La Niña is described as cooler than normal ocean temperatures over the Central and Eastern Pacific. The opposite can be said for El Niño with warmer than normal ocean temperatures in the same areas.
The Easterly trade winds play a significant role in causing these phenomena. During a La Niña year, the Easterly Trade Winds push warmer waters West towards Asia and Indonesia. This leads to the upwelling of cold water along the West Coast of South America. During this phase, a wetter, stormier pattern develops over the warmer ocean waters of the Western Pacific which eventually allows for the development of storms in the Pacific Northwest and Intermountain West. Temperatures can vary with colder or warmer stretches but a drier weather pattern tends to persist in the Southern U.S. while the Northeast U.S. tends to be colder than normal.
What does it mean for Utah this winter?
Higher chances for storminess in the Northern third of the state and equal chances for the Central and Southern portions of the state. Finally, temperatures will be above normal statewide throughout the winter.