It Is Time To Update The NOAA Precipitation Frequency Information

U.S. Weather Bureau Technical Paper 40 and the NOAA Precipitation Frequency Atlas of the Western U.S. stand out boldly as climate information summaries that have guided the engineering, construction and development of our nation from one coast to another. Their quantitative estimates of rainfall associated with various durations and return periods have provided engineers and flood plain managers with design inputs for constructing infrastructure that has saved hundreds and perhaps thousands of lives since they were published in the 1960s and early 1970s, respectively. They have stood the test of time and continue to be used in most of the country.

Decades of new data are now available, and many dense networks have been operated for many years that can provide more insight to the depth-area-duration relationships for local convective storms that were not available when those reports were written. Perceptions of climate change, whether right or wrong, are also leading to a strong sense that information about today and tomorrow's climate derived only on data prior to 1970 cannot be credible and may be erroneous.

For many runoff design applications, the timing of intense rainfall is much more important to the project design than the total accumulation. As a result, it is critical that the distribution over time of rainfall for low probability events (50-year return period or greater) be re-examined and regionally refined.

As a testimony to the need to update precipitation frequency information, a recent case will be described in which the Fort Collins, Colorado City Council voted to modify the 100-year 2-hour storm rainfall based on local and regional data.