The National Oceanic and Atmospheric Administration (NOAA) says it expects La Nina to gradually strengthen and continue through the winter. During September, La Nina strengthened as indicated by increasingly negative sea surface temperature (SST) anomalies across the eastern half of the equatorial Pacific Ocean.
However, NOAA notes that La Nina is not as strong as it was a year ago. "Roughly one-half of the models predict La Nina to strengthen during the Northern Hemisphere fall and winter," it states. "Of these models, the majority predict a weak La Nina (3-month average in the Niño-3.4 region less than -0.9oC)."
NOAA reminds that weaker second La Nina winter has occurred in three of the five multi-year La Ninas in the historical SST record since 1950. "At this time, a weak or moderate strength La Nina is most likely during the Northern Hemisphere winter," it says.
NOAA says expected temperature and precipitation impacts associated with La Nina are expected to remain relatively weak during the remainder of the Northern Hemisphere early fall, and to strengthen during the late fall and winter. "It is important to note that the strength of U.S. impacts is not necessarily related to the strength of La Nina across the equatorial Pacific," it says. "During October-December 2011, there is an increased chance of above-average temperatures across the mid-section of the country. Also, above-average precipitation is favored across the Pacific Northwest, along with a higher probability for drier-than-average conditions across much of the southern tier of the country."