Monsoonal ‘surge’ pushes moisture levels to 200% of normal in Bay Area

The San Francisco Bay Area is getting a humid start to the workweek thanks to a weather pattern hundreds of miles away that’s driving an influx of sub-tropical moisture.

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In recent days, an extremely strong ridge of high pressure has been building above the Rocky Mountains, causing record-high temperatures in the area. That high pressure is funneling monsoonal moisture from the Gulf of California northward into the state and has increased the risk of dry lightning as well as heavy thunderstorms during the normally dry summer months across parts of the state.

“The monsoonal moisture surge is about 150, 200% of normal for moisture for this time of year,” Rachel Kennedy, a meteorologist at the National Weather Service, said of Bay Area weather. “It’s contributing to that increased humidity.” 

Kennedy said people in the normally cool and dryer Bay Area will start to feel the change on Monday with the sticky humid weather expected to last through Tuesday.

“The more likely thing people will notice is just it’s going to feel humid,” she said. “That might contribute to feeling a bit warmer than it normally would.”

There is a chance of thunderstorms in the area, though Kennedy said we’re unlikely to see downpours in the Bay Area.

“We’re not really expecting to see much rain. It would evaporate before it reaches the surface,” she said. “That being said, you might see a few sprinkles at the surface.”

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The low chance of wetting rain means if storms do develop, there’s a risk of dry lightning that could spark a wildfire. 

“We are really dry this time of year and our fuels are pretty much dried out,” she said. “This is a low probability but high risk scenario.”

There is one visible benefit to the increased moisture levels: colorful sunrises and sunsets.

“Even if it’s overcast, there is enough room underneath the clouds (about 10,000 feet) for the setting sun to illuminate the bottom of these altostratus and altocumulus clouds and create the beautiful sunsets,” the weather service forecast discussion said.