Relationships between WMAXSHEAR parameter and atmospheric patterns associated with tornado outbreaks in USA

Abstract:

Tornadic thunderstorms are likely to form in environments with deep-tropospheric wind shear and large values of convective available potential energy (CAPE). Exceptionally dangerous events, like tornado outbreaks, typically are forced by large-scale mid-troposphere geopotential height patterns, such as a negatively tilted trough. The relationships among atmospheric patterns, wind shear, and CAPE associated with major tornado outbreaks are ripe for study using computationally intensive statistical techniques, which have not been extensively applied for these events. Therefore, in this study we investigate patterns that explain covariability between 500-hPa geopotential height anomalies and a parameter known as WMAXSHEAR, which combines instability (CAPE) and 0-6 km wind shear. We hypothesize that spatial relationships exist between the patterns in both fields (500-hPa geopotential height anomalies and WMAXSHEAR) during major tornado outbreaks that occurred in the contiguous United States (25°N-50°N, 65°W-110°W). We test this hypothesis for May, from 1950 to 2017, using Maximum Covariance Analysis (MCA).

Results of the analysis show that the first three leading MCA modes (i.e., MCA1, MCA2, MCA3) explain 58%, 19%, and 13% of the squared covariance fraction. A robust pattern in the WMAXSHEAR and in the 500-hPa geopotential height anomalies fields has been identified in MCA1. This pattern is characterized by a strong dipole located over the central portion of the US Great Plains (Oklahoma, Texas, and Louisiana) and a pronounced long-wave structure that extends from the Dakotas south-southeastward to the Gulf of Mexico in the 500-gph geopotential height anomalies field. Additionally, the MCA2 and MCA3 capture an eastward shift in patterns of covariability in the WMAXSHEAR and in the 500-hPa geopotential height anomalies fields. The mentioned shift in the patterns associated with tornado outbreaks is important to understand when it comes to estimating potential for increased tornado disasters among vulnerable communities that live outside the region of traditional “Tornado Alley” in the central Great Plains.