Probably less likely after a prolonged period of drought. Although the fuel itself would be easier to ignite by flame, spark or friction, the dry weather would reduce heat production from microorganism growth and hydrolytic reactions which are often precursors to the runaway reaction resulting in autoignition.

Note, however, that droughts are often associated with periods of high temperature. Thus there may be a transition period in the early stages of drought in which both high temperatures and moist conditions exist, favoring autoignition.