Analysis of the Outflow Environments of Harvey, Irma and Maria and Why It Was Especially Favorable

The outflow environments of Harvey, Irma and Maria had one thing in common: There was extremely low outflow resistance to divergent outflow almost in all directions. This appeared to be attributable to unusually low horizontal shear, particularly to the north, that enabled the storms, during their intensification phases, to trap their own low potential vorticity outflow over considerable radial distance enabling the Carnot cycle radiative outflow leg to to operate at maximum efficiency. The interesting question is: “Why, then and there?”. We have found evidence of an interesting poleward shift in the potential energy bubble in the tropical UTLS in mid August. Just prior to the shift, a zonal low amplitude high index wave train, centered at near 25 degrees latitude and completely above 10 km AGL formed in the baroclinic gradient between the cold tropical tropopause and UTLS near the equator and the less cold UTLS near the tropical tropopause fold. We are investigating the role played by this wave train in the homogenization and perhaps poleward spread of the tropical UTLS and the role that may have played in setting the stage for the following 3-4 weeks of unusually strong hurricane activity.