A high pressure system predicted to move into northern California today will bring the first heat wave of the season to Ventura County and the rest of the state, the National Weather Service reported.

The offshore flows resulting from the high atmospheric pressure will block cool air and moisture from coming in from the ocean and push temperatures into the 90s in local valleys beginning today, said Bill Hoffer, a spokesman for the National Weather Service in Oxnard.

The heat wave is expected to last through Sunday, with the highest heat predicted for today, Friday and Saturday.

Weather service forecasts show consistent 90-degree heat in local valleys and mountains through Sunday. Triple digit temperatures are possible in Simi Valley, Hoffer said. On the county coast, high temperatures in the 80s and low 90s are expected.

Sea breezes are expected to blow into the county beginning Monday, and temperatures should drop off Tuesday.

The Ventura County Fire Department is boosting staffing levels for the first time this year to deal with the increased danger of quick-spreading blazes during the heat wave, said Capt. Barry Parker, a spokesman for the Ventura County Fire Department.

Extra dispatchers, hand crews, helicopters and water carrying vehicles will be at the ready, he said.

Compared to other parts of the state, Ventura County is expected to get off fairly easy during this heat wave.

Temperatures are expected to creep into triple digits in the San Fernando Valley, and excessive heat watches are scheduled for a large swath of northern California in the coming days.

State agencies on Wednesday issued a statement urging residents to protect themselves from heat related injuries and avoid wasting electricity to reduce the risk of power outages.

The state Department of Public Health advised people to limit outdoor activities during the hottest parts of the day, turn on cooling devices such as air conditioners and drink increased amounts of fluids. The health department recommends drinking water — not alcohol or caffeinated such as soda and coffee.

The Governor's Office of Emergency Services suggests residents set thermostats between 78 and 80 while they are at home, and 85 while they are away.

Under regulations adopted in 2005, employers at outdoor worksites are required take basic steps to prevent heat-related illness, such as making water available to employees and providing access to shade.