276 - Changes in Upper Midwestern Extreme Rainfall Depth Estimates from TP40 to Today: Is Climate Change to Blame?

Estimating the amount of precipitation during extreme rainfall events is critical when designing stormwater control measures for infrastructure projects. The standards that engineers use to estimate design storms like the 100 year, 24 hour event have changed over time, and understanding why estimated depths may have increased or decreased can shed light on what future estimates may look like. This research looks specifically at the changes between Technical Paper 40 (TP40) published in 1961, and Volume 8 of the National Oceanic and Atmospheric Administration’s Precipitation Frequency Atlas of the United States (NOAA Atlas-14) published in 2013. Comparing the two reports, Upper Midwestern states had an average increase in the 100 year, 24 hour storm event of 0.5 inches, which at first glance simply be chalked up to our changing climate and Atlas-14’s more recent period of record. However, Atlas-14 improved on TP40 in many ways, but the number of methodological differences between the reports meant that this increase couldn’t be attributed to any one change in methodology. To determine what role the longer period of record played in the increase, the methods of both reports were replicated and applied to different station datasets to better understand the relative impact of new computational methods employed in Atlas-14, additional precipitation stations constructed since the publication of TP40, and the additional years of data between Atlas-14 and TP40. Audience members can expect to improve their understanding of the evidence for temporal changes in extreme rainfall events in the Upper Midwest.