The governor of Texas on Monday declared states of disaster in 24 counties, citing the severe weather and flash flooding that have killed at least two people.
The state has been pounded by tornadoes, heavy rainfall, thunderstorms and flooding that forced evacuations and rooftop rescues and left thousands of residents without electrical power.
In declaring the states of disaster in 24 counties, Texas Governor Greg Abbott said: "The State of Texas has taken brisk action in dispatching all available resources to aid those affected by this severe weather system.
"My thoughts and prayers are with all the communities that are suffering as a result of this weather disaster, and I am grateful for the first responders who have worked tirelessly to provide shelter, care and resources to all impacted areas," he said.
Widespread severe thunderstorms were forecast for Monday in north-central and northeast Texas and southern Oklahoma, likely bringing destructive winds, tornadoes and hail, the National Weather Service said.
The Weather Service issued severe thunderstorm and flash-flood warnings as well as tornado watches throughout the region.
The most intense storms threatening Texas and Oklahoma were likely to strike in the late afternoon and early evening, according to Accuweather.com meteorologist Brian Lada.
"Damaging wind gusts and frequent lightning strikes can occur," he said.
Parts of the area have received more than a foot and a half of rain since May 1, which is six times more than the area typically receives in all of May, Accuweather said.
The governor's office said the severe weather could continue through the week.
Meteorologists said soil was saturated from heavy rainfall over the past three weeks, leading to the dangerous flash floods.
An unidentified man was found dead from the flooding in San Marcos, and in Oklahoma, a firefighter died when he was swept into a storm drain.