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# Synthesis of Extreme Storm Rainfall and Probable Maximum Precipitation in the Southeastern U.S. Pilot Region (NUREG/CR-7133)

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## Publication Information

**Manuscript Completed:** July 2021**Date Published:** April 2023

**Prepared by:**R. Jason Caldwell, Victoria L. Sankovich, and John F. England, Jr.

U.S. Department of the InteriorBureau of ReclamationTechnical Service CenterWater and Environmental Resources DivisionFlood Hydrology and Emergency Management GroupDenver, Colorado 80225

Elena Yegorova, NRC Project Manager

Office of Nuclear Regulatory ResearchU.S. Nuclear Regulatory CommissionWashington DC 20555-0001

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## Abstract

Data and analyses of ten recent extreme storms in North and South Carolina have been completed by Caldwell et al. (2011). These new storms may influence Probable Maximum Precipitation (PMP) estimates in the region. This report builds on the storm analyses to illustrate some potential impacts to design precipitation estimates and to introduce some alternative, risk-based perspectives, including precipitation frequency. The major objectives of the work were to: (1) describe potential impacts of new storms on existing PMP estimates; (2) present some sensitivities of PMP estimates; and (3) provide preliminary probability estimates of PMP.

The approach that is used is to examine in finer detail the impacts of Hurricanes Floyd and Fran on PMP estimates, based on new Depth-Area-Duration (DAD) and in place storm maximization results from these storms. Brief comparisons are made to 2011 storms. The sensitivity of PMP estimates to several factors is illustrated, including examination of potential climate influences and trends in moisture and surface observations. Approximate PMP exceedance probabilities for example sites within the Carolinas Pilot Region are estimated using regional precipitation frequency analysis.

New data analyses of ten tropical cyclones (TC) suggest that Hydrometeorological Report 51 (HMR 51) PMP values are too low for durations greater than 12 hours and area sizes greater than 5,000 mi2 (12,950 km2) along the coastal Carolinas, based on analysis of Hurricanes Floyd and Fran (Caldwell et al, 2011). Other durations and area sizes are unaffected in this location. HMR 51 may need to be updated for coastal areas in Carolinas. There are unknown impacts in the Carolinas because envelopment, transposition and orographic effects are unclear due to the limited sample. These factors, if included, would tend to increase PMP estimates in some locations. Storm Depth-Area Duration maximized values are somewhat sensitive to radar rainfall biases and use of maximized moisture. Using a median moisture maximization ratio, Floyd is still close to HMR 51 PMP. No significant trends were found in sea surface temperature (SST) and dew point grids. This suggests stationary series for storm maximization. There is a potential for increased temporal clustering of TC events in August-September based on recent storms in 1999, 2004, and 2011. Longer-duration rainfalls (> 72 hr) and soil moisture for runoff may be changing factors.

PMP ratios to 1/1000 Annual Exceedance Probability (AEP) 24 hour rainfall ranged from 2 to 6 times. PMP 24 hour, 10 mi2 (26 km2) return periods ranged from 10-5 to > 10-7. HMR 51 PMP estimates might be high in the Piedmont region based on NOAA 14 point frequency estimates. Existing regional precipitation frequency methods exist and can be used for a transition from a design maximum, PMP-focused assessment to probabilistic assessments. Considerations for future work should address several critical aspects, including orographics with the use of numerical models (e.g. WRF); transposition effects; and larger-scale community efforts on data collection and synthesis.

Page Last Reviewed/Updated Monday, April 24, 2023