During the spring and early summer of 2011, the U.S. Northern Plains states of Montana, North Dakota, South Dakota, Iowa, Nebraska, and Missouri endured torrential rains, heavy snowmelt, swollen rivers, and near-record floods. During this same period, the south central and southwestern states were locked down in a 10-month-long drought, with record heat and massive wildfires. This split personality in the weather is largely attributable to La Niña, a climatic phase in which the equatorial Pacific Ocean turns cooler than normal, creating a weather pattern that sends heavy rain to the northern states and dry conditions to the U.S. southern tier.

Arizona and New Mexico wildfires. The Wallow fire in Arizona’s White Mountains burned for more than a month, blackening 866 square miles (2,240 square kilometers) (553,000 acres) (224,000 hectares) of national forest land before containment. High heat, strong winds, rugged terrain, and lack of rain made the blaze especially difficult to contain for the 1,300 firefighters on the line.

In neighboring New Mexico, the Las Conchas fire had burned 114,000 acres (46,000 hectares) of Santa Fe National Forest as it approached within 12 miles (20K) of the Los Alamos National Laboratory. More than 1,000 firefighters set up a containment line to keep the flames from reaching the nuclear facility’s waste storage area. A University of New Mexico geologist who studies the history of wildfires, stated that the behavior of southwestern firestorms in the last few decades “is at least as severe and maybe more so than anything we’ve seen since the last ice age.” Los Alamos Fire Chief Donald Tucker said, “We’ve seen fire behavior we’ve never seen down here, and it’s really aggressive.”

Texas drought. Texas has experienced the longest, most persistent drought in the state’s history. The entire state of Texas plus 32 counties in adjoining states were declared a disaster area by the U.S. Dept. of Agriculture. Since August, 2010, the state had been plagued by heat, high winds, and lack of rain. Between November, 2010, and June, 2011, Texas wildfires had burned 3,300,000 acres (1,335,000 hectares). Parched grazing land forced ranchers to thin their herds by prematurely sending cattle to slaughter. Farmers throughout the state suffered extreme crop losses. One farmer said, “It’s so dry, the grass just crackles under my feet.” The June, 2011 high temperature recorded in one Texas city was 117F (47C). All farmers and ranchers in the disaster area are eligible to apply for aid from the U.S. Dept. of Agriculture.

Missouri River flood. As heat and flame seared the southwest, the states in the northern plains fought to keep surging rivers from overflowing their banks. The 2010-2011 snowpack in the Rockies and other western mountain ranges was much heavier than normal, as was the spring runoff. Also, the stormy weather that brought tornadoes to some southern and Midwestern areas brought extremely heavy rain to the northern plains. The rain and runoff filled lakes and reservoirs to overflowing, and released extremely high volumes of water into the Missouri’s tributaries and into the river itself. The USDA estimated that the Missouri overflowed its banks and levees in several key places in Iowa, Nebraska, and Missouri, inundating 550,000 acres of farmland, and submerging a number of rural homes and grain storage and processing facilities.

Souris River flood. The Souris is a Canadian river with its source in Saskatchewan. It runs west to east, dipping south through Minot, North Dakota, then looping back north into Canada to join the Assiniboine River that empties into Lake Winnipeg. The Souris’s volume increases dramatically as it runs south into North Dakota. In this wetter than normal year, the river at maximum flood stage crested 4 ft (1.2m) above a record set 130 years ago. 3,000 homes in Minot were flooded out and 12,000 people evacuated to higher ground.

The role of La Niña. Approximately every 5 years, the ocean water in the tropical Pacific around Australia and Indonesia warms or cools at least 0.5 degrees C (0.9 degrees F). When it turns warmer, the condition is called El Niño. When it cools, it is a La Niña condition. This warming or cooling of the tropical ocean is accompanied by an atmospheric change in the western Pacific called the Southern Oscillation. High pressure sets in during an El Niño, and low pressure during La Niña. The combination of El Niño/La Niña and the Southern Oscillation is referred to as ENSO.

Mainly, El Niño/La Niña conditions impact the weather in countries bordering the Pacific Ocean. During La Niña, South America experiences drought conditions, and Australia and Asia very wet conditions. In the past, La Niñas have lasted 6 to 9 months. In recent years, they have been stronger and lasted longer. The current La Niña that has been the cause of the northern flooding and southern fires and droughts began in May, 2010, and lasted till June, 2011, at which point it began to weaken, but the damage had been done.

Typically, during a La Niña, Western Canada, the Pacific Northwest, Northern California, and the northern Midwestern states have above-average precipitation, while the southwestern and southeastern states have below-average precipitation. This time, the north was excessively wet and cool while the south was excessively dry and hot, indicating that these phases are hitting harder and lasting longer. Some scientists attribute this to global warming, but that is still under study. Also under study are the mechanics of the phenomena. Meteorologists, oceanographers, and other disciplines know what ENSO does, but can’t yet fully explain exactly why it does what it does.

If ENSO behaves as it has in the past, there will be a period of “normal” weather, during which ENSO will not be a factor. That will be followed by an El Niño, when conditions reverse. The northern tier will experience drier than normal conditions, and the southern tier will be wetter than normal.

The 2010-2011 La Niña produced a series of natural disasters while in full sway, including fires, droughts, tornadoes, and floods that killed hundreds and displaced thousands. Most people will be glad to see it go.