La Niña has arrived, bringing California the possibility of a relatively dry winter. With the state entering its fifth year of drought, the National Weather Service made it official Thursday, following weeks of speculation, declaring that the country will see a winter of La Niña. That’s a weather phenomenon associated with cooler-than-average temperatures in the surface of the eastern tropical Pacific Ocean near the equator. Jim Mathews, a weather service meteorologist in Sacramento, said the phenomenon is expected to bring somewhere between a normal or somewhat drier winter.