A Slantwise Showalter Index Based on Moist Symmetric Instability: Results for Central Alberta

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Abstract
The Slantwise Showalter Index (SSI) is defined as follows: A parcel from 85 kPa is lifted along a surface of constant angular momentum dry adiabatically until saturation occurs, and then pseudo‐adiabatically to 50 kPa. The slantwise lifted 50‐kPa temperature is then subtracted from the observed 50‐kPa temperature. A negative value for SSI indicates the potential for moist symmetric instability. Since a deep layer of moist‐symmetrically unstable air is likely to produce significant overturning, we explored the usefulness of SSI to indicate conditions of steady rain. A complete year of twice‐daily soundings from Stony Plain, Alberta, provided the experimental dataset. During the winter, SSI values were positive indicating that the 85–50 kPa layer was moist‐symmetrically stable, with very few exceptions. During the summer, however, about 5% of the soundings indicated deep layers of unstable stratification with respect to slantwise overturning (i.e. negative SSI values). Negative SSI values, combined with relative humidity values ≥ 60% at low levels, proved to be useful for identifying a few of the observed summertime rain events.