An Analysis of the 1 October 2017 Derecho in Southern Brazil

On the afternoon of 1 October 2017, a damaging quasi-linear convective system (QLCS) that had atributes of a derecho resulted multiple severe wind reports, including three significant severe wind gusts (65+ kt) in Southern Brazil (SB). The pre-convective environment was characterized by an elevated mixed layer (EML) with base at nearly 750 hPa, considerably lower than the climatological 600–650 hPa EML base east of the Andes during spring. The objective of this study is to present an analysis of the synoptic and mesoscale environments preceding derecho onset, identify EML formation mechanisms, and establish how the anomalously low EML height influenced the severity of the event. The CFSv2 dataset was used to construct a synoptic-scale analysis of the event. The lapse rate tendency equation was evaluated to determine the evolution of the atmospheric stability field. Data from the Brazilian radar in Santiago was also used. Air parcel trajectories are derived from the HYSPLIT lagrangian model run with reanalysis data.

A high-amplitude, synoptic-scale midlevel trough amplified over Chile and Argentina on 30 September 2017 along an anomalously intense, zonal subtropical jet over the southeastern Pacific. This midlevel trough moved northeastward and triggered deep lee cyclogenesis (< 996 hPa) east of the Andes in northwestern Argentina, resulting in a strong poleward-directed low-level flow (40–50 kt at 850 hPa) over SB. The environment ahead of the midlevel trough where the QLCS formed in northeastern Argentina was characterized by surface-based CAPE values > 3000 J kg^–1 and 0–6-km wind shear values ~45 kt, which are highly conducive for severe storms. The QLCS formed over northeastern Argentina and crossed the Brazilian border near 1530 UTC 1 October as a mature, forward-propagating system with attendant cyclonic bookend vortex on its southern end. A rear-inflow jet observed in Doppler radial velocity was greater than 35 m s^–1, suggesting that the rear-inflow jet descent was the primary cause of strong wind gusts at surface.

The backward trajectories of air parcels in the EML (3000 to 5000 m height) at 1200 UTC 1 October 2017 in northeastern Argentina originated between 4000–6000 m within the zonal subtropical jet over the southeast Pacific Ocean. These parcels ascend before crossing the Andes between 30°S and 33°S in the second half of 29 September, descend east of the Andes while turning anticyclonically, and then ascend and turn cyclonically hours before reaching northeastern Argentina. A calculation of the lapse rate equation terms suggests the ageostrophic differential temperature advection is the main reason for the observed lapse rate steepening immediately east of the Andes. This lapse rate steepening process was possibly enhanced in lower levels due to the presence of an anomalously strong upper-level jet exit region over the Andes. The upper-level jet exit in this region causes a thermally indirect circulation east of the Andes, with poleward-directed ageostrophic flow associated with warm advection in the lower levels and equatorward-directed ageostrophic midlevel flow associated with cold advection in the midlevels, steepening the lapse rates.