The storms that slammed into Texas and Oklahoma over Memorial Day weekend may signal the beginning of an El Niño year, but their unusual ferocity could be due to climate change, scientists said Wednesday.

Torrential rains and tornadoes killed at least 17 people in the Southwest, with the death toll expected to rise as authorities in Texas continue to search for about a dozen missing people. In Houston, the nation’s fourth-largest city, record flooding damaged an estimated 4,000 structures, including homes and businesses, and led to about 1,000 calls for help, according to city officials.

Texas Gov. Greg Abbott on Monday expanded a state of emergency, which was declared earlier in the month because of severe weather, to at least 40 counties. President Barack Obama responded by assuring Abbott that the federal government would assist in the state’s recovery.

Some of the worst-hit areas of the Southwest have received more than 18 inches of rain since the beginning of May — six times as much as the area typically receives in the month.

Scientists on Wednesday warned that the heavy rains might signal that 2015 will be marked by El Niño, a warm phase in the equatorial Pacific Ocean characterized by intense, often dangerous storms.

“It does seem to be at least linked to El Niño, because when you have El Niño, generally speaking, you will see a signal in the springtime — a wet signal in Texas and into Oklahoma,” said Tom DiLiberto, a meteorologist at the National Oceanic and Atmospheric Administration Climate Prediction Center.

The Climate Prediction Center, which says El Niño occurs every two to seven years, has forecast an 80 percent chance that one will occur this year. If so, DiLiberto says, residents across the Southwest can expect intense weather.

“Basically what happens is you see a stronger southerly jet stream and a storm track that brings more storms across the South, with an increase in precipitation,” he said.

Although the causes and effects of El Niño are not yet fully understood by scientists, they know that the weather phenomenon results from a coupling of oceanic and atmospheric conditions, DiLiberto added.

Though the ocean warming that causes El Niño may occur naturally, climate change, which is warming ocean temperatures globally, could intensify storms, putting residents caught in their paths in even greater danger, said Mark Crane, a professor of earth and climate sciences at Columbia University. “One of the things that happens with climate change and is very clear is that oceans have all gotten warmer,” he said.

While El Niño and global warming may combine to direct severe storms toward the Southwest, the phenomenon may bring relief to drought-stricken California, DiLiberto said.

“Usually with El Niño, what you see is a wetter signal for California,” he said. “Just because we have El Niño doesn’t mean we will observe any specific weather condition, but generally speaking, there’s a wetter tilt in the odds for the California range.”

Other potential effects of El Niño years include fewer than usual Atlantic Ocean hurricanes but more in the eastern Pacific, Crane said.

“But you know Hurricane Andrew occurred in an El Niño year with below-average number of hurricanes, and it was a bad one,” he said, referring to the Category 5 hurricane that slammed into southern Florida in August 1992.

With wire services