La Niña suggests a dry stretch for Central Coast, but here’s what could change that
The plentiful rain and snow of December decreased the drought severity throughout California. This week’s U.S. Drought Monitor showed the entire Central Coast to be in moderate drought after being classified as severe last week.
Unfortunately, the Eastern Pacific High off the California coastline has locked the storm door shut to Central California. It has forced the upper-level winds and the storms they bring into Northern California, Oregon, and Washington in a classic La Niña pattern.
In fact, much like November when our area was parched, western Washington and parts of Oregon have once again reported record amounts of rain and snow that have caused floods and landslides over the past week as a series of tempests slammed into the Pacific Northwest.
Along the Central Coast, we are on the cusp of another winter dry spell.
These arid winter periods are common, even during the wettest years. A late December and early January dry spell often produce an interesting switch in the weather pattern between the beaches and the coastal and inland valleys.
The interior valleys are frequently hot and dry in the summer months, while the beaches are socked in with fog and mist. It is not uncommon to see beach temperatures more than 50 degrees cooler than the inland valleys during the afternoon.
However, the opposite can occur at this time of the year. As the Santa Lucia (northeasterly) winds blow through the coastal canyons and pass toward the beaches, they dry and warm the airmass, leaving behind clear skies. On the other hand, the inland valleys are often sheltered from these offshore winds, and the force of gravity carries the cooler and denser/heavier airmass to the valley’s floor.
As the overnight temperatures decrease, water vapor condenses onto microscopic particles in the air, and the valleys begin to take on a grayish tint. When the air has reached its dew-point temperature, the relative humidity is at 100%, and water droplets become visible to the naked eye as fog and mist. As the fog layer deepens, it starts to creep toward the ocean but never makes it. A terrible cousin of coastal fog, tule fog, often develops in the San Joaquin Valley and can persist for days.
Along the Central Coast, it appears that what I call the “ridiculously resilient ridge of high pressure,” made famous by the previous drought, will continue. Historically, La Niña conditions typically produce lower-than-average winter rainfall.
However, the Paso Robles Airport has seen 7.2 inches about 179% of normal at the end of December. Cal Poly, the home of climatology for San Luis Obispo, has seen 12.3 inches of rain or about 170% of average.
With that said, there have been La Niña years that have seen well-above-average rainfall for unanticipated and quickly changing atmospheric conditions. One could be developing as a write this column, and here is why:
The Madden-Julian Oscillation (MJO) — unlike El Niño and La Niña, which stay anchored in the same geographic area — 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. There is growing evidence that what happens in the tropics can influence our weather along the Central Coast.
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 very strong 1982-83 El Niño event, which led researchers to believe that the pattern could enhance the amount of rain in California. The MJO is 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 that the dynamical models exhibit a large spread regarding the evolution of the MJO over the next two weeks, with a meandering intraseasonal signal depicted in the GEFS and ECMWF ensembles. In other words, the forecast remains uncertain.
A split in the polar jet stream can develop if it does 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 days of rain.
With that said, only time will tell if the MJO eventually propagates across the International Date Line and helps to direct precipitation to the Central Coast.
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