Unfolding North American spring weather extremes along a scale ladder.

Springtime weather extremes have profound socioeconomic consequences across North America. While their dynamical linkages with synoptic- and mesoscale systems are well-studied, there is limited understanding of their variability on subseasonal and longer timescales. Here, we unfold the multi-layered dynamics of these extremes along a "scale ladder" built around canonical modes of subseasonal variability over North America in spring. The four leading subseasonal modes are characterized by localized geopotential anomalies over the North Pacific and northeastern North America, and two branches of zonally propagating Rossby wave packets. These modes modulate the activity of wet, dry, and wind extremes across North America, causing up to a twofold change in their occurrence frequencies. Over four decades, these modes have shown distinct "active" and "muted" periods, leading to decadal-scale changes in extremes. The multiscale perspective highlighted here provides a dynamics-based guideline for dissecting long-term changes of extremes and extracting potential sources of predictability across timescales.