Slantwise convection occurs at times in the atmosphere, resulting in mesoscale bands of moderate to heavy snow in the winter. Such convection results from the release of conditional symmetric instability (CSI). One way to diagnose the presence of CSI is to examine the surfaces of geostrophic momentum ( Mg ) and equivalent potential temperature ( ) in a cross section, and determine the regions where the slope of the latter is greater than that of the former. (Technically, this provides regions of PSI (potential symmetric instability), but we will refer to them as regions of CSI for the purposes of this document.) This criterion is complicated to apply, however. Another more direct way to determine regions of CSI is to use the equivalent potential vorticity (EPV) (Moore and Lambert, 1993; McCann, 1999). Regions where EPV < 0 are those in which the atmosphere has conditional static instability, conditional symmetric instability (CSI), or inertial instability. Inertial instability rarely if ever occurs on the synoptic scale at mid-latitudes, so we consider that in regions of negative EPV, parcels will be accelerated away from their original positions due to the other two instabilities. If EPV < 0, accelerations due to CSI will predominate only if the atmosphere has conditional static stability. In other words, by definition, regions where EPV < 0 in a conditionally stable area are regions of CSI.

These ideas were discussed in the MSC/COMET Winter Weather Course that took place at COMET in Boulder in February, 2002. As pointed out by several of the course participants, many Canadian weather centres have no way to diagnose CSI. As a result, it was decided to develop a chart that could address this issue. This was done at the Canadian Meteorological Centre in early March, 2002.

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