The Cleveland National Forest issued an emergency closure for Three Sisters and Cedar Creek Falls until Thursday due to scorching temperatures.
The two trails were closed Monday and officials expect both to reopen Friday.
This comes just three weeks after the trails' last closures due to heat.
Much of the Three Sisters Falls' trail is in direct sunlight and can be extremely dangerous for hikers and animals during this time, the Cleveland National Forest said.
Most of San Diego County has been under an Excessive Heat Warning since Monday morning, according to the National Weather Service (NWS).
"We're going to see these temperatures really climb and reach their peak on Wednesday," NBC 7’s Weathercaster Llarisa Abreu said.
NWS predicted the heat wave to last until 8 p.m. Thursday.
Temperatures could rise into the mid-90s in the western valleys and even into the triple digits for the inland valleys.