Midlatitude extreme weather events are responsible for a large part of climate-related damage, yet our understanding of these extreme events is limited, partly due to the lack of a theoretical basis for midlatitude extreme weather.  While high-impact midlatitude weather events (e.g., blocking, heat waves, or atmospheric rivers) are generally manifested in the form of large-amplitude Rossby waves, the interpretations are often restricted to the linear Rossby wave dispersion relationship.  A new diagnostic of midlatitude weather events is introduced in this work using the theory of the local finite-amplitude wave activity (LWA).  It is found that the LWA climatology and its variability associated with the Arctic Oscillation (AO) are broadly consistent with the previously reported blocking frequency in the literature.  There is no statistically significant evidence either in winter or in summer for a hemispheric-scale increase in wave amplitude associated with recent Arctic warming.  Interestingly, there seems to be a local anticorrelation, as expected from theory, between the zonal wind anomaly and LWA amplitude for the patterns associated with the AO as well as for the linear trends.  Furthermore, LWA can identify robust changes in wave amplitude at the regional scales that may be related to previous studies on midlatitude extremes.  Given the lucid linkage to weather extremes, further investigations are warranted into the application of LWA to understanding midlatitude extremes in a changing climate.