Relationship of the Subtropical Jet Stream to Severe Local Storms

A series of five case histories illustrates the apparent effect of the subtropical jet stream on severe local storms. In these cases, the polar and subtropical jet streams are in a diffluent configuration to the east of an advancing trough. Severe storms, particularly tornadoes, are shown to occur between the jet streams and ahead of the surface front. The central observation is that the growth of vigorous squall lines and severe weather are sharply inhibited at and to the south of the subtropical jet. Thunderstorms do occur south of the subtropical jet, but rarely produce severe weather in this synoptic configuration. Speculations are presented which suggest how the subtropical jet might influence divergence, stability, and even surface heating to contribute to an environment favorable for severe storms to the north, but unfavorable to the south of the jet axis.