A Lagrangian Technique for Moisture Attribution of Winter and Spring Severe Local Storms over the Contiguous United States

What are the sources of moisture that contribute to severe local storms? Using the NOAA Air Resources Laboratory HYSPLIT air parcel trajectory model and a Lagrangian-based moisture attribution algorithm, a climatology of the moisture origins of winter and spring severe thunderstorms has been developed. The climatology reveals that moisture contributing to tornadic storms originates from the Gulf of Mexico (GoM) and beyond, including the western Caribbean Sea and western North Atlantic Ocean. Moisture sources for non-significant severe and non-severe events differ in geographical extent, generally in closer proximity to the contiguous United States (CONUS). Moisture source variability in relation to the Loop Current (LC), a variable oceanic current in the eastern GoM, is also analyzed. The length of this record also allows for long-term changes in the origins and atmospheric transport patterns of moisture for severe local storms to be explored.

This study is motivated by the need to better understand the origins and transport of moisture contributing to severe local storms in both forecasting and research applications. Boundary layer moisture is vital to the development of hazardous convection, thus it is essential to demarcate the source regions and processes governing the advection of water vapor in the troposphere. It has been generally agreed that synoptic scale advection of moisture from the GoM precedes severe convection development, with prior studies relating warmer sea surface temperatures to increased severe thunderstorm occurrence. However, results presented herein highlight the need to look beyond the GoM basin. The GoM LC has also been related to severe convection frequency, but previous research failed to quantify the extent of this contribution. The methodology introduced herein allows for moisture attribution of severe local storms, an important step in bridging the gap between the synoptic scale hydrological cycle and high impact CONUS weather events.