This document describes the relationship between rainfall and moth catch and how it can be used to forecast an outbreak of fall armyworm, using conditional probabilities to predict armyworm larvae outbreaks. The data were from a new initiative – a community-based way to forecast an outbreak of armyworm in Tanzania, in which the outbreak risk for a village is determined locally from a single moth trap and rain gauge located within the village. It was found that, following a positive forecast, an armyworm outbreak was approximately twice as likely to occur as would be expected by chance.