Abstract
AbstractSevere thunderstorms can have devastating impacts. Concurrently high values of convective available potential energy (CAPE) and storm relative helicity (SRH) are known to be conducive to severe weather, so high values of PROD = (CAPE)1/2 × SRH have been used to indicate high risk of severe thunderstorms. We consider the extreme values of these three variables for a large area of the contiguous United States over the period 1979–2015 and use extreme-value theory and a multiple testing procedure to show that there is a significant time trend in the extremes for PROD maxima in April, May, and August, for CAPE maxima in April, May, and June, and for maxima of SRH in April and May. These observed increases in CAPE are also relevant for rainfall extremes and are expected in a warmer climate but have not previously been reported. Moreover, we show that El Niño–Southern Oscillation explains variation in the extremes of PROD and SRH in February. Our results suggest that the risk from severe thunderstorms in April and May is increasing in parts of the United States where it was already high and that the risk from storms in February is increased over the main part of the region during La Niña years.
Highlights
Annual losses from severe thunderstorms in the United States have exceeded $10 billion in recent years (Saville 2019)
We quantify the effects of time and El Niño–Southern Oscillation (ENSO) in the generalized extremevalue (GEV) location parameter and study their significance, using q 5 0.05 and q 5 0.2, corresponding to control of the false discovery rate (FDR) at the nominal levels 5% and 20%
In each case we first discuss PROD, which is the main variable of interest for severe thunderstorm risk, and consider convective available potential energy (CAPE) and storm relative helicity (SRH)
Summary
Annual losses from severe thunderstorms in the United States have exceeded $10 billion in recent years (Saville 2019). In addition to economic losses, 2011 was marked by 552 deaths caused by tornadoes. These economic and human impacts are a strong motivation to study how and why U.S thunderstorm activity varies from year to year and region to region. Inadequacies in the length and quality of the thunderstorm data record present substantial challenges to addressing these questions directly (Verbout et al 2006; Allen and Tippett 2015; Edwards et al 2018). In the United States, a severe thunderstorm is defined to be one that produces a tornado, hail greater than 1 in. A supercell is a thunderstorm with a deep, long-lived rotating updraft
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