Thunderstorms are favored when the center of the western North American summer 500-mb high pressure cell moves north of the U.S.-Mexico border, resulting in moist east to southeast flow into southern California and northern Baja California Norte in the 700-500 mb layer. This coincides frequently with instability over the mountains and deserts and occasionally with convergence boundaries between the northwestward-moving Gulf of California moisture surges and eastward-moving Pacific Ocean sea breezes. Moisture from decaying tropical systems over the East Pacific will typically enhance the convection about twice per year, usually in late summer. The favored season for southern California and northern Baja California convection is early July through mid September.
The National Weather Service Forecast Office in San Diego (WFO SGX) is creating a "lightning MOS" to improve forecasts of convection over the mountains for the GFS and WRF models. The goal is to have probabilities of lightning (and frequent lightning) generated locally with each model run for each forecast zone in the WFO SGX County Warning and Forecast Area (CWFA) plus extreme northern Baja California. Initial results show that the variables that correlate best with summer lightning are 700 and 600 mb relative humidity, convective available potential energy (CAPE) over the deserts and wind direction and velocity at 700, 600 and 500 mb.