Now I Know: SoCal Thunderstorms

Southern California’s monsoon and thunderstorm season got underway last Monday, and if you were paying attention, you felt it. At dawn the sky above was dull and gray — the so-called June Gloom — but by 10:00 am the marine layer had been beaten back out to sea. High altitude high pressure air had slipped in from the deserts to the east, sending the cooling gloom away. Temperatures have risen steadily since and everyday thunderstorms form over the mountains that ring the Los Angeles basin (such as seen from the beach, here).

Thunderstorms require three things to form: moisture, air instability, and a lifting force.

Moisture can come from oceans or from remnants of hurricanes or other storms.

Air instability occurs when a cool dry pocket of air rests over a warmer, wetter pocket of air; given a sufficient nudge, the warm, moist air will rise.

Lifting force usually comes from heat, whether by temperature differential (uneven heating of the ground, which creates thermals) or by boundaries between pockets of air (warmer air will rise over cooler; the heat differential between drier and wetter air will force the wetter air up; and the outflow boundary, on the edge of a thunderstorm, is cooler than the surrounding air, creating more thunderstorms). Lifting force can also come from topography — wind moving up mountain slopes and through canyons will provide the push the moist air needs to get aloft.

Once aloft, the air rises and cools, condensing its moisture into water droplets, which form clouds. More rising air pushes some droplets still higher, where they continue to cool and grow. Some fall as rain. Some freeze and fall and melt back into rain. Some freeze and fall and stay frozen and fall to the ground as hail.

The high pressure that arrived on Monday gave the thunderstorms two of the three necessary ingredients: air instability and lifting force. Without the marine layer to keep us cool, temperatures began to rise (in Downtown LA it was 75 on Monday, 86 on Tuesday, then 94, then 92, and 93 today). While heat differentials promote thundercloud formation, our topography probably plays a larger role. The Los Angeles basin is ringed with mountains — the San Gabriels to the north, the San Bernardinos to the northeast, the Santa Anas to the southeast. Air moving onshore from the ocean rises up the slopes and through the canyons, pushing warmer, moister air up. Upslope thunderstorms are common in the Rocky Mountains and the Sierra Nevadas and they occur here, too. We also have several convergence zones — regions where breezes, after having diverged around mountains and through canyons, collide and force air upward. One such convergence zone is near Lake Elsinore (the Lake Elsinore Convergence Zone), on the eastern side of the Santa Ana Mountains, where this supercell thunderstorm and funnel cloud was videotaped.

For the third element — moisture — we return to the opening line of this post and that word “monsoon”, which seems out of place when discussing Southern California. The North American, or Arizona, or Southwest monsoon occurs when high pressure moves over the southwest from the south and intense summertime heating of the desert creates rising air and low pressure. The monsoon takes place between June and mid-September. The result is a change in direction of prevailing, low level air flow that brings moisture up from the Gulf of California and the Western Gulf of Mexico. This moisture is then forced up by heat differentials and upslope winds to form into water droplets and clouds and eventually thunderstorms.

Other than that, all I know about our monsoon and thunderstorms is that mid-September is a long way off.