LOS ANGELES - The Latest on heat, humidity and lightning in Southern California (all times local):

10:45 a.m.

Lifeguards have reopened all Los Angeles County beaches that were closed due to lightning.

The beaches were cleared for about an hour Tuesday morning after lighting was observed.

Southern California is hot and humid as monsoonal moisture flows into the region.

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10:20 a.m.

Lifeguards are clearing a long stretch of Los Angeles County beaches due to lightning as monsoonal moisture flows into Southern California.

The county Fire Department tweets that lightning was observed at midmorning Tuesday and beachgoers are urged to find shelter indoors until it passes.

Heat and humidity are building due to high pressure over the Four Corners region.

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7 a.m.

Southern California is hot and humid as high pressure centred near the Four Corners region maintains a flow of monsoonal moisture.

The National Weather Service says a heat advisory will be in effect in San Fernando Valley communities from late Tuesday morning until 9 p.m.

Highs will range from the 90s to 103 degrees (32.2 to 39.4 Celsius), and forecasters warn that the combination of the hot temperatures and humidity will make heat illnesses possible.

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The mountains will see scattered thunderstorms.

Elsewhere, forecasters say an excessive-heat watch will be in effect in the San Joaquin Valley and the west side hills Thursday afternoon through Sunday evening.