Heavy Rains and Flooding in Montana: A Case for Operational Use of Symmetric Instability Diagnosis

Abstract

A strongly developing cyclone east of the Rockies moved northward along the Montana/North Dakota border between 24?26 September 1986 and deposited over 1 in. of rain throughout much of eastern Montana. Included in this area of precipitation was an embedded heavy band, with rainfall amounts exceeding 7 in. over a 36-hour period. Often when significant banding exists across a relatively homogeneous precipitation area, symmetric instability may exist. Slantwise convection associated with this area of instability may be the source of the precipitation banding. Conceptual models are described, based on available synoptic data and computer derived fields, which suggest the frontogenetic dynamics necessary to utilize the symmetric instability in the form of banded convection. The potential for operational use of symmetric instability concepts are discussed with respect to today's and the future operational environment.