RIVERSIDE (CBS) — An excessive heat warning denoting potentially deadly conditions will be in effect over vast areas of the Southland Wednesday and Thursday as a result of the heat wave gripping the region.

Additionally, thunderstorms could develop in the afternoon and evening hours in mountain and desert areas, creating a risk of wildfire, the National Weather service warned. These could be largely dry thunderstorms — fire-sparking lightning not accompanied by rain, said NWS meteorologist Rich Thompson.

“Hot temperatures and increased humidity … will combine to create dangerous conditions in the afternoons and early evenings” today and Thursday, according to an NWS advisory.

The excessive heat warning will be in effect from 11 a.m. Wednesday to 7 p.m. Thursday in the Santa Clarita and San Fernando valleys and the Los Angeles and Ventura county portions of the San Gabriel mountains.

No such warning has been issued for the Antelope Valley. It will be somewhat dryer there than in the other valleys, keeping heat index values — how hot it really feels — just under the warning threshold, according to the NWS.

Also on tap Wednesday for San Fernando and Santa Clarita residents is air quality in the unhealthful range, according to the Air Quality Management District.

The NWS forecast highs today of 71 in Avalon; 73 at LAX; 81 in Newport Beach; 87 in downtown L.A; 91 in Long Beach; 93 in San Gabriel; 94 in Anaheim and on Mount Wilson; 96 in Burbank; 99 in Pasadena; 106 in Palmdale; 107 in Woodland Hills; and 108 in Saugus and Lancaster.

“Heat exhaustion is possible, especially if engaging in strenuous outdoor activities. This weather could be deadly for unprepared campers or hikers,” according to an NWS advisory.

To guard against heat exhaustion or heat stroke, the NWS advised residents to schedule strenuous activities for early mornings or the evening, wear light, loose clothing, and drink plenty of water.

“To reduce risk during outdoor work, the Occupational Safety and Health Administration recommends scheduling frequent rest breaks in shaded or air-conditioned environments,” the advisory noted.

“Anyone overcome by heat should be moved to a cool and shaded location. Heat stroke is an emergency — call 911.”

NWS meteorologists said monsoonal moisture filtering in from the southeast would create the possibility of thunderstorms in the mountains and deserts this week starting today.

“Given the hot and dry conditions, the possibility of thunderstorms will result in a heightened fire-weather risk,” an advisory said.

Thompson said he expects any thunderstorm to be mostly dry today, wet and dry Thursday, and mostly wet Friday.

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