The heat wave that is finally breaking in San Diego may be partly responsible for the rough tropical weather expected to hit the region on Friday.
National Weather Service forecasters say San Diego could still get drenched as Hurricane Kay begins to weaken and move west.
“The rain’s going to be widespread on Friday and Friday night,” National Weather Service meteorologist Alex Tardy said.
Rainfall amounts are unclear, but, because the storm is swirling in a counterclockwise pattern, more moisture will fall in the county’s desert region than in the coastal zone.
Flash flooding is a concern because a lot of rain will likely fall in a short amount of time.
Hurricane Kay is already pounding the central Baja California coast. Big waves are pummeling coastal communities, and rain is drenching the peninsula.
Wind is also buffeting the Mexican coast, and forecasters say it will get windy in San Diego, as well.
The tropical storm comes on the heels of one of the region’s longest and most intense heat waves.
“From Salt Lake, Sacramento to San Diego, Los Angeles and most of those areas in between, its top-three hottest for a seven-day stretch,” Tardy said. “In some cases, like Salt Lake and Sacramento, No. 1.”
The high-pressure area, or heat dome, allowed the hot weather to sit over the west, and also likely pulled the hurricane north toward San Diego.
“This requires this sort of harmony between the atmospheric circulation providing this northward flow along the western margin and then also a storm kind of getting caught up in that,” said Dan Cayan, a climate researcher at the Scripps Institution of Oceanography.
Cayan said it was not common for hurricanes to form south of San Diego and then track north along Mexico's coast. Storms typically drift west toward Hawaii.
He is tracking surface temperatures in the western Pacific and Western Atlantic, which are up 4 degrees Fahrenheit compared with the average temperatures of the rest of the oceans. The large pools of warmer water can have an impact on things such as currents and habitats — and they can also change how weather systems react to the ocean and atmosphere.
“We have not had that much storminess over these last several months, which allows the heat to accumulate at the surface,” Cayan said. "It doesn’t get stirred up, and consequently we get sea surface temperature anomalies that are extremely warm.”
Though the ocean is getting warmer in spots — and generally warmer overall — the Pacific waters near San Diego are still too cold to feed a hurricane. If a tropical storm were to move north, it would quickly lose strength as it hit the cooler waters, Cayan said.
The monsoonal weather pattern that brought humidity to San Diego during the heat wave is helping bring the storm’s fringe into the region.
County officials were handing out free sandbags at several locations and allowing residents to fill them.
Robert of Spring Valley picked up a dozen because he is worried that mud will flow from his sloping backyard onto his porch and into his house.
“We don’t have that moisture in the ground,” he said. “Rain's going to come, and it’s going to oversaturate, and things are going to slide. Things are going to get slippery and nasty.”
Mud will accumulate, he said, and flow into the door, if there are no sandbags to keep it at bay.
