Produced using data from the Climate Prediction Center, the telltale sign of La Niña is clearly evident in the cooler-than-average surface temperatures of the Pacific Ocean, right around the equator. This latest iteration of La Niña began in 2010's summer, then dissipated somewhat in the spring of 2011, and finally came roaring back late this summer. The CPC has forecasted the climate pattern to reign through the winter, ratcheting up the chances that the terrible drought in the South will persist.
Here are the details from NOAA:
This animation shows daily analysis of the sea surface temperatures from the NOAA AVHRR satellite sensor over the course of 2011. Values are shown as the temperature relative to the 1971-2000 average, where blue is cooler than average, red is warmer than average. The animation starts out showing a strong La Niña, but by May, warmer than normal waters appear in the equatorial Pacific - characteristic of an emerging El Niño. However, the El Niño did not gather strength, and towards the end of the animation the cooler than normal conditions associated with the current La Niña can be seen to reappear.