The Unusual Behavior and Precipitation Pattern Associated with Tropical Storm Ignacio (1997)

Abstract

Abstract A case study of eastern North Pacific Tropical Storm Ignacio (1997), which brought rainfall to the southwestern United States as a tropical cyclone and to the northwestern United States as an extratropical cyclone, is presented. This tropical cyclone formed from a region of disturbed weather, rather than a tropical wave, outside the typical eastern North Pacific genesis region and intensified into a tropical storm coincident with the passage of an upper-tropospheric trough. Moisture transported from Ignacio along an outflow jet associated with the trough resulted in precipitation in Mexico and the southwestern United States. As Ignacio moved north and away from the trough, this tropical cyclone weakened and eventually underwent extratropical transition over the open ocean, in contrast to climatological eastern North Pacific tropical cyclone behavior. Ignacio then strengthened as an extratropical cyclone due to favorable baroclinic conditions and the passage of another upper-tropospheric trough before making landfall on the northern coast of California, bringing rain to the northwestern United States. Ignacio’s remnant moisture eventually merged into a slow-moving midlatitude low pressure system that developed after interacting with the extratropical remnant of Hurricane Guillermo.