Probable Cause of a Significant Precipitation Episode in the Western United States

Abstract

This synoptic study for the period 22 September to 3 October 1974 involves a case analysis of an unforeseen major precipitation episode. This apparently resulted from the merging of an inactive propagating short?wave trough that moved cast-northeastward out of the subtropics and an inactive extratropical low moving southeastward. Prior to the amalgamation of the two systems, weak upward vertical motion fields allowed only specks of high clouds to be associated with each. Almost immediately with the merging of the two systems, a rapid increase in upward vertical motion around the upper-level extratropical system took place, as evidenced on VHRR satellite imagery by the sudden development of rather organized middle and high cloudiness. Within 24 hours, a distinct vortex comprising all cloud levels was evident on satellite photographs. During the next 24 hours, a major rain-producing storm developed. The major impact of this storm was an abrupt end to the California dry season.