A satellite view of the moody river that brings heavy rains to California
Satellite images Wednesday noon show an intense low-pressure system rotating counterclockwise off the coast of Northern California. As it spins, it attracts a robust plume of moisture from the Pacific Ocean called atmospheric flux.
Atmospheric fluxes are concentrated streams of water vapor about 100 to 250 miles wide in the middle and lower layers of the atmosphere. They can move water at a rate equivalent to 25 Mississippi rivers or 2.5 Amazon rivers per second, according to Marty Ralph, an expert at the Scripps Institution of Oceanography in San Diego.
They are like a continuous channel of moisture flowing non-stop across the ocean. When this moisture-bloated atmospheric current hits the mountainous coastal landscape of California, it is propelled up and over the higher ground. This is called an orographic lift.
(Paul Duginski/Los Angeles Times)
The moist plume cools as it gains altitude, and the moisture condenses and falls as precipitation. Atmospheric flux in this satellite image will drop south toward Los Angeles on Thursday.
Rainfall rates are expected to peak between 0.5 and 1.0 inches per hour in most areas. However, there is a possibility these rates could reach as high as 1.25 inches per hour – which would increase the risk of mud and debris flows in recent fire areas.
“This is a dangerous weather system, a very significant flooding situation that is developing later tonight and Thursday morning,” said National Weather Service meteorologist Eric Boldt in Oxnard.
After the heaviest rains, there is also a low risk of thunderstorms.
“These could create their own very brief heavy downpours, but these cells will be fast-moving and short-lived,” Boldt said. “So the risk of flooding is still there, but we will be monitoring this closely for possible heavy rain and gusty winds.”
Although strong atmospheric flows can cause extreme rainfall with catastrophic flooding and mudslides, many are weak, providing beneficial rain to drought-stricken California.
According to the National Weather Service, on average, about 30 to 50% of the West Coast’s annual precipitation comes from just a handful of atmospheric rivers.
Mountain slopes that face the ocean and are positioned as obstacles to oncoming atmospheric fluxes tend to receive the heaviest rainfall, while some areas such as San Jose and parts of the Salinas Valley, for example, lie in the rain shadow of these mountains and receive less rainfall as a result.
An atmospheric river meeting the Central Coast will continue through the Central Valley and climb the western slope of the Sierra Nevada. These peaks are so high and give such a workout to atmospheric flow that almost all residual moisture is wrung out of them, suffocating the mountains in a blanket of snow due to the cold at high altitudes. But the Great Basin of Nevada beyond the Sierra to the east lies mostly in a gigantic rain shadow.
A look at the map shows that California’s deserts are a product of mountain ranges, including the Transverse Ranges to the south. They are starving inland areas of the moist flow from the ocean.
The mountain snowpacks serve as the state’s water bank in good conditions, and the snow gradually melts during the warm months, replenishing streams and reservoirs.
Sometimes the humidity is tropical, originating near the Hawaiian Islands. Because of their origin, such systems are getting warmer, resulting in higher snow depths.
An atmospheric river like this is popularly known as the “Pineapple Express”.
“All Pineapple Expresses are atmospheric fluxes, but not all atmospheric fluxes are Pineapple Expresses,” said Drew Peterson, meteorologist for the National Weather Service in Monterey.
Times contributor Luke Money contributed to this report.