All the right ingredients combined for Tuesday's killer tornadoes, especially warm moist air and a shifting weather pattern courtesy of the La Niña phenomenon. Just one thing was off: the calendar.
The Feb. 5 killer tornadoes -- at least the 15th-deadliest U.S. outbreak on record -- had all the earmarks of a batch of twisters usually seen in March, several meteorologists said.
It was farther north than most February tornadoes and stronger, said Joseph Schaefer, director of the Storm Prediction Center in Norman, Okla.
Tornadoes do happen in February, but a study by Schaefer two years ago found that winter tornadoes in parts of the South occur more frequently and are stronger when there is a La Niña, a cooling of Pacific waters that is the flip side of the better known El Niño.
But Tuesday's weather violence was noteworthy. February tornadoes usually pop up near the Gulf Coast, not in Kentucky or Tennessee, said University of Oklahoma meteorology Prof. Howard Bluestein.
Part of the explanation is record warmth. It was 84 degrees in Oklahoma before the storm front moved through on its path of destruction. On Tuesday, 97 weather stations broke or tied records in Arkansas, Alabama, Tennessee and Kentucky -- the hardest-hit states.
While La Niña doesn't specifically cause tornadoes, it helps shift the jet stream, pushing storms from the West and moisture from the Gulf into the necessary collision course over the South, Schaefer said.
Like El Niño, it happens every few years, and it's been changing global weather patterns for a few months now, strengthening in January, said Mike Halpert, deputy director of the Climate Prediction Center, which monitors La Niña.
