For the first time we demonstrate an indirect, rather than direct, role of quasi-resonant amplification of planetary waves in an unprecedented summer weather extreme. We find that there was an interplay between a persistent, amplified large-scale atmospheric circulation state and soil moisture feedbacks as a pre-cursor for the June 2021 Pacific Northwest “Heat Dome” extreme heat event. An extended resonant planetary wave configuration prior to the event created an antecedent soil moisture deficit that amplified lower atmospheric warming through strong nonlinear soil moisture feedbacks, favoring this unprecedented heat event.