Thunderstorms over Bay Area’s Diablo Range mountains possible as moist air continues to circulate

What many in the Bay Area may have hoped would be a brief tangle with steamy, muggy air has lingered longer than  National Weather Service forecasters expected.

Now residents and meteorologists are bracing for the moisture-filled air to deliver its most potent weather so far — in the form of lightning and thunder.

The weather service issued a severe weather outlook for Thursday afternoon in Bay Area mountains along the Diablo Range. Thunderstorms will be possible through the early evening and could bring areas of heavy rainfall, small hail, lightning and rural flooding.

The area covers the mountain range from Coalinga to Santa Rosa.

“We’ve got that lingering upper-level moisture, and we have that daytime heating,” NWS meteorologist Matt Mehle said. “That’s what is allowing for the thunderstorms to develop.”

It also was expected to bring muggy conditions into Thursday night, he said. The temperature Thursday in Brentwood was expected to peak at 98, while Livermore was expected to hit 93 degrees. Morgan Hill and Walnut Creek were expected to reach 89 degrees, San Jose was forecast for 83 and Oakland for 73.

The upper level moisture is left over from the disturbance caused by the development and quick stall of tropical storm Eugene last week. Mehle said the disturbance in that weather pattern has caused the upper level of the atmosphere to continue to collect moisture from systems near New Mexico and Arizona.

“They are waves of moisture,” Mehle said, comparing it to an ocean wave, only in the atmosphere. “With each wave comes more moisture.”

More moisture also is on the way, he said. Meteorologists are tracking Hurricane Hilary, which began as a tropical storm but reached hurricane status early Thursday. The weather service is currently tracking the Southern Calfornia-bound hurricane on a path that will have it come close to San Diego by sometime late in the weekend.

“If its track stays the same, the question is, how much moisture will our weather pattern here in the Bay Area get out of that initial surge?” Mehle said. “We’ll be influenced by the western edge of it.”

In the meantime, Mehle said the persistent moisture in the upper level will begin to move east by the start of the weekend, and the weather should be closer to the Bay Area norm until any influence from Hurricane Hilary arrives.