Temperatures could reach triple digits Monday in the San Diego County deserts, and the mercury will be on the rise all week throughout the region, according to the National Weather Service.

Conditions will be relatively mild west of the deserts Monday, but building high pressure will cause temperatures to rise throughout the county until Friday, forecasters said. By midweek, high temperatures could be 15-20 degrees above average in most areas, forecasters said.

The NWS issued a heat advisory that will be in effect from 10 a.m. Tuesday to 9 p.m. Friday in the western valleys.

An excessive heat watch will also take effect Tuesday morning and last until Friday evening in the county deserts.