The Climate Prediction Center last week released the latest outlook for El Niño this winter and made quite a few headlines throughout the meteorological community. Here’s why:
The so-called “Spring Predictability Barrier” has passed. In spring, the El Niño Southern Oscillation (ENSO) is often in transition from one phase to another. For example, a La Niña phase could be decaying and passing through neutral condition to an El Niño condition, or vice versa. Of course, as we get closer to winter, the models become more accurate because there’s less time for inaccurate oceanographic and atmospheric data to be amplified at model initialization.
The Climate Prediction Center is currently advertising a neutral condition — the infamous “El Nothing” continuing through summer. However, the chance for El Niño will increase to 50 percent during fall, and 65 percent during winter 2018-19, which is excellent news if you’re hoping for increased amounts of precipitation. The most important aspect of this prediction is how strong of an El Niño event this will eventually become. Typically, along the Central Coast, the higher the classification of the El Niño event, the more rain it will create.
Unfortunately, most of the models and ensemble packages are advertising a weak El Niño condition, which like neutral conditions — or El Nada — typically doesn’t produce any reliable seasonal rainfall predictions along the Central Coast. Remember, El Niño is not some gigantic vortex of gloom that’s lurking somewhere in the equatorial Pacific. El Niño is an area of above-average sea-surface temperatures (SST) in a central equatorial region of the Pacific Ocean called Niño 3.4.
Since 1950, NOAA has used Niño 3.4 as the standard for classifying El Niño (warmer-than-normal SST) and La Niña (cooler-than-normal SST) events. The fortunetelling SST cycles in Niño 3.4 are categorized by the amount they deviate from the average SST — in other words, an anomaly over a three-month period. A weak El Niño is classified as an SST anomaly between 0.5 and 0.9 degrees Celsius. A moderate El Niño is an anomaly of 1.0 to 1.4 degrees Celsius. And a strong El Niño ranges from 1.5 to 1.9 degrees Celsius. A very strong El Niño anomaly is anything above 2.0 degrees Celsius (or 3.6 degrees Fahrenheit). Neutral conditions range between plus 0.5 and minus 0.5 degrees Celsius anomaly levels — or the sector between El Niño and La Niña.
There is still a ray of hope, as several of the numerical model runs are indicating a moderate El Niño developing this winter, which could mean a good chance of above-average rainfall this upcoming rain season. Jan Null, a former National Weather Service lead forecaster and PG&E meteorologist, is recognized as an expert on El Niños and La Niñas and their relationship to California’s weather.
According to Null’s studies, which can be viewed at ggweather.com, a moderate El Niño condition historically produces about 113 percent of average rainfall along the coast from San Francisco to San Luis Obispo County. Farther south, it generates approximately 124 percent of average precipitation along the coastline from northern Santa Barbara County toward San Diego.
So how do these above-normal seawater temperatures in an area so far away have such a profound effect upon California’s weather? The answer is in the winds, or should I say the upper-level winds. These warmer waters in the eastern Pacific produce a more considerable amount of evaporation. As this water vapor ascends into the atmosphere, it often condenses into thunderstorms and releases tremendous amounts of latent heat, which further decreases the atmospheric pressure. This area of low pressure, in turn, changes the path of the southern branch of the polar jet stream, pulling it farther southward toward the Central Coast.
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