On Thursday, September 10, 2020, NOAA issued a La Niña advisory. La Niña conditions are here and there is a 75% chance they will persist through the winter. So what exactly is La Niña and what does it mean for the United States in general and for the Pacific Northwest?
La Niña is a cooling of the equatorial Pacific and is based on water temperatures between 5°North latitude and 5°South latitude. A La Niña is characterized as weak (temperature 0.5°C to 0.9°C below normal) moderate (temperature 1.1°C to 1.4°C below normal), or strong (temperature 1.5°C to 1.9°C below normal). These anomalies must be persistent over several months. The NOAA depiction (above) gives a general idea of what may be expected. Statistically, La Niña results in cooler and wetter conditions during winter than normal in the Pacific Northwest and warmer and drier than normal conditions across the southern tier of the United States.
On the above chart for September 10, 2020, we can see very strong negative anomalies in parts of the Equatorial Pacific.
La Niña and snow in the Pacific Northwest (PNW)
Taking a look at snow in the Pacific Northwest (PNW) – we see that average snowfall at SeaTac Airport has decreased substantially over the past 70 years. We can examine two periods of 32 years each – the winter of 1949-50 through the winter of 1979-80 and the winter of 1980-81 through 2019-20. (Snowfall was not reported at SeaTac from the 1996-97 through the 2003-04 seasons). For the earlier period, average snowfall was nearly 15” per year. For the latter, it was less than 8”per year. The contrast is even more striking when we look at the seasons from 1949-50 through 1971-72. That 24 year period saw an average of over 17” of snow per year with eight seasons reporting over 20”. For the next 40 years of record there were only three seasons that exceeded 20”.
Of the years with snowfalls greater than 20”, five of them occurred during periods of weak or moderate La Niñas including the two winters when SeaTac reported over 60” of snow (1949-50 and 1968-69). During 13 other La Niñas, seasonal snowfall remained below 20”. So, we can see that, at best, La Niñas are only weakly associated with snowy winters.
Other factors that influence seasonal snowfall in the PNW include the rise of average winter temperature – about 3°Fahrenheit since the 1950s and the Pacific Decadal Oscillation which deserves a discussion of its own.
September 2020 ENSO update: La Niña is here! | NOAA Climate.gov
One important global impact of La Niña is its effect on the Atlantic hurricane season.La Niña reduces wind shear—the change in winds between the surface and the upper levels of the atmosphere—allowing hurricanes to grow.The likelihood of La Niña was factored into NOAA’s August outlook for the Atlantic hurricane season, which favored an “extremely active” season.