Significant Flooding Strikes Central Indiana
The first half of August 2026 has brought repeated heavy rainfall to the state of Indiana, with the area in and around the capital city of Indianapolis experiencing particularly strong flooding. Numerous roads are closed, and a bridge in Carmel, IN, just north of Indianapolis, has been washed out. Multiple fatalities have already been reported, and continued storms on Sunday 16 August are exacerbating an already challenging situation.
A sustained river flooding event builds over an extended period of time, and this event is no exception. We’ll start our analysis by looking at the cumulative rainfall graph for Indianapolis dating back to 31 July 2026 as recorded by the invaluable xmACIS2 site. So far in August 2026, Indianapolis has seen three separate days with at least one inch of rainfall, including a whopping 2.09 inches on the 1st of the month. As a result, Indianapolis has experienced more than twice the normal amount of rainfall up to this point in August.
All of that water has to go somewhere, and so it rushes into the streams and rivers of Central Indiana. This plot shows the time series of water flow in the White River just north of Indianapolis dating back to August 1, roughly contemporaneous with the precipitation graph above. Note the logarithmic scale. It’s easy to see how the initial heavy rainfall on the 1st caused the water levels to rise and the flow rates to increase over the next few days, but an extended period of over a week without rain caused a gradual slowdown in the water flow. However, once the rain returned it came back in a big way and the water levels quickly exceeded the system’s capacity to absorb them. Between the 11th and the 15th flow rates increased by a factor of 70.
Perhaps the most important question is: what’s been causing all this rain? Here’s where our satellite tools can provide valuable insight. The CIMSS MIMIC-TPW2 product helps identify areas of elevated moisture by compositing microwave observations of total precipitable water (TPW) with gridded wind data to depict how the moisture evolves over time. Recall that TPW is the measure of how much precipitation in a column of air could be produced if all of the water vapor in that column condensed into liquid and fell to the ground.
Here’s the TPW plot for the contiguous United States on 1 August at 0000 UTC (31 July at 8:00 PM EDT) showing the state of the atmosphere just ahead of that 2 inch rainfall day. Note the strong moist plume that runs up the lower Mississippi valley and into Illinois and Indiana. These TPW values are nearly tropical in magnitude. (That’s no exaggeration: compare the values of the TPW in the Midwest to the bullseye of Tropical Storm Genevieve around 20 N 130 W).
Similarly high levels of TPW have been present over the past several days. Here’s an animation of the CIMSS MIMIC-TPW2 product for Sunday 16 August, covering the most recent round of storms over Indiana. Again, these storms have quite high TPW values, which are made possible by warm August air with a high capacity for elevated moisture levels, significant moisture advection from the Gulf, and high transpiration from the Midwest’s corn fields.
Let’s take a closer look at the most recent round of storms to hit central Indiana. The GOES-19 (GOES East) true color view shows a band of deep moist convection heading east-southeast from Illinois into most of central Indiana. Numerous overshooting tops are visible which become easier to see as the loop goes on; this is more likely due to the setting sun casting longer shadows than it is any trait of the clouds themselves.
The IR sandwich product can be a quite useful tool for assessing the strength of daytime convection. In the IR sandwich, colors representing the infrared window brightness temperatures are overlaid on top of the high resolution visible channel. Lower (warmer) clouds remain unshaded, but deeper (colder) clouds have colors associated with them. Since the overshooting cloud tops are going to be the coldest parts of the cloud, they’re going to appear as a different color. This makes the clouds appear to have more texture than the true color product shows, making it easier for forecasters to identify where the most vigorous convection is. In essence, this unites some of the best aspects of the visible band (high spatial resolution) with the infrared (quantitative information about cloud temperatures).
Satellites can also help identify where the flooding is taking place. Back in July, the CIMSS Satellite Blog discussed the ABI Flood Mapping package deployed by the CSPP Geo group. In short, by comparing geostationary observations of surface water and comparing those to a baseline level of known bodies of water, it’s possible to determine the extent of surface flooding. By applying this technique to the geostationary ABI instrument, forecasters and hydrologists can track flooding on fine timescales and, perhaps more importantly, be more likely to dodge the clouds that would otherwise inhibit the observations. Here’s an animation that shows how that works in practice. This animation stretches from 1500 to 2000 UTC on 16 August 2026 (11:00 AM to 4:00 PM EDT). Tan represents dry land, gray depicts missing data due to clouds, and blue shows where surface water is expected. The other colors represent various degrees of flooding. As the clouds clear we see flooded areas emerge in central Indiana, especially around the White River which cuts northeast to southwest across Indianapolis and central Indiana.
Both the IR sandwich and the ABI Flood Mapping products are available from SSEC’s RealEarth data viewer.