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. In this letter, the local finite-amplitude wave activity (LWA) of Huang and Nakamura (2015) is introduced as a diagnostic of the 500 hPa geopotential height for characterizing midlatitude weather events. It is found that the LWA climatology and its variability associated with the Arctic Oscillation agree broadly with the previously reported blocking frequency in the literature. There is a strong seasonal and spatial dependence in the trends of LWA in recent decades. While there is no observational evidence for a hemispheric-scale increase in wave amplitude, robust trends in wave activity can be identified at the regional scales, with important implications for regional climate change.