Temperatures are climbing back into the triple digits in desert communities and humidity is one the rise across Southern California.
The significant warming trend is due to an upper-level high-pressure system strengthening over Northern California and Nevada combined with a weak northerly wind component at the lower levels. Widespread triple-digit heat is expected across the lower mountains, Antelope Valley and interior valleys.
"It will be above-average heat for us all throughout the week," said NBC4 forecaster Shanna Mendiola.
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By Tuesday, an increase in monsoonal moisture will potentially lower temperatures a few degrees across Los Angeles and Ventura counties, but the increased humidity will lead to uncomfortable conditions.
An excessive heat warning issued by the National Weather Service will be in effect until 10 p.m. in the Antelope Valley, where highs will range between 106 and 109 degrees. A slightly less serious heat advisory will be in effect from 10 a.m. to Tuesday night in the San Gabriel Mountains and the Santa Clarita Valley amid highs ranging from 95 to 106.
People who work outdoors were urged to save strenuous activities for the early morning or evening, wear light-colored, loose-fitting clothing, and drink plenty of water. Temperatures inside vehicles, even if the windows are partially open, can quickly rise to life-threatening levels.
The NWS forecast a mixture of partly cloudy and sunny skies today in L.A. County, along with highs of 77 at LAX; 80 in Avalon; 84 in Long Beach; 85 in downtown L.A.; 90 in San Gabriel; 92 in Pasadena and Burbank; 95 on Mount Wilson; 100 in Woodland Hills; 105 in Saugus; 107 in Palmdale; and 108 in Lancaster.
Sunny skies were forecast in Orange County, along with highs of 70 in Laguna Beach; 73 in San Clemente; 76 in Newport Beach; 85 in Irvine; 86 in Anaheim and Mission Viejo; 89 in Fullerton; and 90 in Yorba Linda.
Temperatures will stay the same in some communities Tuesday, climb in others by up to 4 degrees and decline by a few degrees in some areas, including Lancaster, where a high temperature record for a July 30 was matched Sunday, when temperatures reached 106 degrees, the same as in 2000.