La Niña is a component of the natural see-saw of oceanic and atmospheric conditions in the tropical Pacific characterized by anomalously cold ocean temperatures in the central and eastern tropical Pacific. These anomalously cold temperatures are evident in sea surface temperature anomalies from earlier this month. Note in particular, the tongue of anomalously cold water extending along the equator from South America to the dateline.
|Source: Climate Prediction Center
Another way to look at this is in terms of hight anomalies at upper levels. Areas in warm colors below correspond to anomalous ridging, and areas in cool colors anomalous troughing. At upper-levels, the primary circulation features are an anomalous trough near Hawaii, ridging in the North Pacific, and troughing over northwest North America.
Thus, this is a pattern that doesn't fit the average La Niña pattern all that well. That isn't to say La Niña isn't playing some role. It could be playing an important role, with our use of relationships based on averaging past events the real problem. On the other hand, it is also possible that we need to look at what is happening from a broader, global perspective. Some discussion of this topic is provided by the California Weather Blog. This is an area of active research, and one that will probably get even more attention after this winter, which has generated some remarkable weather extremes across the United States.