Lightning spotted off coast of Marin County as monsoonal moisture brings chance of thunderstorms to Bay Area

Monsoonal moisture from the desert Southwest pushed into Northern California overnight, bringing a chance for thunderstorms to the San Francisco Bay Area on Monday, the National Weather Service said.

"We have a pretty good stream of moisture coming into our area from down south, and we have a low pressure system off the coast so that's helping create some extra lift to maybe produce some thunderstorms," said Weather service forecaster Sarah McCorkle.

The weather service spotted a lightning strike just after 6 a.m. off the coast of Marin County. The storm cell weakened as it moved on land, bringing scattered light rain to the North Bay but no lightning strikes, McCorkle said.

Rainfall amounts in the North Bay ranged from 0.01 to 0.05 inch.

Light rainfall was also spotted in parts of the East Bay just before 7:30 a.m. and ABC 7 meteorologist Drew Truma reported rain falling over the San Francisco International Airport.

"We have the chance of thunderstorms across the Bay Area through the early morning," McCorkle said at 7 a.m.. "After 2 p.m., the chance for thunderstorms really drops off and everything moves out of our area."

She added: "On our radar right now I see some rain moving into our area in the next few hours. There's about a 15% chance we could get a lightning strike or two . There is a chance for some isolated showers reaching the ground."

The chance for rain drops off on Tuesday and seasonable temperatures with morning and evening fog are in the forecast through the week. Afternoon highs are expected to be in the 60s to 70s and in the 90s inland.

A monsoon refers to a seasonal reversal of wind patterns over a region. The summer monsoon is associated with an increase in thunderstorm activity; in North America, winds pull moisture from the Pacific Ocean and the Gulf of Mexico and transport it into the Southwest U.S.

The summer monsoon is a typical weather pattern that is most pronounced in the interior Southwest states of Arizona and New Mexico. Sometimes the moisture pushes northward into California.

When monsoon moisture flows into Northern California, lightning is usually more common in the Sierra Nevada than along the coast due to the mountains' lack of a marine layer and its topography. The air mass is forced to lift, intensifying the thunderstorms.