LOS ANGELES — Monsoonal moisture streaming across Southern California triggered thunderstorms and flash flood watches and warnings again Wednesday while much of the northern half of the state baked in high heat.

Severe thunderstorms began popping up in the mountains of northern San Diego County and in adjacent Riverside County during the early afternoon as the atmosphere became increasingly unstable.

Flash flood watches issued for a vast swath of the Southern California interior were elevated to warnings in the Palomar Mountain and Temecula areas.

Meteorologists warned of winds up to 60 mph (96 kph), hail and potentially deadly lightning. Up to 2 inches (50 millimeters) of rain fell in an hour, the San Diego National Weather Service office tweeted.

It was the second day of severe weather in the region, which has sweltered in unusually high humidity.

On Tuesday, the threat of lightning forced evacuation of the shoreline in Huntington Beach where thousands of people were gathered for the Vans U.S. Open of Surfing, while inland areas were whipped by tree-snapping winds and downpours that unleashed torrents of water through neighborhood streets.

To the north, skies were clear but conditions were very hot under a strong ridge of high pressure.

Triple-digit temperatures brought excessive-heat warnings in the northern third of the state except for the immediate coast.

Elsewhere, heat advisories were issued for portions of the northern Sierra Nevada, Central Valley and southwestward to the Big Sur coast region, excluding San Francisco and Monterey bays.

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