August 9, 2007 / 1:40 PM / 12 years ago

Weather Service raises La Nina probability

NEW YORK (Reuters) - The U.S. National Weather Service on Thursday predicted slightly increased chances of greater than 50 percent that the La Nina phenomenon would develop during the next couple of months.

In the meantime, its Climate Prediction Center projected neutral conditions prevailing through August.

Recent atmospheric patterns are consistent with movements toward La Nina conditions, despite their falling short of temperature thresholds for the weather anomaly, the Climate Prediction Center said.

La Nina — Spanish for “little girl” — is an unusual cooling of Pacific Ocean surface temperatures, which can trigger widespread changes in weather around the world, including a higher-then-usual number of hurricanes and storms.

The La Nina weather pattern occurs about every three to five years and often follows El Nino, a warming of Pacific waters, which can also wreak havoc on weather around the world.

The latest atmospheric conditions and forecast models suggest a slightly greater than 50 percent chance of La Nina developing during the next couple of months. Historically, August through October has been a critical period for the onset of La Nina events, the government group said.

Nearly all of the models predict below average sea surface temperatures for the remainder of the year, in some El Nino regions, which extend from the United States to South America and Asia.

Recent forecasts range from neutral to La Nina conditions, with a majority of dynamic models indicating a more immediate transition to La Nina, sometimes referred to as a Pacific cold episode.

Over the past several months, however, those models have consistently predicted a stronger and more rapid cooling than has actually occurred.

By contrast, a majority of statistical models indicate more neutral conditions, but some forecast weak La Nina conditions during the fall or winter.

For example, low-level easterly winds remained stronger than average in the west-central equatorial Pacific and heat movement within the atmosphere remains suppressed across most of the equatorial Pacific.

Also, the report said, upper-ocean heat in the central equatorial Pacific region remained below average.

Collectively, oceanic and atmospheric conditions show neutral conditions continuing during August.

Also on Thursday, the National Oceanic and Atmospheric Administration predicted the Atlantic hurricane season would be slightly less active this year than first predicted, with up to nine hurricanes expected to form.

Still, “the conditions are ripe for an above normal season,” NOAA meteorologist Gerry Bell said.

So far this hurricane season, which ends on November 30, there have been three named storms in the Atlantic — Andrea, Barry and Chantal — but no hurricanes.

La Nina last occurred from 1998 to 2001 and resulted in drought across much of the western United States.

The CPC forecast is available on: here

Additional reporting by Christopher Doering

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