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, said several meteorologists.
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 Nino. In 1971, a deadlier February outbreak in the Mississippi Delta killed 121 people.
But Tuesday's weather violence, which killed at least 50 people, was noteworthy. February tornadoes usually pop up near the Gulf Coast, not in Kentucky or Tennessee, said University of Oklahoma meteorology professor 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.
Meteorologists are quick to say they cannot blame global warming. There is not enough good data over enough years with weather events as small as tornadoes, to draw such conclusions.
But there were plenty of the standard ingredients for tornado formation Tuesday: a strong storm system coming from the west (this one buried parts of Colorado in snow); warm air near the ground; high winds; and warm moist air coming north from the Gulf of Mexico.
La Niña shifts jet stream
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, said Schaefer.
Like El Nino, 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.
Preliminary figures for January — the numbers usually drop after closer scrutiny — show 136 tornadoes, five tornado deaths and three killer tornadoes. The average from 1997-2007 is 40 tornadoes, three deaths, and three killer tornadoes. The record for most tornadoes was 212 in January 1999.
Between 1997 and 2007, the average February has 30 tornadoes, killing 9 people. Early reports tallied 68 tornadoes so far this month.
"We're off to a big start for the year," said Greg Carbin, a meteorologist at the storm center.
Get used to this because the patterns that lead to tornado outbreaks seem to be here for a while, meteorologists said.
"As long as the pattern remains the same it can be very active," Schaefer said. "It's not a time to let down your guard."