Observation-Based Estimates of Present-Day and Future Climate Change Impacts on Heavy Rainfall in Harris County

Abstract

This report describes the results of an extreme value analysis of precipitation in and around Harris County, Texas, in order to determine whether the newly-promulgated NOAA Atlas 14 rainfall design values are valid in a changing climate. The analysis in this report is based on the original NOAA Atlas 14 data set as well as a set of composite stations for the Gulf and Southeast Coasts. As of this writing, this report and its findings have not yet been peer-reviewed. • The recent upward trend in extreme precipitation in the Houston area has contributed to extreme rainfall design values in the area that far exceed those of comparable locations. This is in part due to some stations not having a sufficiently lengthy set of observations and in part due to southeast Texas receiving more than its fair share of storms. We assess that the design values of 100-year rainfall amounts would be 7% smaller if a longer period of record was available at all observation locations. • Coastal southeast Texas has the largest single-day and multi-day return values anywhere along the Gulf and Atlantic coasts for return periods of 100 years or more. This is in part due to some recent storms that could have occurred anywhere along the Gulf coast concentrating their activity around Houston. There is no known factor that would make storms such as Harvey more likely to happen in Texas than elsewhere along the northern Gulf Coast. We assess that extreme rainfall risk in Southeast Texas should consider storms from a broader portion of the Gulf Coast, decreasing return values by an additional 1%-18%, with the larger values applying to the larger return periods. • A robust upward trend in extreme precipitation is present across the southern and southeastern United States. The trend is larger in southeast Texas, but we have no reason to expect that climate change would cause trend variations on such a small scale. Using averaged trends across areas near the Gulf Coast, we assess the best estimate of the climate-driven trend in southeast Texas to be 11%-15% over the past 60 years, with the remainder of the observed trend caused by regionally unusual storms (like Harvey) that are not likely to recur in the same places. • The three factors listed above effectively cancel each other out for 2-year return values. We assess that the present-day nonstationary return values are approximately equal to the stationary estimates of NOAA Atlas 14 for 2-year return periods. • Because of the three factors listed above, the NOAA Atlas 14 100-year and 500-year return values generally overestimate the present-day and near-term future extreme rainfall risk in and around the Houston area. We assess that for 100-year return periods, current nonstationary values are still about 10-12% below the NOAA Atlas 14 values. • The historic upward trend is very likely to continue with global warming. Because of this, we assess that NOAA Atlas 14 return values underestimate the intensity of all future 2-year rainfall events in the Harris County area. We also assess that, depending on the rate of future warming, the nonstationary 100-year return values will exceed the NOAA Atlas 14 values around the middle of the 21st century.