The majority of San Diego County was under a heat advisory Tuesday, on what was expected to be the hottest day of the week amid dangerous fire weather conditions. 

The heat advisory was in effect from 10 a.m. to 5 p.m. for the mountains, deserts and inland valleys, including the cities of Vista, San Diego, Escondido, El Cajon, San Marcos, La Mesa, Santee and Poway.

High temperatures surpassed 100 degrees in Santee (101) and Anaheim (103). Miramar, Montgomery Field, Brown Field, National City, Lake Elsinore, Fallbrook, Valley Center, San Marcos, Escondido, Rancho Bernardo, Poway, Lemon Grove and Rancho San Diego all saw highs between 95 and 99 degrees.

A record daily high of 84 degrees was recorded at the Oceanside Harbor, topping its previous high of 78 degrees in 2016.

The high temperatures come amid gusty Santa Ana winds, which, when combined with high temperatures and low humidity, could quickly spark and spread wildfires. Humidity levels Tuesday could drop to about 10 percent, the NWS said. 

Fire danger will elevate beginning late Wednesday when a Fire Weather Watch will take effect for Southern California. The warning will be in effect until Friday evening. 

A fire sparked Tuesday on base at Camp Pendleton and grew to around 80 acres in about 5 hours. A large plume of smoke was visible from various parts of the county but officials said the fire posed no threat to the public.

Another fire sparked in the river area near homes on Hazard Center Drive. A nearby resident was alerted to the fire by her companion dog and immediately grabbed a hose and started protecting her residence.

Another fire sparked in the riverbed less than a mile away. Both started near knwon homeless encampments. Fire investigators have not confirmed either fire was started by transient activity.

This week's extreme heat could cause heat exhaustion or heat stroke. The National Weather Service urges residents to drink plenty of water, stay in air-conditioned rooms and stay out of the sun as much as possible.

NBC 7 Weathercaster Ashley Matthews pinpointed some coastal areas that are predicted to have the lowest high temperatures as possible "cool off spots." Those beaches include Encinitas, with a forecasted high of 77 degrees, and La Jolla, with a forecasted high of 78 degrees. For the South Bay, Imperial Beach is expected to see a high temperature of 83 degrees.  

For a list of cool zones in San Diego County, click here.