the region, flooding / flash flooding is the primary hazard with storms through the morning hours. Current radar trends and recent CAM guidance suggest Warren, Fountain, Vermillion, Montgomery, and Tippecanoe counties face the highest threat in the near term.

DISCUSSION (Today through Friday)

Issued at 312 AM EDT Sat Aug 15 2026

Through 7AM:

Rapid convective development is occurring upstream from central Illinois westward along and north of a quasi-stationary front at the nose of a 30-40KT southwesterly low level jet. This activity will expand and push eastward through daybreak, likely making it into northwestern portions of the forecast area. Modest effective shear values and weak mid-level lapse rates suggest very limited severe potential.

The atmosphere across the region remains deeply and anomalously moist, however, with precipitable water values near the top end of the climatological envelope. Highly efficient rainfall rates continue to be a concern, owing to this deep moisture, and deep warm rain processes, as freezing levels remain at or above 15,000 feet. Needless to say, much of central Indiana, particularly central and northern portions of the area, remains waterlogged tonight, and any additional convective rainfall will rapidly pose a flash flood threat.

7AM through Noon:

The aforementioned convection will continue to push across the area through the morning hours, with CAMs suggesting the northern half to two thirds of central Indiana likely to see additional storms/rainfall. Primary concerns will be heavy rain and isolated strong to damaging wind gusts, but due to strong low level shear an isolated tornado cannot be ruled out entirely.

Noon through Saturday night:

Additional diabatic pressure losses over northern Indiana throughout the day will continue an MCV with a SE progression this afternoon. This will likely lead to additional convection NE of the I-74 corridor with isolated damaging wind gusts possible once again. Given the orientation of this newly formed MCV to the low level jet, training thunderstorms will be possible with a moderate (level 3 of 4) risk for excessive rainfall. This area will have very low flash flood guidance values due to prior rainfall and therefore flash flooding will likely have an extremely rapid onset. There is some uncertainty with the specific placement of the precipitation axis with the afternoon rainfall, with the best case scenario keeping the precipitation axis north and east of the highly vulnerable region - recent high resolution model runs are somewhat less promising in this regard, however.

Once the MCV exits the region, height rises once again should limit convection, keeping any rainfall mostly as a light shower during the Saturday evening hours. That said, the heavy rainfall threat will sadly not be over, as the nocturnal LLJ is expected to ramp up again late Saturday night with moderate isentropic lift and additional scattered to numerous convective showers and thunderstorms.

In additional to the rainfall mentioned earlier, southerly winds will bump temperatures up over the south and southwest portions of the state, especially where cloud cover will be less abundant. This is expected to create a window of 100-106 heat index values Saturday and Sunday afternoon along and south of a line from Sullivan to Bloomington to Seymour and thus a Heat Advisory has been issued for this area from 11AM today until 8PM Sunday.

Sunday and Sunday Night...

Northwest flow aloft will continue into the latter portion of the weekend, with one or more upper level disturbances passing through the area bringing continued threats for thunderstorms and heavy rainfall. Extension of the ongoing flood watch into Sunday may eventually be needed but will forgo this for now with the pressing concern of ongoing threats and potential additional flash flooding today at top of mind.

Instability will again be very strong in an extremely moist airmass, though deep layer shear will be more modest. The gradual passage of a cold front during the mid to late day hours along with enhancement from any additional potential MCVs will serve to focus possible severe potential along with additional heavy rainfall and flash flooding threats. The primary severe threat would be damaging winds, though again an isolated tornado cannot be entirely ruled out. The more significant and pressing threat will remain flash flooding given the extreme amounts of rainfall this week and extreme sensitivity to additional high rainfall rates with convection.

Monday and Tuesday...

An area of surface high pressure will mercifully bring at least a temporary end to the repeated rounds of thunderstorms early in the week, with dry weather and near normal temperatures likely early Monday into late Tuesday.

Wednesday...

Unfortunately, an additional round of thunderstorms appears likely Wednesday into Wednesday night as another cold front and upper level wave accompanying a strong jet streak aloft passes through the region. Plentiful moisture will be available, though precipitable water values may be slightly more modest (70-80th percentile climatologically). However, somewhat stronger deep layer shear may be available. Another round of severe storms will certainly be possible, though predictability is greatly limited given the sensitivity to prior rounds of storms and model inconsistencies 5 days out.

That said, even with a couple of days to dry out, flash flood vulnerability will remain quite high in the face of any additional convective rainfall given the extreme antecedent conditions, and we will need to remain vigilant through at least mid week for these concerns.

Thursday Onward...

Amplification of the upper ridge over the western CONUS and southward displacement of both the frontal zone and deeper moisture by surface high pressure and upper level troughing suggest milder temperatures and mostly dry conditions late week and into the weekend, though some slight chances for showers or storms may linger across the southern portions of central Indiana. It will take quite some time for antecedent conditions to significantly improve, unfortunately, and any additional convective rainfall with periodic frontal passages or upper level waves in the broader cyclonic flow next weekend and into the following week will pose at least modest flash flood threats.

AVIATION (06Z TAF Issuance)

Issued at 128 AM EDT Sat Aug 15 2026

Impacts:

- Thunderstorms likely at all sites in the 10-19Z time frame depending upon the site, earlier in the north and later in the south

- Patchy MVFR fog possible near daybreak where convection has not yet impacted

Discussion:

Moist low levels may promote some patchy fog development near daybreak at sites where convection has not yet reached.

Thunderstorms are developing upstream and are expected to progress into central Indiana late tonight into the midday hours on Saturday. Will carry TEMPO groups at all sites during the most likely hours for storms, though it is possible that training occurs and multiple rounds impact the sites. Periods of MVFR or worse conditions may occur with the storms, though fluctuations are likely. Ceilings may be MVFR for a time in the wake of the activity before returning to VFR.

Winds will be calm to light and variable overnight, becoming more southerly and then southwesterly with time. Winds are likely to again go light and variable Saturday evening.

IND WATCHES/WARNINGS/ADVISORIES

Flood Watch through Sunday morning for INZ021-028>031-035>049- 051>057-065.

Heat Advisory from 11 AM this morning to 8 PM EDT Sunday for INZ060>063-067>071.