Sweltering heat, storms and possible twisters were expected to hit the southern plains and south-eastern states on Memorial Day, on the heels of deadly tornadoes and flooding.

Temperatures around 100F (38C) were forecast from Jacksonville, Florida, up the south-east into Macon and Savannah in Georgia and on to Charleston, South Carolina, the National Weather Service (NWS) said.

“This is super hot for this time of year,” said John Deese, a NWS forecaster in Peachtree, Georgia, near Atlanta. “This is a heat wave across the south, and it’s going to be here for a while.”

Deese predicted high temperatures through the week, staying in the mid to high 90s.

The risk of strong tornadoes is moderate but they remain possible through the week for south-eastern plains states hit by lethal twisters last week, forecasters said.

A severe tornado killed two people in El Reno, Oklahoma late on Saturday, injured at least 29 people and left hundreds homeless, officials said. Rescue workers searched for survivors in the rubble of the small community near Oklahoma City.

The NWS said a second tornado touched down in the Tulsa area early on Sunday, damaging structures, uprooting trees and toppling power lines.

Pete Snyder with the NWS said officials had confirmed that a tornado caused damage in the Tulsa suburb of Sapulpa and surrounding areas. Snyder said the area also experienced damage from straight-line winds that officials say exceeded 80mph.

The tornado was spawned by a powerful storm system that rolled through the state. Four more people were killed in the storm in Oklahoma, CNN reported.

At least seven people were killed by storms last week.

States including Nebraska, Oklahoma and Kansas, as well as parts of Ohio, will remain under flood watches and warnings through the week as rains, wind, hail and possible twisters are in the forecast, said David Roth of the NWS Weather Prediction Center in College Park, Maryland.

Parts of Arkansas, meanwhile, are preparing for what meteorologists predicted will be the worst flooding in recorded history along parts of the Arkansas river over the coming week.

The NWS said in the statement that levee “over topping” is likely with “significant impacts to life and property across a very large area”.

The Arkansas river reached 38.2ft on Sunday near Fort Smith, surpassing the historic crest of 38.1ft in April 1945.

Spokeswoman Karen Santos said the city of 80,000 residents on the border with Oklahoma was in “preparedness and warning mode”. She said one home was completely submerged and about 500 homes either had water very close or in them. Authorities predicted hundreds more homes and businesses would flood by the time the river crests there Tuesday at 42.5ft.

The River Spirit Hotel and Casino has flood waters surrounding it in Tulsa, Oklahoma. Photograph: Tom Gilbert/AP

Across the river, the tiny town of Moffett, Oklahoma, population about 120, was submerged by Saturday afternoon, Sequoyah county emergency management director Steve Rutherford told the Times Record in Fort Smith.

On Sunday afternoon, a national guard helicopter was sent to rescue two Army Corps of Engineers workers who had become trapped in a building as the Arkansas river rose, said Arkansas Department of Emergency Management spokeswoman Melody Daniel.

“The river had risen and spread to a point where the lock and dam building itself was no longer accessible by boat or road,” said Daniel, who took video of the rescue at the Trimble Lock and Dam. She said there were also several road closures.

The water flowing into the Arkansas river has come from rains in south-east Kansas and north-eastern Oklahoma, said NWS meteorologist Willie Gilmore.

“All that water funneled down into the tributaries that go into the Arkansas river,” Gilmore said.

In Tulsa, authorities advised residents of some neighbourhoods to consider leaving for higher ground because the river is stressing the city’s old levee system.