The devastation is stunning -- homes and lives shattered as the deadliest swarm of twisters in three years battered up to 15 states.
This could turn out to be among the top 10 three-day outbreaks for number of tornadoes, though experts can't be sure until all the reports are sorted, said Greg Carbin of the federal Storm Prediction Center in Norman, Okla.
While tornadoes occur regularly, their power always shocks.
This time it was storms battering their way from Oklahoma to North Carolina, claiming at least 45 lives, almost half of those in North Carolina. It was the deadliest since Feb. 5, 2008, when 57 died in the "Super Tuesday" primary election day tornadoes in the Southeast.
May is the busiest month for twisters, but they surge in April. Most early spring tornadoes strike the Southeast and South Central states.
For about the past 30 years, the United States has averaged 135 tornadoes in April, the highest number being 266 in 1974, according to Jake Crouch of the National Climatic Data Center in Asheville, N.C.
In these latest storms, the National Weather Service is investigating 267 preliminary tornado reports -- including 97 Saturday in North Carolina. But many of those will turn out to be duplicates, Carbin said.
Teams are assessing the damage, but he said it can take several days to more than a week for a final determination.
In this case, the storm system first developed over the Pacific and intensified Thursday when it got to the central Plains where the dry western air collided with the warm humid air from the Gulf of Mexico.
From there, the storms developed "pretty much as expected" over Oklahoma, Carbin said. Overnight, the storms merged into a fast-moving front crossing Arkansas and into the Mississippi River Valley, drawing fuel from daytime heating, continuing on into Mississippi and Alabama, then into the Appalachians by Saturday morning. Then the storms again strengthened with daytime heat for the third day of tornadoes, hitting North Carolina hardest.
That intersection of dry and wet air masses sets up Tornado Alley in the country's center.