A La Nina occurs when certain parts of the Central Pacific Ocean cool by half a degree Celsius (0.9 degrees Fahrenheit) compared to normal. The world had been flirting with one this year and the National Oceanic and Atmospheric Administration declared Thursday that La Nina conditions have formed. But it's likely to be not very strong and may disappear in the next few months, based on multi-factor computer model forecasts by NOAA and Columbia University, said Michelle L'Heureux, lead scientist on the NOAA team that studies both La Nina and El Nino. สล็อต วอเลท