Relief workers and clean-up crews defied thunderstorms Thursday while families and friends gathered for the funeral of one of the Oklahoma tornado's youngest victims.

Heavy rain and lightning at dawn threatened to upset Moore's long trail back to normality, with police at one point closing flash-flooded streets in the residential city of 56,000 struggling to get back on its feet.

The tornado, one of the most powerful in recent years, killed 24 people, injured 377, damaged or destroyed 1,200 homes and affected an estimated 33,000 people in this Oklahoma City suburb, officials said in their latest update.

Initial damages have been estimated at around $2 billion.

"My biggest concern is not necessarily for the structure, but for people's contents," general contractor Lane Yeager, scrambling to patch the roof of a stricken home, told AFP during a break in the thunderstorms.

"They just survived the tornado. Now they're going to have more problems (with rain damage). If we can get the lightning to let up for a little while, we're going to try to cover that up so they don't have further damage."

"It's wet and it's cold," said Andy Loyd, helping to repair his daughter's house.

"But it kind of helps us to find where the holes are in the roof. It's going to get dry (later) and we'll get it secured and wait for the insurance companies to come in -- and then we'll rebuild."

By mid-afternoon, the skies had brightened, enabling utility workers to resume fixing downed power lines.

Volunteers from the American Red Cross, the Salvation Army and other charities fanned out with food, water and tools. Insurance claims adjusters tallied up losses.

In an Oklahoma City funeral home, about 250 family and friends together mourned the death of Antonia Candelaria, age nine, one of the seven children killed when the roof collapsed at Plaza Towers Elementary School.

"We will miss our precious little 'Ladybug' every day, but will rejoice for the day we will be reunited with her again someday," the girl's family wrote in an online obituary.

Pupils returned to Plaza Towers on what officially marked the last day of school, to collect belongings and see their teachers one last time before going on summer vacation.

President Barack Obama is scheduled to visit Moore on Sunday to comfort survivors and take stock of the destruction and clean-up. The tight-knit community suffered a similarly powerful tornado in 1999 that killed 41 and another in 2003.

"We're not only at the stage of recovery and getting our community back together. We're also at the stage of healing for our citizens," said Oklahoma Governor Mary Fallin in announcing a public memorial service for late Sunday.

The United States experiences three out of four tornadoes in the world, but the one that hit Monday was unusually powerful when it touched down with little advance notice, cutting a 17-mile (27-kilometer) swath of destruction.

It followed roughly the same track as the 1999 twister, yet very few homes in Oklahoma -- and neither of the stricken schools -- had purpose-built storm shelters.

Vendors of such underground shelters for private residences say demand has surged since the latest tornado, but Fallin was non-committal on whether government should help fund such facilities.

"It think it's important for Oklahoma to talk about that," she said.

Oklahoma state legislators -- otherwise bitterly divided over such issues as abortion -- came together to draw $45 million from the state's "rainy day" emergency fund to help tornado victims.

On the Enhanced Fujita scale that gages a tornado's strength based on the damage it causes, the twister was an EF-5, the highest possible level, said Kelly Pirtle of the National Weather Service's Severe Storms Laboratory.