California is preparing for major weather changes: rain, wind, fire weather

It’s been almost two weeks since San Francisco has had any measurable rainfall, but that will change by the end of this week.

A stubborn ridge of high pressure that stubbornly persisted and contributed to persistently unhealthy air quality is finally disappearing. As this system breaks down and the weather becomes more active, there will be rapid changes in the Bay Area.

Gusty offshore winds are expected to begin the week, followed by a strong storm system that will bring widespread rain through the end of the week.

On Monday, a low pressure area will move quickly from the Pacific Northwest into the interior of the western United States. In its wake, a new high pressure area will form by Tuesday. This rapid transition from low to high pressure will create an appropriate pressure gradient and set the stage for the development of offshore winds across the state.

Northeast winds with gusts of 20 to 30 mph are expected to develop in the Bay Area Monday afternoon, particularly at higher elevations in the North Bay, East Bay and Santa Cruz mountains. Fortunately, the region received several inches of rain just a few weeks ago, easing fire weather concerns. Southern California, however, won’t be so lucky.

Southern California largely missed the generous rains that recently flooded much of the state. The low rainfall was not enough to completely eliminate the fire risk.

As high pressure builds over the interior Monday and into Tuesday, strong Santa Ana winds will intensify in the mountains surrounding Los Angeles and San Diego. Warnings are in effect for both regions and critical fire weather conditions are expected on Tuesday. The more exposed areas may experience wind gusts of up to 70 miles per hour.

This time the high pressure ridge will not last long. A weak low pressure system will move into the Bay Area beginning late Wednesday, increasing the chance of light rain or drizzle through Thursday. But that’s just the beginning.

A much stronger storm system will arrive on Friday, expected to bring a good drenching to the region and heavy snow in the Sierra.

San Francisco: Morning haze will persist at low levels but should disappear completely by the afternoon, leaving mostly sunny skies. Temperatures will range from the low 60s at Ocean Beach to the mid 60s downtown, with northeast winds of 10 to 20 mph. Expect mostly clear conditions overnight with persistent gusty winds and temperatures dropping into the 40s.

North Bay: The morning haze in the valleys quickly clears and gives way to a mostly sunny sky. By the afternoon, northeasterly winds will increase with gusts between 15 and 25 mph. Highs will range from the low 60s in Napa and Sonoma to the mid to upper 60s in San Rafael and Santa Rosa. Overnight the sky will remain mostly clear and temperatures will drop to 30 to 40 degrees.

East Bay: Areas west of the Berkeley Hills may experience some haze in the morning, clearing in the afternoon. Mostly sunny skies will prevail, with temperatures reaching the low to mid 60s in Oakland, Hayward and Livermore. Northeast winds will be gusty, particularly at higher elevations, reaching speeds of 15 to 30 miles per hour. Conditions will remain mostly clear overnight with winds gradually easing and low temperatures in the upper 30s to mid 40s.

Pacific coast and peninsula: Morning haze and low clouds are expected along the coast and peninsula, with clearing expected first on the peninsula and partial clearing along the immediate coast later in the day. Highs will range from the low 60s in Half Moon Bay and Pacifica to the mid 60s in South San Francisco and Redwood City. Northeast winds will blow at speeds of 10 to 25 miles per hour. Skies will remain mostly clear overnight except along the coast, with lows below 40°C.

Santa Cruz and South Bay: Morning haze and low clouds will persist near the Santa Cruz coast, dissipating by late morning. Winds will remain light and temperatures will rise to near 70 degrees in Santa Cruz and the Santa Clara Valley into the upper 60s. Expect mostly clear skies overnight with lows in the lower to mid 40s.

Reach Greg Porter: [email protected]