Observation-Based Estimates of Present-Day and Future Climate Change Impacts on Heavy Rainfall in Harris County
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Date
2020-08-07
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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 As of this writing, this report and its findings have not yet been peer-reviewed.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.
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Keywords
Extreme rainfall, Climate change