Recurving eastern North Pacific tropical cyclones

Most eastern North Pacific tropical cyclones form off the Mexican coast and track due west into the central Pacific bothering nothing but the ocean flora and fauna. However, a few tropical cyclones that form close to the Mexican coastline in the later part of the hurricane season track anomalously towards the north around the western edge of the southward retreating subtropical high and/or in association with a digging mid-latitude trough off the coast of California. Approximately every other year, one (or more) of these tropical cyclones (or its remnants) survives to the Mexico/United States border and accounts for 25-100% of the warm season precipitation over the southwest United States. Furthermore, about half of these systems maintain their tropical moisture and low-level vorticity signatures and can be tracked thousands of kilometers downstream over the continental United States.

Analysis of these long-lived systems using a dynamic tropopause perspective reveals a rich array of synoptic- and large-scale flow patterns associated with their introduction to, and interaction with, the mid-latitudes. The exact nature of the interaction depends on the strength and scale of both the tropical cyclone remnant and the mid-latitude feature it interacts with, the distance between the systems, and whether the large-scale pattern is progressive or highly amplified and blocked. These interactions include significant amplification of the downstream flow pattern over the continental United States, becoming wrapped up in a developing mid-latitude system, or remaining as a separate vorticity maximum and exhibiting the classic signatures of extratropical transition. Representative examples of each of these instances will be explored in my presentation as well as the implications of these recurving tropical cyclones on mid-latitude weather forecasting.