Most of San Diego County will be under an Excessive Heat Warning this week as a high-pressure system sends temperatures 10 to 15 degrees above average for this time of year.
The heat wave began to move into the region on Monday, bringing a wave of extreme temperatures to all areas of San Diego County except the coast and upper mountains.
"Only the immediate coast and high mountains will escape the very hot temperatures," the National Weather Service said.
An Excessive Heat Warning has been issued from 10 a.m. Monday to 8 p.m. Thursday for those areas.
The hottest temperatures are expected between Tuesday and Wednesday, and will likely be in the mid-90s in the western valleys and upwards of 100 degrees in the inland valleys. Temperatures could get as high as 106 degrees.
Oceanside and Escondido both tied the record for highest overnight low Monday night at 70 and 71 degrees, Dagmar Midcap said. Chula Vista and Vista also tied the record for the highest low temperature with 70 and 67. Until Thursday, the low deserts should expect temperatures 117 degrees and up with overnight lows in the 90s. In the inland valleys temperatures will be between 95 and 107 degrees.
"We're going to see these temperatures really climb and reach their peak on Wednesday," NBC 7’s Weathercaster Llarisa Abreu said.
While the heat wave is at its peak, the California USO has asked the state's residents to conserve energy to avoid overtaxing the electricity grid. The Flex Alert will be in effect from 5 to 9 p.m. on Tuesday and Wednesday.
It is possible the cities of San Diego, Vista, Chula Vista, Escondido, Ramona and several others will break records during the warning period, the NWS said.
On Monday, two San Diego County cities tied heat records -- Ramona reached 102 degrees tieing its 2006 record and Borrego reached 117 degrees tying its 2014 record.
Meteorologist Dagmar Midcap said she expects records to be broken on Tuesday when temperatures will be even higher.
While the coast and the region's highest elevations in the mountains are excluded from the warning, those areas will be under a heat advisory, the NWS said.
The heat wave prompted Cleveland National Forest official to shut down two popular hiking trails through Thursday, Three Sisters and Cedar Creek falls.
Abreu reminds residents to drink lots of water, remember to not leave children or pets in cars, stay inside as much as possible and limit time outside as heat illnesses are likely during the warning period.
The county's official list of "Cool Zones" for 2018 include community centers, recreational centers, libraries, senior centers and museums.