Beyond the Mean: Trends in United State Convective Environments

Recently, there has been considerable speculation as to whether there is a definable change in the occurrence of severe thunderstorms and their associated environments. In this presentation, we examine the statistical properties of trends in atmospheric variables favorable to the development of severe convection from the North American Regional Reanalysis for the period 1979-2012. Contrasting previous studies, here the full parameter distributions are assessed via quantile analysis as well as devolving the trends to a regional basis. Further, to address recent questions into the shifting seasonality of severe thunderstorm observations, the analysis is used to assess whether a definable shift to the favorable environmental space has occurred.

Our results show that over the recent period there are CAPE increases for the Midwest and Central regions particularly during the spring, along with sharp increases in Storm Relative Helicity (SRH) for all seasons. In contrast, the commonly applied 0-6km vertical wind shear parameter (S06) does not show a clear signal, while 0-1km vertical wind shear parameter (S01) shows positive trends in all seasons and regions (~1% per year). Seasonally, this analysis has identified a shift toward greater SRH, S06, and S01 through all percentiles towards the earlier parts of the year. When this shift coincides with higher values of CAPE which occur earlier in the year as well, this reflects a seasonal shift which extends the severe convective season in the last 17 years, increasing risk to the general public and consistent with the shifts seen in tornado observations. Shifts that are conditioned on already favorable environments reveal that generally, increases in the parameters have the largest slope at the extreme quantiles, suggesting that not only has the season shifted, but the shift is manifesting itself in the extremes, as has often been posited in warming climate scenarios for quantities such as temperature or precipitation. However, the strong increases identified to SRH and low-level wind shear go against the prevailing hypothesis for a warming climate. This suggests that in the current environment, changes to the low-layer shear may be having the strongest influence on severity or volatility, contrasting the projected increases to CAPE.