Searing temperatures caused fires and power outages around Napa County Thursday, with more high temperatures on the horizon.

A large oak fell on utility lines near Lake Hennessey Road, sparking a fire near the shoreline, CalFire reported Thursday.

The 11:30 a.m. fire burned less than 5 acres along Chiles Pope Valley Road near Sage Canyon Road, said CalFire Battalion Chief Ed Shew.  It was contained within 90 minutes.

The power lines draped a nearby fence and PG&E crews had to deactivate power before firefighters could attack the fire, Shew said.

Eighty customers lost power on Sage Canyon Road, Long Ranch Road, Chiles Valley Road and Pope Valley Road, PG&E spokeswoman Jana Morris said. Power was scheduled to be restored at midnight.

The oak may have been the victim of sudden oak death, said Shew, referring to the disease that weakens and kills oak trees.

Chiles Pope Valley Road was closed for about two hours.

Also on Thursday, one of the main links between St. Helena to Santa Rosa was blocked to commuters after an early morning fire broke 3 miles east of the Sonoma-Napa county line.

Spring Mountain Road, which becomes Saint Helena Road, closed to all but fire trucks and local traffic after the fire broke at 9 a.m. near Tarwater Road, CalFire said.

As of 6 p.m., the fire burned more than 80 acres, threatening houses and outbuildings, CalFire said.

Power failures left businesses and houses without power in Napa and Calistoga.

In Calistoga, nearly 200 lost power after 8 a.m. The power failure remains under investigation.

In one incident, 117 Calistoga customers lost power a little after 8 a.m., Morris said.

As of 3 p.m., power could be restored at 7:45 p.m.

In another incident, about 70 other customers lost power at 4:30 a.m., according to PG&E. The customers were expected to have power again late in the afternoon.

In Napa, 22 downtown homes and businesses went dark after a transformer failed, according to PG&E. All but four customers should have power back by 9 p.m., Morris said.

Silverado Middle School reported a power failure at 4:30 a.m.

Power was reestablished except for two wings and the school day was not affected, said Vince Meyer, administrator of general services for the Napa Valley Unified School District.

The power failures remain under investigation.

The oppressive heat sent some residents to the hospital. As he waited for the first heat stroke patient to arrive at Queen of the Valley Medical Center, Dr. Paul Kivela said the emergency room staff treated a series of minor heat related cases Thursday.

Another heat advisory was expected Friday as temperatures were expected to reach 99 degrees in Napa, according to the National Weather Service.

To avoid heat-related health illnesses, people should remain in air-conditioned rooms, drink plenty of fluids, avoid alcoholic beverages and wear light-colored clothing.