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LOS ANGELES (FOX 11 / CNS) - A dangerous level of heat will persist Friday across much of the Southland, then the weeklong heat wave will come to an end -- sort of, forecasters said.

With triple-digit highs forecast in several communities, an excessive heat warning now in its 5th day is scheduled to be in effect until 8 tonight in the San Fernando, Santa Clarita and San Gabriel valleys and both the San Gabriel and Santa Monica mountains, although National Weather Service Dave Bruno said he expects the warning in those areas to be converted to a less serious heat advisory before then, possibly as early as this morning.

In the Antelope Valley, an excessive heat warning is expected to be in force through Saturday.

No heat advisory of any kind has been in effect since yesterday morning in the L.A. coastal zone -- the coast, beach cities, metropolitan Los Angeles, Downtown L.A. and the Hollywood Hills -- thanks to a developing marine layer.

In most communities, the heat wave will be considered over after today, in part because overnight temperatures will decline, becoming more bearable, but daytime temperatures will remain 4-8 degrees above normal through the middle of next week, Bruno said.

The prevailing conditions create an ``increased potential for serious heat-related illnesses, especially for the young and elderly, those performing outdoor activities, as well as those without access to air conditioning,'' warned an NWS statement that also cited ``increased potential for power outages.''

The statement urged people who work outdoors to wear lightweight, loose-fitting clothing and drink plenty of water. It added:

``Never, ever, leave people or pets in enclosed vehicles, even for a short period of time'' because vehicle interiors can quickly become deadly hot.

The NWS mostly forecast partly skies in L.A. County today and highs of 78 degrees in Avalon; 79 at LAX; 83 in Long Beach; 92 on Mount Wilson; 93 in San Gabriel; 94 in Burbank and Pasadena; 102 in Saugus and Woodland Hills; and 108 in Palmdale and Lancaster. Roughly the same temperatures will prevail through at least Thursday.

A combination of sunny and partly cloudy skies were forecast in Orange County, along with highs of 76 in San Clemente; 78 in Newport Beach; 79 in Laguna Beach; 86 in Anaheim; 87 in Irvine and Fullerton; 90 in Mission Viejo; and 93 in Yorba Linda. Temperatures will remain at roughly the same levels through at least through Thursday, although they'll dip by a few degrees Saturday.