This week's killer freeze in the U.S. was no surprise.

The government and private meteorologists saw it coming, some nearly three weeks in advance. They started sounding the alarm two weeks ahead of time. They talked to officials. They even issued blunt warnings through social media.

And yet catastrophe happened. At least 20 people have died, and 4 million homes lost power, heat or water at some point.

Experts said meteorologists had the science right: The math-oriented atmospheric physics for the forecast and the social sciences on how to get their message across were accurate.

"This became a disaster because of human and infrastructure frailty, a lack of planning for the worst-case scenario and the enormity of the extreme weather," said disaster science professor Jeannette Sutton of University at Albany in New York.

The event shows how unprepared the nation and its infrastructure are for extreme weather events that will become bigger problems with climate change, meteorologists and disaster experts said.

The insured damage, which represents only a fraction of the real cost for the nearly weeklong intense freeze starting on Valentine's Day weekend, is probably $18 billion, according to a preliminary estimate from risk-modeling firm Karen Clark & Company.

So why were many entities unprepared?

One of the main problems was the Texas power grid, which is overseen by the Electric Reliability Council of Texas (ERCOT).

Sutton said there was "a huge failure" on that part of the infrastructure.

Texas is the only state in the continental United States with an independent and isolated grid. It allows the state to avoid federal regulation but also severely limits its ability to draw emergency power from other grids. 

According to economists, ERCOT relies on a wholesale electricity market, where free-market pricing provides incentives for generators to provide daily power and to make investments to ensure reliability in peak periods, while other states in the U.S. maintain a capacity market – a system that provides payments to operators to be on standby to supply power during severe weather events.

The grid operator's chief executive officer, Bill Magness, told reporters on Thursday that the agency prepared based on past cold outbreaks and "this one changes the game because it was so much bigger, so much more severe, and we've seen the impact it's had."

Essentially, it was so big it wasn't planned for "is not a great way to plan," Sutton said, "especially if we are supposed to learn from our failures."

Also, this was so unusual that residents had no idea how to handle it, Sutton said. It simply wasn't something they had experienced before.

People think they know cold, so they likely judged the forecasts based on much milder chills, said Kim Klockow-McClain, who heads the National Weather Service's behavioral insights unit, which focuses on how to make forecasts and warnings easier for people to understand and act on.

The forecast also included snow and ice that probably got people's attention more than the temperature drop, Klockow-McClain said.

"Human beings, we live our lives as though we are not at risk," Sutton said. "We come up with all kinds of rationale for 'we're going to be OK.'"

(With input from AP)