The Eastern Pacific High was anchored off the California coastline and combined with transitory high-pressure systems over the Great Basin — the area between the Sierra Nevada range to the west and the Rocky Mountains to the east — created persistent Santa Lucia northeasterly (offshore) winds. These winds brought in a cold air mass that created frosty mornings and bone-dry weather after Christmas to the Central Coast.
Then a significant change occurred in the first part of 2019; the Eastern Pacific High weakened and shifted, allowing the upper-level winds (jet stream) and the storms it carries and nourishes to travel at high speeds across the Pacific from west to east toward the Golden State. Currently, these upper-level winds are about 150 mph at 30,000 feet. With tailwinds like this, a passenger jet flying from Japan to California would see ground speeds (how fast the aircraft is moving over land or ocean) faster than the speed of sound.
Many of the weather models are predicting a cumulative rainfall total ranging between 4 and 8 inches from Saturday through next Monday (Jan. 14) for most of the Central Coast — with rain or showers occurring just about every day. Not only will we receive significant rain but heavy snow in the Sierra Nevada along with periods of gale-force southerly winds and high sea swells along the coastline.
So why did the Eastern Pacific High weaken and open the storm door to an impressive series of storms that will march across the Pacific into California through mid-January, if not longer?
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At this time, nobody knows for sure, but it could be the following oceanographic, and atmospheric, phenomena. Here is why.
The first possible cause is El Niño (warmer than average sea surface temperatures in eastern Equatorial Pacific). 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.
However, despite the fortunetelling from the warmer-than-average sea surface temperatures (SST) in the central equatorial region of the Pacific Ocean, “the patterns of convection and winds are mostly near average over the tropical Pacific,” according to NOAA’s Climate Prediction Center.
In other words, the atmosphere is not yet responded to the warmer-than-average temperatures such as trade winds weakening. Their most current report dated Dec. 31 states that “El Niño is expected to form and continue through the Northern Hemisphere winter 2018-19.
Another conceivable reason is the Madden-Julian Oscillation (MJO).
Unlike El Niño and La Niña, both of which are a standing pattern (in other words, they stay fixed in the same geographic area) the MJO is a large traveling pattern of increased rainfall and thunderstorm activity that propagates eastward at approximately 8 to 18 mph across the tropical parts of the Indian and Pacific oceans.
In 1971, Roland Madden and Paul Julian stumbled upon the pattern when analyzing wind anomalies in the tropical Pacific. However, little attention was paid to the oscillation until the strong 1982-83 El Niño event, which led researchers to believe that the pattern may have enhanced the amount of rain in California. The MJO is also called the “30- to 60-day oscillation” and the “30- to 60-day wave.” The latest MJO forecast from the Climate Prediction Center in Silver Spring, Maryland, indicates an oscillation is present in the western Pacific but its forecast remains uncertain.
There is growing evidence that what happens in the tropics can influence our weather along the Central Coast. Models suggest the oscillation will travel eastward. Historically, especially during El Niño phases, as the Madden-Julian Oscillation moves across the Pacific Ocean, a split in the polar jet stream can develop. The southern branch of the polar jet stream can extend far out over the Pacific toward the coast of California. That river of air in the upper-atmosphere can steer moist, subtropical air toward California and can bring several days of rain.
Another cause for above-average rain is another great oceanographic cycle that can orchestrate changes in our weather: a longer lasting cycle called the Pacific Decadal Oscillation or PDO.
While the ENSO phase typically lasts from eight to 13 months, the PDO alternates between a warm phase (positive) and a cooler (negative) phase that can last a few years to decades.
Unlike El Niño, which focuses on SST in the central equatorial region of the Pacific Ocean, the PDO is classified by seawater temperatures throughout the northern Pacific Ocean. According to Josh Willis, oceanographer and climate scientist at NASA’s Jet Propulsion Laboratory in Pasadena, the PDO shifted to the positive phase. Historically, the positive phase of the PDO typically enhances the effects of El Niño events.
Last, but perhaps not least, is the Arctic vortex that could split into three sections creating a blocking high in the Gulf of Alaska that may continue to help steer storms to California and the potential for frigid weather along the East Coast.
All of these or just one or two of these coupled conditions between the ocean and atmosphere may help to produce above-average rainfall for California this year. However, when trying to predict that far into the future, there are no guarantees.