Heavy rains poured down Sunday night on the Texas Gulf Coast and throughout eastern Texas, leaving about a thousand houses flooded and hundreds of people in need of rescue.
As much as 37 centimeters (more than 14 inches) of rain fell in the hardest hit areas of Texas.
The heaviest downpours came in the early morning hours Monday, along with hail and small tornadoes in some areas. By daylight, city officials in Houston were scrambling to deploy rescue teams all around the city, and Mayor Sylvester Turner was asking citizens to stay home.
"With respect to our situation, there is water all over the place, flooding all over the place," he told residents.
The mayor said rescue crews had used high-water vehicles early on, and then used boats in heavily-flooded areas to rescue 400 people — many of whom are now in shelters. Underpasses on many major streets and highways are flooded and sections of Interstates 10 and 45 are closed.
In one area, rescuers in boats reported seeing several horses struggling to keep their heads above water as they searched for a way out of the fast-moving current. At least one dog was rescued, but crews primarily focused on saving humans.
Turner asked citizens to remain calm and patient as city, county and state officials respond to the disaster.
"We are deploying resources; we certainly want to make sure people are safe, especially our elderly and people who are living by themselves,” he said. “That is the number one priority."
Hundreds of flights were delayed or canceled at Houston's two large airports, but neither of the airports closed.
However, more trouble is on the way, as additional rain is expected into Tuesday.
Official flood warnings extend through Louisiana and Arkansas and into southern Missouri, but the biggest danger remains in the Houston area, where the rain-soaked ground cannot absorb the heavy rains that are expected. The new downpours could cause already-swollen rivers and bayous to overflow their banks and further swamp residential areas.
WATCH: Video footage of Texas flooding
