San Diego is not known for very exciting weather in June, where a typical day begins with a layer of marine stratus clouds that burns off to hazy sunshine in the afternoon hours, with the clouds returning in the evening. Rain is a rarity during the summer months in Southern California.
This June, though, definitely went out with a bang. In connection with an early start to the North American monsoon, there had a persistent strong southeasterly flow aloft across the region for a few days, which can be a recipe for thunderstorms to form over the mountains. The moist flow contained so many middle and high clouds that there was not enough sunshine to provide the necessary heating. That changed on the night of Tuesday June 29, though, strong thunderstorms over old Mexico pushed an outflow westward. That outflow, in combination with relatively clear skies the next morning, was a perfect recipe for getting convection going that resulted in severe thunderstorms. The strong easterly winds aloft were able to bring the storms westward to the coastal sections where they normally don’t occur, so the highly populated western third of San Diego county was treated to a round of severe thunderstorms.
At my house we got 0.64″ of rain in 20 minutes, with an instantaneous peak rain rate of 6 inches per hour. The official recording station for San Diego was largely bypassed by the storms and only mustered 0.04” of rain, but that was still enough to set a record for the date, since in the previous 165 years of record keeping no measurable rain had ever fallen in San Diego on June 30!
I’ve included a photo I took of what appears to be a horizontal funnel cloud that formed near the convergence zone of the sea breeze and the easterly flow, taken about an hour before the thunderstorms got going in earnest. Such funnel clouds are not uncommon in growing cumulus clouds and aren’t much of a threat to people on the ground, but they do indicate vertical wind shear, which is a necessary ingredient for tornadoes to form. In this case no tornadoes were reported, but lightning started numerous fires and flash flooding occurred in a few areas, mostly though the storms just provided an exciting end to what is ordinarily the dullest weather month in San Diego.
James D. Means, Ph.D.