Slow-moving planetary waves of high amplitudes are often associated with persistent surface weather conditions. This persistence can lead to extreme weather events with potentially serious implications for society and nature. Quasi-resonant amplification (QRA) of planetary waves has been proposed as a mechanism to generate high-amplitude hemisphere-wide patterns of wavenumbers 6–8 in the Northern Hemisphere (NH) summer. Here this mechanism is studied in both hemispheres. Analyzing 1979–2015 reanalysis data, evidence for QRA in the Southern Hemisphere (SH) is found for wavenumbers 4 and 5. It is shown that the difference in resonating wavenumbers between hemispheres stems from the different magnitude and latitudinal shape of the respective zonal-mean zonal winds. During resonance events a strong and narrow jet is observed in both hemispheres, whereas the emergence of a second zonal mean jet at high latitudes (i.e., a “double jet”) is seen in the NH only. Strong and narrow jets can form stable, zonally oriented waveguides, which are an essential prerequisite for resonance. After investigating the waves’ preferred phase positions during QRA, a bimodal behavior is identified for wavenumbers 4 and 5 in the SH and for wavenumber 6 in the NH, whereas wavenumbers 7 and 8 exhibit a single preferred phase position in the NH. Composite plots of meridional wind and temperature anomalies during periods when the resonating wave is in the preferred phase position indicate those regions that are most likely to experience heat extremes. These regions include western North America, western Europe, and central Eurasia in the NH and Patagonia and Australia in the SH.