The Unusual Tornadoes in Chile in May 2019: Forecasting Challenges from the Synoptic, Mesoscale, and Subseasonal Scales

In the presence of vertical wind shear, moisture, instability, and a source of lift can lead to damaging weather. However, environments not characterized by high values of CAPE or vertical wind shear often receive less attention, despite supporting hail, strong winds, and tornadoes. There thus remains a need to tornado events in regions where they are not common, to highlight the synoptic and mesoscale conditions under which they form and provide forecasters important context by which to anticipate future events. In this study, the tornadoes of 30-31 May 2019 in south-central Chile (37S) were investigated. The synoptic-scale context of the two tornado events were analyzed using observations from several sources: (1) CFSRv2 reanalysis of geopotential height and u- and v-wind components at 500 hPa, mean sea level pressure (MSLP), 10-m wind u- and v-wind components, 2-m dew point and equivalent potential temperature, surface-based CAPE, and 850-hPa u- and v-wind components; (2) Infrared brightness temperatures from the GOES-16 satellite at 10-min intervals; (3) Lightning flashes detected by the GOES-16 Geostationary Lightning Mapper (GLM) and the World Wide Lightning Location Network; (4) Daily values of sea surface temperature from the NOAA OISST record; and (5) Surface observations every 5 minutes from two surface observing stations within 15 km of each tornado.

Mesoscale conditions local to each tornado were analyzed using output from the Weather Research and Forecasting (WRF) Model version 4.0. The simulation was performed using three nested domains with horizontal resolutions of 9, 3, and 1 km, 55 irregularly spaced vertical sigma levels, and a 50-hPa model top. The simulation was initialized two days before the first tornado and was run until the day after the second tornado. Initial and boundary conditions (every 6 h) were provided by the National Centers for Environmental Prediction (NCEP) Final (FNL) Analysis. The subseasonal context of the two tornadoes was evaluated by comparing the synoptic-scale conditions observed in the middle troposphere (500 hPa) and surface (MSLP) during late May 2019 with the average conditions seen in composite analyses based on the phases of the Madden-Julian Oscillation (MJO). Results indicate that the tornadoes of May 2019 shared some similarities with synoptic-scale, mesoscale, and subseasonal conditions associated with tornadoes in other regions, particularly tornadoes in low-CAPE, high-shear environments. Results also indicate a potentially important role of flow blocking by the Andes in creating a barrier jet that could have increased values of low-level shear. Those results will be presented and discussed.