The National Weather Service is warning that weekend thunderstorms could bring "critical fire weather conditions."

The National Weather Service is warning that weekend thunderstorms could bring "critical fire weather conditions."

The weather service's Medford office posted a fire weather watch and a special weather statement this afternoon.

Both warn that thunderstorms will build up in Northern California and Southern Oregon Saturday, increasing through the day. The storms will continue and become wetter through Sunday, delivering heavy rains to some areas.

The storms are the result of moist, unstable air moving in from Arizona and Nevada in what is known as a monsoonal flow. The phenomenon happens each summer, but rarely this early, meteorologist Mike Johnson said. It is more common in July and August.

— Anita Burke