Dry conditions and triple-digit temperatures forecast for interior portions of Northern California mean critical fire danger over the weekend, Cal Fire and the National Weather Service said.

A Red Flag Warning has been issued for six Northern California counties, including Lake and Colusa, for Friday and Saturday, the state firefighting agency said.

Weather officials said everyone, but especially children and the elderly, should take care to avoid excessive heat exposure given the prospect for record-breaking temperatures.

Pets and livestock also are susceptible and should be kept sheltered and hydrated, the weather service said.

Forecast highs included 105 degrees in Clearlake on Friday and 107 on Saturday; 102 Friday and 104 Saturday in Middletown; 105 Friday and 104 Saturday in Ukiah; 101 both days in Cloverdale; 99 Friday and 100 Saturday in Healdsburg; 98 Friday and 100 Saturday in Calistoga; and a relatively cool 93 Friday and 94 Saturday in Santa Rosa.

Breezy conditions in some areas contributed to the Red Flag Warning, which covers portions of six counties: Shasta, Tehama, Glenn, Butte, Lake and Colusa counties.

There also is the possibility of thunderstorms in some areas this weekend, especially Sunday in the Sierra.

Cal Fire officials said they planned to beef up staffing in stations around high-risk areas.

A Spare the Air Day also was in effect Friday for the greater Bay Area because of hot temperatures and degraded air quality.

Residents are urged to avoid any unnecessary motor vehicle use.