In southern California there seems to be a tornado maximum, or most favored single region for tornadogenisis near 29 Palms, which is a military base in southern San Bernardino County to the east of the San Bernardino mountain range. This is very likely maximized by local geography and synoptic/mesoscale features, with the Salton Sea and low desert region directly south of the area and a ridgeline coming up from SE to NW toward the San Bernardino mountains to the west. It seems that when other conditions line up and combined with SE low level and uninhibited inflow to storms that develop in this area, tornadoes do occur. I have spoken to people who have chased this region and witnessed the tornadoes here.
The more favorable patterns for tornadic activity in the southern California deserts associated with monsoonal surges have seemed to occur when there is substantial and deep low-level moisture, characterized by 60+ degree dewpoints and some semblance of enhanced shear, usually if the region is undergoing a transition-phase from SE to SW flow aloft, as by this time copious moisture has been able to become established and if a trough grazes the region from the west veering and increasing the upper flow storms have been known to become tornadic. This is a setup similar to the one found east of the Rockies where wind shear and instability is introduced. Additionally, convergence zones/wind shift boundaries/outflows can contribute to the tornado factor. It's very possible that many of the tornadoes associated with the monsoon storms are of the "landspout" variety, caused by intense instability, lapse rates, and vertical vorticity stretching, but there is also evidence suggesting supercellular storms with attendant mesocyclones do occur as well. The tornadoes of California, whether occurring during the cold-season upper level (coldcore) lows or the warm season monsoonal flows, have been recorded to be generally weaker than their Midwestern counterparts, with the F-0 to F-1 variety dominant, and the occasional tornadoes up to F-2 strength. However, there are conflicting sources online that suggest there may have been 2 F-3 tornadoes recorded in California, depending on the site that lists the stats the same tornadoes are listed F-2s on other sources. One of them occurred in Orange County back in February 1978, and the other occurring in Riverside County on August 16, 1973. The latter would have been during either a monsoonal or tropical moisture surge. The area that it is recorded to have occurred in is in a sparsely populated region and for it to have picked up such a rating means that it must have hit something that would equate to F-3 damage. I just went back and looked at the archived upper-air charts via the Plymouth State Weather Center for 8/16/73 and the mid-upper flow does not stick out as anything that one would write home about. If anything, it's a very weak flow environment. I wonder if there is any more information about this event anywhere.
Another important thing to remember is that these desert regions are very sparsely populated, with less-than-ideal road networks and a SIGNIFICANTLY fewer quantity of weather spotters than places in the Plains and Midwest have. As such and in all likelihood there may have been significantly more tornadoes that have occurred than appear in the offical record books.
