A swift-moving flood that swept through the Hill Country of Texas on Friday, killing at least 79 people and leaving many more missing, was a flash flood.

Forecasters warn that slow-moving storms could bring heavy downpours to already saturated areas, increasing the risk of flash flooding.

People awoke from water rushing around them during the early morning hours of July 4, all along the Guadalupe River in the Texas Hill Country. Residents were seemingly caught off guard, but warnings had been issued days and hours before floodwaters began carrying away homes,

Morning thunderstorms are expected across the Hill Country while rain chances will increase along I-35 later in the day.

Intense rainstorms are becoming more frequent in most of the U.S. — though experts say where they occur and whether they cause catastrophic flooding is largely a matter of chance.

Crissy and Avi Eliashar bought their home in Jonestown, Texas, 13 years ago. They never had a problem with flooding until water washed away their home, car and all their possessions early Saturday morning.

Severe thunderstorms will head east by Saturday, impacting eastern Michigan and parts of northern Indiana. Rain from 1 to 2 inches is possible as the system tracks east, but some areas could see isolated amounts of 3 to 5 inches through the weekend.

Thunderstorms and moisture from the Gulf were among the ingredients that led to catastrophic weather and flooding in the Hill Country.

As a climate scientist who calls Texas home, I can tell you that the Hill Country of Texas is no stranger to flooding. Meteorologists often refer to it as “Flash Flood Alley” because of its steep terrain, shallow soils, and its history of sudden and intense rainfall.

North Texas will see a hot and humid Friday followed by increasing chances of rain and storms through the weekend.