September Heat Wave Settles In for Rest of Week

Temperatures and humidity levels will rise to uncomfortable levels in Southern California Tuesday at the start of a heat wave expected to last until early next week.

Excessive heat warnings and heat advisories will go into effect for some inland communities. An excessive heat warning for the San Gabriel and Santa Monica mountains in both Los Angeles and Ventura counties will be in effect from 11 a.m. until 8 p.m. Thursday.

Highs around 105 degrees are expected at low elevations.

"It's going to heat up very fast as we head into the afternoon," said NBC4 forecaster Crystal Egger.

A National Weather Service heat advisory is set to take effect at noon and extend until 6 p.m. Thursday in the valleys around Riverside. The NWS forecast highs today in the mid 90s in metropolitan Los Angeles and the low 100s in the San Gabriel, San Fernando, Santa Clarita and Antelope valleys as well as inland Orange County.

Temperatures Tuesday and during the rest of the heat wave will be running 10 to 15 degrees higher than normal. The high heat is being attributed to high pressure developing over the region, combined with the fact that only weak winds are blowing in from the ocean.

Humidity levels are expected to be at the 50 percent level in the coastal plains -- the norm is 20-30 percent -- and around 40 percent in valley areas, where percentages in the teens would be considered normal. The humidity is attributed to lingering moisture from Hurricane Linda.

A flash flood watch for the mountains, the San Gorgonio Pass and the Coachella Valley will also extend from noon through Thursday evening.

Along the coast, high surf is expected through Tuesday night as a result of a long-period swell in the Pacific. The highest surf -- 3 to 6 feet, with 7-foot sets -- will be on south- and southwest-facing beaches. Strong and frequent rip currents will continue to be a hazard.

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