these phenomena are “high impact” in that they affect life, property, and the economy. All pose the same problems for forecasters: time of initiation, intensity and intensity variations, and end time. And, although some of the phenomena near the top of the list are large and persist for days, mesoscale features embedded within them, especially convective elements, cause most of the havoc.

Observations useful in the context of this study are equally useful for monitoring phenomena that lie outside the time-space envelope considered

FIGURE 2.1 Time and space scales associated with the “high-impact” weather phenomena that are discussed in Appendix A and summarized here. NOTES: The scale is logarithmic in both directions. Common units of time are noted on the vertical coordinate. Common notions of size are listed on the horizontal axis. The sizes and lifetimes associated with each phenomenon are typical but not necessarily definitive. Not portrayed is the size of mesoscale features that may be embedded within the larger events.