An atmospheric river taking aim for California was expected to bring heavy rains to an already-drenched San Diego County Thursday, creating the possibility for flooding, downed trees and big waves for the first week of the New Year.
The atmospheric river, which is a term used to describe heavier-than-normal rains that pull moisture from the tropics, inundated Northern and Central California on Wednesday, prompting evacuations, causing power outages and other damage. The storm was expected to bring potentially-damaging showers to San Diego County in the early-morning hours Thursday then again from around 10 a.m. to noon before letting up.
San Diego Weather Forecast
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The system was expected to weaken by the time it hit San Diego County, but will still bring downpours across the county through Thursday afternoon. This storm alone could bring up to an inch of rain to parts of San Diego County.
"We're kind of getting the tail end of this. This system really focused on Northern California moving into Central California, portions of LA County and this is what we have in San Diego County to start out on this Thursday," NBC 7 weathercaster Brooke Martell said.
By 1 p.m., coastal areas had seen about a quarter-inch of rain, while some inland valley areas saw anywhere from a quarter to three-quarters of an inch. Palomar Mountain received more than an inch of rain while Julian and Mount Laguna saw about a half inch. See more rainfall totals here.
Local
The storm left roads saturated during the morning commute. At least one person was injured when a vehicle went off westbound SR-78 in Vista and overturned into a ditch around 9 a.m.
An SDPD officer and an MTS trolley collided in downtown San Diego, but it was not clear if the weather was a factor.
The rain was expected to saturate most of the coast, inland and mountain communities until about noon, Martell said.
Along with the rain comes gusty winds, particularly along the coastline where winds will be in the 30 to 35 mph range. A wind advisory is in effect until noon Thursday for the coast, mountains and valleys of San Diego.
The mountains will see some snow from this system, but only at the highest elevations. Peaks around 6,000 feet will get a dusting but it's elevations above 7,000 feet that could see anywhere from 6 to 16 inches of fresh powder.
Rain will move away from the region Thursday night with a dry weekend ahead. Don't get too used to the sun, though; another storm is expected early next work-week, forecasters say.
Once the storm is gone, expect the surf to elevate. Rough waters were already reaching San Diego County beaches Thursday morning, prompting San Diego lifeguards to close the Ocean Beach pier to foot traffic. Even bigger waves of about 10 to 16 feet are expected to arrive late Thursday with the biggest swells expected Friday morning. A high surf warning is in effect from Thursday afternoon through Friday.
That storm surge could also bring flooding to coastal areas, especially Friday morning, prompting the National Weather Service to issue a coastal flood advisory.
Preparing for Potential Storm Damage
The state predicts the third in a series of atmospheric river storms could be damaging. Trees are already stressed due to three years with limited precipitation. Now, suddenly saturated ground and heavy wind mean they're more likely to fall over, possibly knocking down power lines or creating flood hazards, said Karla Nemeth, director of the state’s Department of Water Resources.
Gov. Gavin Newsom on Wednesday declared an emergency declaration in anticipation of any potential damage. The declaration allows for the state to utilize the National Guard's disaster response, ask the Federal Highway Administration for assistance to repair roads and releases funding for other emergency response needs.
“We anticipate that this may be one of the most challenging and impactful series of storms to touch down in California in the last five years,” said Nancy Ward, the new director of the California Governor’s Office of Emergency Services.
While the north was expected to get the brunt of the system, potentially-damaging showers were still expected in San Diego County starting in the overnight hours Thursday.
The San Diego County Department of Public Works had crews out Wednesday to clear culverts and roads from debris that could potentially create floods. For days, workers with plows tended to snowy roadways and sanded icy spots. The team also responded to any fallen trees or rock and mudslides from previous storms.
The county urged residents to prepare their own homes for the storm by clearing debris that could cause flooding and placing sandbags where necessary (the county and city of San Diego provide free sandbags to residents). Drivers should avoid areas of standing water and flooded roadways.
Even after the storm, flooding is possible along the coast as the beaches see waves up to 16 feet. A coastal flood advisory is in effect through Friday.
Staff was preparing for potential flooding from a storm surge. Del Mar Lifeguard Chief Jon Edelbrock said the winds, waves and tides could create the perfect combination for damage to the already-fragile bluffs. A team, including members of the Scripps Institution of Oceanography, will be monitoring the area using a radar system during the storm.
"It's been a number of years since we've had as significant as a surf advisory. I believe it's been five or 6 years, like 2017. The last time we saw kind of a series of swells coming through with this type of size and energy coupled the rain and the coincidence with our spring high tides," Edelbrock said.
What is an atmospheric river?
Three recent storms hitting California this week are considered atmospheric rivers, which is a long narrow band of condensed water vapor carried from the tropical regions near the Earth's equator towards the poles that can produce heavy rain and snow.
They're invisible to the naked eye -- the water is moved over the ocean in the form of water vapor, not a "river" in the way we think of them on land. They tend to move through the atmosphere in streams between 250 and 375 miles wide.
Temperatures at the equator tend to run hotter, and just like in grade-school science lessons, hotter temperatures cause water to evaporate into the atmosphere. Warmer air can also hold more water vapor. When an atmospheric river moves inland and hits mountains, the water vapor rises and cools, producing heavy precipitation.
The strongest atmospheric rivers can move anywhere between seven and 25 times as much water as the flow of the Mississippi River, which is the second longest river in North America and has a watershed that reaches 32 states, according to the national park service.
According to the National Oceanic and Atmospheric Administration, atmospheric rivers "are a primary feature in the entire global water supply and flood risks, particularly in the western U.S."
Atmospheric rivers contribute up to 50% of California's annual precipitation and drive about 84% of flood damage in the western U.S. While recent heavy rains have brought a degree of relief to the drought-imperiled California, scientists warn that the storms won't immediately alleviate severe deficits in the state's groundwater and soil moisture.