From the Portland National Weather Service:
The hot, dry, unstable air mass over Western Oregon and Washington will begin to break down as an upper level low moves through the area. However, before the dry air mass moderates, the upper low will provide the potential for thunderstorm development. Very hot and dry conditions are a potent combination for fires.
A Red Flag Warning means that critical fire weather conditions are either occurring now or will shortly. A combination of thunderstorms, dry fuels and relatively low humidity create explosive fire growth potential. The Red Flag Warning is in effect from 11:00 a.m. today to 11:00 p.m. Tuesday for thunderstorms and hot and dry conditions.