SAN DIEGO — Temperatures could creep into the triple digits Thursday in portions of the inland valleys, while extreme heat will blanket desert areas until cooler conditions arrive heading into the weekend, according to the National Weather Service.

The NWS issued an excessive heat warning that will last until 9 p.m. Thursday for desert areas.

A heat advisory will also last until 9 p.m. Thursday for the inland valleys, including El Cajon, Santee and Escondido.

A high pressure system is centered over northern Baja California, but a trough of low pressure will move inland on Friday and bring cooler temperatures this weekend, forecasters said.

The deserts could see a high of 120 degrees, while temperatures in the inland valleys will be in the high 80s to low 90s and the areas near the foothills could reach 101, forecasters said.

The NWS advised that those who work outdoors should avoid the midday sun, wear light, loose-fitting clothing and drink plenty of water.

Nighttime lows are expected to remain in the low 80s in the deserts through Thursday night, meaning the minimal cooling at night could pose a health risk to those who don't have access to air conditioning because the body needs time to cool down from the day's heat.

The NWS also urged residents to drink plenty of fluids, stay out of the sun and check up on relatives and neighbors.

High temperatures near the coast could reach 78 degrees, with the mercury set to climb to 88 further inland and reach 102 in the mountains. 

Here's a list of designated Cool Zones sites that provide refreshing relief for people during hot summer months.