La Nina conditions — an unusual cooling of ocean surface temperatures off the western coast of South America that is believed to have significant effects on North American weather patterns — continue to develop across the equatorial Pacific, according to the National Oceanic and Atmospheric Association (NOAA).

NOAA and other forecasters have said a weakening El Nino — the warming of surface temperatures in the central and eastern equatorial Pacific Ocean — and above-average sea surface temperatures in the Atlantic could be primary drivers behind an increase in Atlantic hurricane activity this year, and the development of a La Nina event could further drive tropical storm activity (see NGI, May 31). Those same conditions are likely to bring warmer-than-normal temperatures to much of the United States through September, according to some forecasters (see NGI, June 28).

“The majority of models indicate the onset of La Nina conditions,” according to NOAA’s Climate Prediction Center. “Most of these models indicate La Nina conditions will strengthen and persist at least through Northern hemisphere winter 2010-11.”

While the rate of the sea surface temperature decrease slowed during June, it continued across the equatorial Pacific with negative anomalies expanding across the central and eastern Pacific, NOAA said. The change in surface temperatures, combined with enhanced convection over Indonesia, an expanding area of suppressed convection over the western and central equatorial Pacific, enhanced low-level easterly trade winds and anomalous upper-level westerly winds reflect developing La Nina conditions, according to the forecaster.

The maturing La Nina could bring with it above-normal temperatures for the southern and eastern United States, and an increased chance of precipitation for the Gulf Coast and Southeast.

The consensus forecast for the 2010 Atlantic hurricane season points decidedly towards an unusually active season. NOAA expects the 2010 season to be “active to extremely active,” with 14-23 named storms, including eight to 14 hurricanes, three to seven of them intense (Category Three or greater). WSI Corp. has increased its Atlantic hurricane forecast three times this year, saying in its most recent forecast that it expects 20 named storms, including 11 hurricanes, five of them intense, with a greater-than-normal chance of landfall along much of the East Coast.

Colorado State University forecasters have said they expect a “very active” season (see NGI, June 7) and Chief Long Range Forecaster Joe Bastardi has said the collapsing El Nino pattern could help make 2010 one of the most active seasons on record (see NGI, May 24).

The first hurricane of the 2010 Atlantic season, Alex, was a Category 2 hurricane when it made landfall in northeastern Mexico June 30. It cause only a relatively minor amount of gas production shut-ins in the Gulf of Mexico.

In a separate study released last week, NOAA said last month’s combined global land and ocean surface temperature made it the warmest June on record. Worldwide average land surface temperature last month was the warmest on record for June and the April-June period, and the second warmest on record for the year-to-date (January-June) period, NOAA said.

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