California to Be Impacted by Category 4 Atmospheric River, Heavy Rain Expected

A weather system moves by the Golden Gate Bridge as seen from Alcatraz Island in San Francisco, on Aug. 17, 2020. Justin Sullivan/Getty Images

An atmospheric river—a weather system that moves a high concentration of water vapor outside of the tropics—is expected to bring rainfall to certain regions in California this week, with temperature drops expected in major cities like Los Angeles.

A winter storm system had already hit places like the Bay Area on Monday, bringing with it a Category 4 atmospheric river. By Tuesday, the river could bring up to one to three inches of rain to coastal regions and three to five inches of rain to higher elevations, the National Weather Service (NWS) told CBS San Francisco. The weather service expects the Monday–Tuesday storm to kick off a wet week.

“Like the Jelly of the Month Club, a stout jet across the entirety of the Pacific Basin will be the gift that keeps on giving as it delivers additional waves of moisture across the West Coast,” the NWS said.

“Potentially up to three additional waves of moisture will be possible before the New Year with systems possible again on Thursday, Friday, and Saturday and additional waves arriving through the first week of January.”

Though the NWS has not determined the timing of the upcoming storms, it estimates that they would likely be “cooler and wetter” than the storm on Tuesday.

The agency also expects the next “upper-level trough” moving in around Dec. 30, which will provide the “next healthy dose” of rain for the Bay Area region.

Weather This Week

The NWS is predicting temperatures to drop in Southern California as a storm system sweeps through the region on Wednesday. For Tuesday, 15–20 degrees of cooling is predicted.

A low-level pressure system is expected to pull a plume of moist air over California on Tuesday and early Wednesday. Santa Barbara and San Luis Obispo counties are predicted to receive one and a half to three inches of rainfall on Tuesday, with places like the foothills of Santa Lucias getting up to five inches of rain.

In downtown Los Angeles, the temperature is expected to drop from the 70s on Monday to 60s on Tuesday and Wednesday and finally to the upper 50s on Thursday and Friday.

“The storm door will remain open through the rest of the week with a chance of rain every day,” the NWS said, according to KTLA. “Showers may continue at times through Friday, with another significant storm possible this weekend.”

Beneficial Atmospheric River

Though atmospheric rivers can wreak havoc, they are critical to California because these rivers are the source of water supply. Not only do the atmospheric rivers fill up water reservoirs but they also build up snowpack, which steadily supplies California with water.

According to an estimate, around 30–40 percent of droughts in California were ended by atmospheric rivers. Capturing and storing water from atmospheric rivers has become increasingly crucial in the state as it experiences alternating dry and wet conditions.

During Water Year 2020, the water supply at Lake Mendocino rose by almost 20 percent thanks to utilizing weather forecasts to determine storms.

However, it is also important that forecasters are reliably accurate in predicting where the atmospheric rivers are going to make landfall, failing which it can even end up creating a catastrophe.

In an interview with the San Francisco Chronicle, flight meteorologist Lt. Col. Ryan Rickert pointed out that if those who manage water resources do not know that the atmospheric river “is going to hit smack dab in the reservoir, and then all of a sudden it does and they didn’t let any water out, now you’re talking about dams possibly breaking and all kinds of other huge impacts to people.”