The question is perennial: “How many observations do I need, and how dense and how frequent?” The honest answer is “It depends upon the application.” This appendix deals with a single but very important application: observational support of the national infrastructure for weather and climate monitoring and numerical weather prediction. Even for this single application area, the answer to the question depends upon the phenomenon: its size and longevity, which governs its predictability, and whether it has any embedded features that cause localized damage. Consideration of the phenomena is roughly in the order of size/longevity. The list is neither definitive nor exhaustive, but it does cover events that cause the greatest disruption, damage, and loss of life.
Definition: Steady soaking rains, sometimes with embedded showers and thunderstorms, cause flooding of small streams and larger rivers. Rain falling on melting snow exacerbates the flooding.
Size: Typically 300-2000 km across.
Duration: half a day to several days
Geographic preference: West Coast, Southern Plains, Lower Midwest, Appalachians. Rapid melting of a heavy snow cover, especially when accompanied by rainfall, sometimes causes floods in the northern United States,