Temperatures are expected to jump dramatically Friday, pushing the day's high to near-record levels.

Temperatures are expected to jump dramatically Friday, pushing the day's high to near-record levels.

However, thunderstorms on the heels of the heat are what have forecasters worried.

"Things are setting up optimally for thunderstorm development," said Mike Johnson, meteorologist at the National Weather Service office in Medford. "This looks like a big event."

He said forecasters will study computer models in meetings today in an effort to develop a clear forecast to warn firefighters and others over the weekend.

A weather system sweeping in from offshore Friday will boost valley temperatures across Southern Oregon 10 to 15 degrees, this morning's forecast said. Friday's high temperature is predicted to close in on 100 degrees, the day's record, set in 2003. Saturday will be even hotter, with 102 forecast for Medford.

Hot, moist air from Mexico and Arizona, known as a "monsoonal push," looks likely to roll in from the southwest on Saturday, Johnson said.

It will bring prime conditions for forming thunderstorms to Northern California, already hammered by a lightning storm last weekend.

Jackson and Josephine counties also can expect thunderstorms.

"We certainly will get lightning in Medford," Johnson said.

He said wildland fuels in Oregon aren't as dry as those in California, but fire weather forecasters are keeping a close eye on conditions to determine if warnings need to be posted.

By Sunday, the thunderstorms will bring significant amounts of rain, which could cause localized flooding in the mountains, Johnson said.

A chance of rain will remain Monday, as temperatures drop back to the mid to low 90s in the Rogue Valley.

— Anita Burke