"This is going to be our new normal and the effects that we're seeing from climate change are the crisis of our generation," Criswell told co-host Jake Tapper on CNN's "State of the Union."
Severe storms and tornadoes tore through states in the South and Midwest late Friday into Saturday, leaving devastating damage, tens of thousands of homes without power and nearly 100 people dead.
Kentucky Gov. Andy Beshear (D) said more than 80 individuals have died in his state as a result of the storm, while warning that the number is expected to grow larger than 100.
Asked how unusual it is to see such a powerful storm this late in the year, Criswell told Tapper "I think it's incredibly unusual," pointing to the magnitude and timing of the cyclone.
"We do see tornadoes in December, that part is not unusual, but at this magnitude I don't think we've ever seen one this late in the year. But it's also historic. Even this, the severity and the amount of time this tornado, or these tornadoes, spent on the ground is unprecedented," Criswell said.
The FEMA administrator said her agency is working with communities in making efforts to "help reduce the impacts that we're seeing from these severe weather events and help to develop system-wide projects that can help protect communities."
Comment: See also: Record-breaking tornado outbreak hits US: At least 3-dozen tornadoes form, including monster that dragged 4 states - At least 100 killed