Sensitivity of a Winter Tornado Outbreak to Upstream SSTs

Gulf of Mexico (GoM) surface waters can provide ample water vapor for the development of severe thunderstorms over the Southeast United States (U.S.) during winter months, when local moisture availability is scarce over the continent. Despite the low climatological likelihood of winter tornadoes, the interface between sufficient boundary layer moisture, vertical forcing, and favorable kinematics can result in tornadic activity. Such an event occurred on 21-23 January 2017 over the Southeast U.S., when more than 80 tornadoes, 200 injuries, and $1 billion (USD) in property damage was recorded. Sea surface temperatures (SSTs) were also the warmest winter temperatures on record (1982-2019) for the GoM, which raises the question – would this tornado outbreak have occurred with cooler upstream GoM SSTs? To answer this question, we use a two-pronged approach: (1) upstream SSTs were delineated with an air parcel backward trajectory model and (2) the event was recreated with perturbed SSTs underlying advected air masses using convective-permitting model simulations. The use of the NOAA Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT) and a Lagrangian-based diagnostic in the first component of this study allowed us to quantify moisture uptake along retraced air parcel paths. The second component employed the NCAR Weather Research and Forecasting (WRF) model to test the effect of perturbed SSTs. Additionally, various microphysics schemes were tested to quantify uncertainty rooted in microphysical parameterization schemes. This study demonstrates that upstream SSTs can modulate an event’s convective morphology and resultant severe hazards, providing an additional source of predictability for winter tornadoes. More broadly, these results present potential implications of warming oceans and marine basins on downstream convection.