The tornado which tore through the town of Joplin, MO was the deadliest since officials began keping records in 1950, and worries many that an unuusally deadly storm season is underway. As Brian Vastag and Ed O’Keefe reported:

The extraordinary Joplin twister — the single deadliest tornado since officials began keeping records in 1950 — was a rare destructive phenomenon known as a “multi-vortex,” hiding two or more cyclones within the wider wind funnel.

The storm smashed the southwest Missouri city’s hospital Sunday, left nothing but splintered trees where neighborhoods once stood and killed at least 116 people, with the death toll expected to rise. The storm injured an additional 500 people and damaged or destroyed at least 2,000 buildings.

Added to the record 875 tornadoes that tore across the country in April, this latest disaster has experts asking why 2011 has spawned so many deadly storms. While researchers suss out the causes for this year’s record-breaking season, one thing is certain: Unusually big twisters are blasting through heavily populated areas.

“We have had more F4s and F5s than in past years,” said Jack Hayes, director of the National Weather Service, referring to the two most destructive categories of tornadoes. And instead of touching down in farms and fields, storms have hit cities such as Joplin and Tuscaloosa, Ala.

An emerging body of research points to a cyclical drop in temperatures in the Pacific Ocean as part of the answer. Called La Nina, the cycle lasts at least five months and repeats every three to five years. This year La Nina is pushing a strong North American jet stream east and south, altering prevailing winds. The jet stream’s river of cool air high in the atmosphere pulls warmer, more humid air from the ground upward, forming thunderstorm “supercells.”

The seach for survivors in Joplin has reached a fever pitch as law enforcement and volunteers try to take advantage of small windows of good weather. As AP explained:

Crews busted holes in concrete slabs and sifted through strewn home goods Tuesday as rescuers focused on crumpled big-box stores and apartment complexes in Joplin in a frantic search for survivors, after nearly 120 people were killed by the deadliest single U.S. tornado in about 60 years.

One team poked through the remains of a Home Depot store, while others searched a Walmart and wrecked apartments as the clock ticked down on another round of severe storms. A hunt through the rubble using search-and-rescue dogs was planned, and officials expected to test the city’s nine warning sirens while the sun was still shining.

The Storm Prediction Center in Norman, Okla., warned of severe weather starting Tuesday afternoon in a band from northern Texas up to southern Illinois and stretching east into western Kentucky, western Tennessee and northwest Mississippi. Meteorologist David Imy said conditions were ripe for tornadoes in central and eastern Kansas, almost all of Oklahoma and northern portions of Texas.

“It looks like primetime for the greatest tornado coverage and intensity will be between 3 to 4 p.m. and 9 to 10 p.m.,” Imy said. “That will be when the greatest coverage and most intense storms occur.”

Thunderstorms are forecast in Joplin from 6 p.m. to midnight, and there’s a possibility of tornadoes, he said.

Speaking from London, President Obama pledged to visit Joplin on Sunday and meet with survivors of the tornado. As Debbi Wilgoren reported:

President Obama will visit Missouri Sunday to meet with victims of the storm that devastated the town of Joplin, Mo.two days ago and to hear about the disaster response from emergency officials.

Speaking from London, where he is on the second leg of a six-day European trip, Obama said he wanted to let the storm’s victims know “that all of America cares deeply about them and that we are going to do absolutely everything we can to make sure that they recover.”

Noting that the unusually powerful tornado “compounded what has already been an extraordinary storm season throughout the Midwest and in the South,” Obama pledged to support the affected communities “until every house is repaired, until every neighborhood is rebuilt, until every business is back on its feet.”

He said that while in Missouri, he is hoping “to pray with folks and give them whatever assurance and comfort I can that the entire country is going to be behind them.”

More from The Washington Post

BlogPost: Survivors react to Joplin tornado as recovery operations begin

Capital Weather Gang: More tornadoes feared as more storms form over Plains

Video: Raw footage of Missouri twister