The NOAA Climate Prediction Center (CPC) announced this morning that we have entered into a weak La Niña pattern that is expected to last through the winter. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml
But, what does that mean as far as this winter's rainfall across California and the United States? Given past climatology, it can mean almost anything! This is dramatically illustrated by the past two La Niñas, both weak, which occurred in the winters of 2016-2017 and 2017-18. Their rainfall patterns looked dramatically different, especially in the West and California. See https://www.ggweather.com/ca_enso/ca_lanina.html and https://ggweather.com/enso2016/us_lanina.html
For comparison in California, below are the seasonal percentage of normal rainfall for the 11 La Niña events since 1950:
And likewise, the precipitation anomalies for the United States.
The bottom line is that with most ENSO (El Niño Southern Oscillation) events, and especially with weak La Niñas, there is no clear signal that can be used as a predictor. What is often forgotten, is that while ENSO events have a strong influence on a given year's weather, they are not the only game in town and the impact of ENSO is influenced by a myriad of other interacting factors like the North Atlantic Oscillation (NAO), Pacific North America (PNA) pattern, Arctic Oscillation (AO), Pacific Decadel Oscillation (PDO), Madden-Julian Oscillation (MJO), etc. STAY TUNED!
Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services