La Niña winters exhibit significant local enhancement of heavy rainfall in the southwest United States, relative to El Niño. This contrasts with average daily rainfall intensity, which is instead increased during El Niño winters. The present study explores the relationship between heavy rainfall and associated atmospheric circulation patterns. Using composite analysis, we find that heavy rainfall events in the southwest arise from the presence of a persistent offshore trough and simultaneous emplacement of a strong source of subtropical water vapor. Greater intensity of these storms during La Niña is consistent with a deeper offshore trough leading to strengthened moisture fluxes. Composite circulation patterns survive amongst a large degree of synoptic variability, highlighting the importance of understanding this variability when making regional climate predictions.