Static Stability and the 300 mb Isotach Field in the Colorado Cyclogenetic Area

Abstract

The static stability and 300 mb isotach fields for 102 cases of Colorado cyclogenesis during spring and fall are studied. The data sample is divided into cases that develop into either major or minor storms (developing cases) and casts which fail to develop into significant cyclones (non-developing cases). Two stability indices are used: the 800?300 mb temperature difference and a slightly modified form of the Showalter index. The seasonal means, based on a sample of 915 days, show relatively weak stability in the lee of the southern Rockies, which is accentuated in developing cyclones situations. The mean 300 mb isotach field for conditions that lead to developing cyclones shows a well developed jet core propagating eastward into the Colorado region. Contingency tables relating the 300 mb wind speeds at two upstream stations to static stability at two downstream stations show a statistically significant joint occurrence of strong 300 mb winds and weak stability in cases of developing cyclones. Student t tests further indicate that the sample means of the static stability indices are highly significant for both developing and non-developing cyclones. The sample means of the 300 mb winds are statistically significant for developing cases but not for non-developing ones. It is postulated that weak stability in the lee of the southern Rockies is an important ingredient in cases of both developing and non-developing cyclogenesis; however, for developing cases a strong 300 mb wind maximum is also present.