Another occasional winter pattern influencer can be sudden, sharp warming events high in the atmosphere known as sudden stratospheric warmings. These disrupt and weaken the polar vortex, which can then set up blocking patterns that bring much colder air into the U.S. weeks later in winter, regardless of La Niña.

The development of a weak La Niña could mean that weather in the U.S. may look different from times in which a strong La Niña takes hold.

A La Niña winter is still in the cards for the U.S., the Climate Prediction Center said in an updated forecast Thursday. The Center, a division of the National Oceanic and Atmospheric Administration (NOAA),

With the late arrival of La Niña, it is anticipated to be weak. This is due to timing. Typically, ENSO patterns reach their peak strength during the winter months and only grow stronger throughout. With the late arrival, La Niña is running out of time to grow and strengthen.

Back in the spring it was looking like a strong La Niña might shape up come late fall, but since then the odds have decreased quite a bit, as waters along the equatorial Pacific didn’t get as cool as anticipated and westerly trade winds failed to organize.

State climatologists are eyeing historical records of recent La Niña winters in the Pacific Northwest as a clue of what might be waiting in the wings.

The National Oceanic and Atmospheric Administration has updated its forecast for La Niña this winter. NOAA has a La Niña watch in effect.