More rainfall is expected across the region on Friday.

More rounds of excessive rainfall across parts of the Ohio and Tennessee Valleys were expected to trigger flash flooding throughout Friday, according to the Weather Prediction Center.

As of early Friday, a flood watch was in effect through the evening for large swaths of Kentucky, including several of the counties hit by storms on Thursday. The approaching front could bring persistent and heavy rainfall over soil that is already saturated, driving up concerns about additional flooding in the region. A flood warning was also in effect until the afternoon for parts of southeastern Kentucky, including several rivers.

A flood watch was in effect until 10 p.m. Friday for a large portion of West Virginia, where heavy rain was possible. The highest concerns about flooding were in areas already hit by rainfall. Meteorologists throughout the region echoed a warning they have used repeatedly during the past few days: “If you encounter flooded roadways, turn around don’t drown.”

Gov. Jim Justice of West Virginia on Thursday declared a state of emergency for six counties after severe thunderstorms rolled through the region, causing local flooding and disrupting potable water systems.

Similarly, Mr. Beshear declared a state of emergency in the eastern portion of his state, where at least eight people died and dozens of others were rescued from rooftops Thursday after heavy rain swept through the region.

The deadly flooding in Kentucky came days after record rainfall drenched parts of the St. Louis region with up to a foot of rain that quickly flooded interstates and neighborhoods. Two people died, including one who was found on Wednesday in a semi-truck that the authorities said had been submerged in floodwaters. About 70 other people were rescued, and more than a dozen homes experienced “significant flooding,” officials said.

While many factors contribute to flooding, researchers expect that, as the climate warms, flash floods will increase and get “flashier,” meaning their duration will shorten as their magnitude increases. Severe flash floods can be more dangerous and destructive.

“I believe climate change is real,” Mr. Beshear, a Democrat, told reporters. “I believe that it is causing more severe weather. With that said, I don’t know about this one and whether it is or is not connected, and I don’t want to cheapen or politicize what these folks are going through.”

It was unclear how the deaths on Thursday in Kentucky occurred, but Mr. Beshear said it was likely that people died in the middle of the night — “maybe even in their sleep.”

Mr. Beshear also requested federal assistance to respond to the flooding. “The damage suffered is enormous and recovery will be a long-term effort,” he said. “This assistance is critical to our efforts and essential for our people.” He also established a state flood relief fund to help those affected by the floods.

State officials had also recommended that people evacuate the floodplain of Panbowl Lake in Jackson, Ky., because of a “muddy discharge” at a local dam. More than 100 homes and more than a dozen businesses could find themselves in the path of flooding.