Understanding Left-Moving Supercells: Environmental Factors and Forecasting Challenges

TL;DR

This study analyzes environmental conditions and forecasting challenges of left-moving supercells in North America, providing a climatology and identifying key predictors of their strength and hail potential.

Contribution

It offers the first comprehensive climatology of LM supercells, highlighting environmental factors that differentiate their strength and hail production, filling a significant research gap.

Findings

LMs form in environments similar to right movers but with specific differences.

Lapse rates, CAPE, and LCL height are key predictors of LM strength and hail potential.

Longer-lived LMs have weaker CAPE and stronger shear.

Abstract

Left-moving (LM) supercells, characterized by anticyclonically rotating updrafts in the Northern Hemisphere, are significant due to their propensity to produce large hail. Although less common than right-moving supercells, they present notable forecasting challenges and societal impacts. However, despite these impacts, the environments of LM supercells are poorly understood compared to their right-moving counterparts. To address this gap, this research focuses on enhancing the understanding of LM supercells by examining the environmental conditions conducive to their development. A manually compiled and quality-controlled dataset of over 850 LM supercell cases across North America is used to provide a robust sample. Near-storm environments are characterized through the use of RAP/RUC inflow proximity sounding profiles. Leveraging storm properties, including mesoanticyclone strength, hail size, wind speed, and duration, we investigate whether environments can differentiate between these varying strengths and categories, thereby enhancing forecaster awareness. Results show that LMs typically form in environments supportive of right movers, with a key difference being that LMs likely only realize the shape of the hodograph above their LCLs. Lapse rates, CAPE, and LCL height are the best predictors of LM strength and hail potential. LMs with wind reports have drier boundary layer moisture, steeper 0--3 km lapse rates, larger CAPE, and higher LCL heights, leading to increased evaporational cooling. Longer-lived LMs often have weaker CAPE and stronger shear as compared to shorter-lived LMs. These results establish a unique parameter space climatology of LM supercells, thus providing essential forecasting insight and reducing the research gap for these storms.