Temperatures in valley and mountain areas of the Southland again will reach dangerous "excessive heat'' levels today, fall marginally during the week and rise anew next weekend, National Weather Service forecasters said.
Highs will climb into triple digits thanks to a hot air mass resulting from a ridge of high pressure over the Southwestern United States, and high humidity levels will make it feel even hotter, according to the weather service.
"Heat index values, which take into account the air temperature and humidity, are expected to reach 105 degrees in many valley locations in Los Angeles and Ventura counties ... and to around 100 degrees in the lower mountain elevations,'' according to an NWS advisory.
"The combination of very hot temperatures and moderate humidity will combine to create a dangerous situation in which heat illnesses are possible,'' it said.
Glendora is expected to reach temperatures as high as 105 degrees today.
An excessive heat warning, issued by the NWS to indicate health- threatening conditions resulting from high heat and humidity, will be in effect from 11 a.m. to 8 p.m. in the San Gabriel, San Fernando, and Santa Clarita valleys, the L.A. County portion of both the San Gabriel and Santa Monica mountains, and several areas of Ventura and Santa Barbara counties.
The weather service urged area residents to take precautions to avoid heat-related illnesses, including: schedule any outdoor work for early morning or evening; wear light, loose-fitting clothing; and drink plenty of water.
Anyone overcome by the heat should be moved into the shade, and 911 should be called.
Highs are expected to fall by a few degrees on Tuesday and again on Wednesday.
"There will be some cooling both Tuesday and Wednesday, but maximum temperatures will remain above normal and further warming is now forecast for
next weekend,'' according to an NWS advisory this morning.