Large-scale moisture and thermodynamic properties associated with rapid weakening in the eastern North Pacific

It is well-known that sufficiently warm sea surface temperatures, mid-level tropospheric moisture, weak vertical wind shear, and conditional instability are conducive to tropical cyclone development. These conditions generally exist in a portion of the eastern North Pacific, which boasts the greatest genesis density of all tropical basins in the world. However, the sharp SST gradient northwest of the main genesis region in this basin, in conjunction with low relative humidity and increasing wind shear, presents a combination of conditions that are hostile to tropical cyclones, and most systems that move into this region rapidly weaken. Some storms drop from major hurricane to tropical storm status within 24 hours despite never moving over land.

In order to investigate the environmental conditions associated with rapid weakening (RW) in this basin, moisture and thermodynamic parameters are calculated and examined using reanalysis and available observational data. Monthly CAPE and MPI are computed at each grid point, and the resulting patterns are explored in light of seasonal shifts and the influence of ENSO. Individual cases are also investigated. Results that may improve future intensity forecasts in RW-conducive conditions will be presented.