Emergency services and volunteers are still searching for survivors
USPresident Barack Obama is set to visit the tornado-hit city of Joplin in Missouri, where 142 people are now known to have been killed, reports the BBC.
Obama will meet survivors and see the damage for himself.
He will also take part in a memorial service and observe a minute of silence at 1741 local time (2141 GMT) - exactly a week after the tornado struck.
On Saturday, a list of 156 people missing dropped to 105 after more were accounted for, city officials said.
The tornado, with winds of 200mph (322km/h), was one of the most destructive in US history. It injured more than 900 people and carved a swathe of destruction through the city.
Missouri Governor Jay Nixon has declared Sunday an official State Day of Prayer and Remembrance.
"I invite all Missourians to pause and remember their neighbours and draw upon the resources of their faith in support of their fellow Missourians," the governor said in a statement.
US and Missouri flags will fly at half-mast over all government buildings in the state throughout the day.
A rescuer with a dog searches through the rubble in Joplin The tornado destroyed about a third of Joplin
President Obama will attend a community memorial service at the Taylor Performing Arts Centre on the campus of Missouri Southern State University.
During the service, the whole city is expected to fall silent for a minute to honour the victims of the single tornado that devastated Joplin.
On Saturday, city manager Mark Rohr said Obama's visit would be an opportunity to convey the scope of the damage to the federal government.
Rohr also said the death toll now stood at 142 - an increase of three from the previous total.
Among the newly confirmed victims was teenager Will Norton, who was sucked from his father's car as they drove home from his high school graduation.
More than 600 volunteers and 50 dog teams are still scouring the shattered remains of homes and offices for survivors or victims.
"We're going to be in a search and rescue mode until we remove the last piece of debris," Rohr said.
Joplinpolice say they have made 17 arrests for looting.
The US National Weather Service says that 2011 is already the deadliest year for tornadoes since 1950, when precise figures were first kept.
The death toll so far this year stands at 520. The previous highest recorded death toll in a single year was 519 in 1953.
The first funeral of a confirmed victim from the Joplin tornado was held on Friday.
Hundreds of mourners gathered at a church in Galena, just over the Kansas border, for the funeral of Adam Dewayne Darnaby, 27.
So far at least 19 bodies have been released to families, but many are yet to be formally identified.
Officials say that, wherever possible, they prefer to base identifications on DNA, medical records and also distinguishing features such as tattoos and piercings.
However, some families of victims say the delays are adding to their distress.