While wet and snowy weather has dominated the western U.S., persistent drought conditions are likely to linger in the Southern Plains and Southeast through mid to late spring, according to NOAA's National Weather Service. La Ni?a has kept storms and most of their precipitation in the north, leaving the South drier than normal.

A drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation.

It can have a substantial impact on the ecosystem and agriculture of the affected region. Although droughts can persist for several years, even a short, intense drought can cause significant damage and harm the local economy.

"The speed with which the drought developed across the southern United States is rather unusual considering that just last year El Ni?o dominated the region with abundant precipitation," said Bill Proenza, director of NOAA's National Weather Service southern region." Then it was as if a switch was flipped during the summer, changing to La Ni?a conditions."

One of the major aspects of the emergence of La Ni?a was a very busy Atlantic hurricane season, which spawned 19 tropical storms, making it the third most active on record. Despite the large number of storms, only Hurricane Alex and Tropical Storm Hermine produced any appreciable rainfall in the southern United States. Those storms only affected Texas; no significant rainfall from an organized tropical system fell along the Gulf Coast from Louisiana to Florida.

Sparse tropical rainfall and the dry conditions associated with La Ni?a combined to create severe to extreme drought conditions for nearly a third of the South and Southeast by late fall and early winter.

While the drought touches all of the Gulf Coast states, Texas and Florida are the most affected. From October through December, Texas received only five to 50 percent of normal precipitation, with portions of the lower Rio Grande averaging less than five percent of normal. During that period, for example, Brownsville received only 0.14 inches (normal is 6.55 inches) and Del Rio received 0.04 inch (normal is 3.89 inches). To the north in Austin, only 1.55 inches of rainfall was observed, compared to the normal of 8.34 inches.

In Florida, 51 percent of the state was in severe to extreme drought by the end of 2010. Some areas experienced the driest July 1 — December 31 period on record. For example, Gainesville received only 12.95 inches of precipitation, compared to the previous record low of 15.25 inches. The city normally receives 27 inches. Daytona Beach ended the period with 14.71 inches compared to the previous record low of 15.35 inches ? its normal is 30 inches.

In central and southern Florida, the South Florida Water Management District rain gauge network recorded an average of only 2.97 inches during the October through December period, breaking the previous record low average of 4.07 inches. Moreover, the District reports that Florida's Lake Okeechobee ended the year at 12.4 feet, 2.3 feet below average.

Drought happens periodically. In the worse case, it occurs over a prolonged period of time and once fertile land turns into desert. At the worst of the USA's most recent drought, in August 2007, almost 50% of the country was involved.

In early 2010, about 7% of the country was in a drought, according to federal scientists. The only part of the USA in "extreme" drought then was a small fraction of Hawaii.

In 2007, gigantic portions of the Southeast were in the worst drought in more than a century, sparking water wars among Georgia, Alabama and Florida.

La Ni?a has developed 13 times since 1950, and the current La Ni?a ranks as the sixth strongest. The question climate experts are asking now is whether it will fade with the approach of summer or continue into next year.

"Of the five stronger La Ni?as that occurred, four resulted in multi-year events," said Victor Murphy, climate program manager for NOAA's National Weather Service southern region. "If this La Ni?a persists until next winter, the threat of drought conditions in the south extending into next year will be heightened."