Aheat wave is expected to push weekend temperatures in the Rogue Valley to heights not seen since 2010.

The National Weather Service office in Medford said in a special weather statement Friday that a strong area of high pressure will continue to build, pushing temperatures to 105 degrees today.

Temperatures last topped triple digits in Medford on Aug. 25, 2010. The 709-day streak is the second longest on record, after a 732-day run that ended July 25, 1958.

NWS meteorologist Marc Spilde said temperatures could even rival the 108-degree record set for Aug. 4 in Medford in 1998.

"We've been getting lots of sunshine and warming from area pressure causing the heat," Spilde said.

Forecasters issued an excessive heat watch for today for southwest Washington and Western Oregon, including the Portland area and Willamette Valley.

Temperatures in the 90s and 100s bring the risk of heat exhaustion and heat stroke.

The high pressure system will coincide with the arrival of southern moisture aloft late Saturday into Sunday, which is likely to cause thunderstorms across the area, Spilde said. Lightning will bring a risk of wildfires, reported the weather service, which has also issued a fire weather watch for the weekend.

The Oregon Department of Forestry will keep a close eye on any lightning strikes, which are the second most common cause of wildfires after human actions, said ODF spokeswoman Ashley Dubrey.

The ODF uses a lightning map to locate strikes and will send an engine to check for any smoke or hot spots, Dubrey said. ODF crews will continue to be on a 24-hour call schedule this weekend.

The best chance for thunderstorms will be over the mountains of Northern California, particularly western Siskiyou County, the weather service said. The chance of such a storm will begin Saturday evening, then spread north over the Siskiyous, into the Cascades and Eastern Oregon on Sunday. Western valleys, including the Rogue Valley, could see thunderstorms during this time.

Despite the strong likelihood of storms, Spilde pointed out how difficult predicting them can be.

"Thunderstorms are hit or miss," he said. "A few places may not end up with anything."

Should storms appear in the Rogue Valley, the first will have gusty winds and little rain, but Sunday's storms should be wetter, the forecast says. Spilde said the southern moisture should leave the valley by Monday.

Reach University of Oregon reporting intern Josephine Woolington at jwoolington@mailtribune.com. Web Editor Anita Burke contributed to the story.