Subtropical cyclone formation off the coast of Chile
A very unusual subtropical cyclone formed off the coast of Chile during the 07-08 May 2018 period (Weather Channel | Weather Underground). The system transitioned from a typical cold core baroclinic mid-latitude cyclone to a shallow warm core cyclone with some deep convection around the center of circulation. GOES-16 (GOES-East) “Clean” Infrared Window (10.3 µm) images (above) showed the evolution from 06 May to 09 May. The surface report plotted in the lower right corner of the images is Concepción, Chile.A Suomi NPP VIIRS True-color Red-Green-Blue (RGB) image viewed using RealEarth (below) depicted the circulation once it had drifted to a position northwest of Santiago, Chile at 1839 UTC on 08 May.
The hourly MIMIC Total Precipitable Water product (below) initially showed a long ribbon of subtropical moisture which was being transported ahead of a cold front into the baroclinic low on 05 May — then during the transition to a subtropical low, a small pocket of modest TPW migrated slowly northward with the surface circulation.