The beastly storm that caused dangerous flooding around San Diego County Monday was an anomaly that only became fully apparent to forecasters shortly before it struck the region.
Most winter storms originate in the northwest Pacific and spread ashore in California as they drop south toward San Diego. They can siphon moisture from the subtropics along the way, making them stronger. But San Diego typically gets the weakest piece of the storm.
These systems also usually hit San Diego at an angle, amounting to little more than a glancing blow in many cases.
Monday’s storm was different. It originated in the western Pacific and headed directly for northern Baja California and San Diego County. Along the way, the system drew extraordinary amounts of moisture from the subtropics.
“The columns of air coming at us had 250 percent to 350 percent as much water in them as they normally do,” said Liz Adams, a forecaster at the National Weather Service. “That turned into rain.”
The rain ended up falling faster than normal because part of the storm hit San Clemente Island off the coast of San Diego County. The clouds rode up the far side of the island, causing the moisture to condense into rain as it flowed toward the mainland.
By the time the clouds reached the coast, they were unleashing 1 inch of rain or more per hour in many places, notably National City, San Diego, and Point Loma. Rain rarely falls with that intensity in San Diego County.
The downpours were especially strong in Mission Valley, causing the normally weak lower San Diego River to surge rapidly to life and exceed flood stage. People living or moving about in the river bed had to run for their lives or, in many causes, be rescued.
The downpours also overwhelmed city drainage systems, causing street flooding throughout the region. Much of the rain flowed to the coast, flooding parking lots and seeping into buildings.
“The rain was heavier than what we got last August when Tropical Storm Hilary came through,” Adams said.
So why didn’t the National Weather Service fully anticipate what was about to happen?
The problem, Adams said, largely involves forecast models. They’re quite sophisticated. But they don’t always correctly predict the timing and path of a storm, which is what happened here. Forecasters expected most of the storm to go into Baja California.
To a degree, San Diego forecasters also are blind to the west. There are comparatively few weather stations at sea to help them track a system’s size and movement. That isn’t true in cities far from the coast, which can tap dozens and dozens of ground stations that track a storm’s every movement.
