An excessive heat watch will be in effect from 11 a.m. Wednesday to 8 p.m. Friday east of San Diego County’s Interstate 15 due to soaring temperatures that are expected to affect much of the western U.S., the National Weather Service said.

The heat wave will increase the risk of power outages, according to the California Independent System Operator, which is responsible for almost 80 percent of the state’s power grid.

Forecasters say the hot weather will begin Tuesday and peak Wednesday and Thursday, when temperatures could reach 95 to 105 degrees in many inland valley cities. Friday also will be uncomfortably hot. San Diego Gas & Electric said the heat will elevate the risk of wildfires in a county that has experienced about a half-dozen small blazes in recent weeks.

Escondido is projected to reach 100 Wednesday, 105 Thursday and 104 Friday, the weather service said. El Cajon’s forecast is 99 Wednesday and 100 Thursday and Friday.  Borrego Springs will likely range from 111 to 114 during that period.

The temperature also could reach 100 on eastern Interstate 8, below the 4,500-foot level, potentially causing overheating in vehicles that are not well maintained.

The weather will be noticeably milder at the coast but still much warmer than usual for the first week of September. Forecasters said San Diego is expected to reach 83 Wednesday, 86 Thursday and 87 Friday. Forecast models suggest that temperatures could turn out to be higher. The city’s seasonal high temperature is 78.

“Low temperatures will also be elevated across the area, which will limit overnight relief from the heat,” the weather service said.

Forecasters also noted that the average monthly temperature in August at San Diego International Airport was 73.6 degrees, which is 1.2 degrees above normal.