* Likelihood of La Nina seen at 70 pct - BOM
* Alert follows Japan issuing La Nina warning
* La Nina can trigger hot but wet summers in southern hemisphere
* Can trigger cold snowy and winters in northern hemisphere (Adds graphic, detail on potential La Nina impact; changes slug)
By Charlotte Greenfield and Henning Gloystein
Nov 22 (Reuters) - Australia's Bureau of Meteorology has issued an alert for a La Nina weather event starting next month, saying the chance it would take place was triple the normal likelihood.
The weather occurrence La Nina is triggered when sea surface temperatures in the central Pacific Ocean drop to lower-than-normal levels. It generally implies stormier and cold winters in Asia's northern hemisphere and warm but wet summers in the southern hemisphere.
The Australian bureau pegged the chance of a La Nina at 70 percent, adding that all its models reached La Nina thresholds in December with most models suggesting it would last until at least February.
This is a notch-up from Japan's weather bureau, which last week increased the likelihood of La Nina from 50 percent to 60 percent.
"This could lead to a colder-than-usual winter in the northern (hemisphere), which would be positive for coal and gas demand," said Ralph Leszczynski, head of research at shipping brokerage Bancosta in Singapore.
He added that La Nina could lead to dry conditions in many agricultural crop growing regions, which would include the southern United States, and hot but unusually wet weather for most of Australia.
NOT AS BAD AS ITS BROTHER, BUT STILL BAD
While a La Nina, which means little girl in Spanish, tends to be less damaging than the corresponding El Nino (little boy), which occurs when there are warmer waters in the Pacific, a strong event can have a severe impact on agricultural and commodity markets.
In the past, Las Ninas have caused supply disruptions in food crops and mining output in the southern hemisphere, including in Australia.
In the Atlantic basin, La Nina is also known to trigger hurricanes.
The BOM said in the alert on its website that the event would likely be weaker and shorter than the strong La Nina that occurred between 2010 and 2012 and brought devastating floods to Australia.
A U.S. government weather forecaster this month said La Nina conditions are predicted to continue through the Northern hemisphere winter 2017-18.
(Reporting by Charlotte Greenfield in WELLINGTON; and Henning Gloystein in SINGAPORE; Additional reporting by Keith Wallis in Singapore; Editing by Adrian Croft and Richard Pullin)
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