The new chart covers the “northern latitudes” of North America, in other words, Alaska, Canada and the northern part of the U.S. lower 48. The basic field on the chart, in a lavender colour, is the area where EPV < 0 somewhere in the 700-500 mb layer, based on the GEM regional forecast model. Progs are produced for 6-hour intervals from 00 hr to 48 hr. The 700-500 mb layer was chosen since forecast experience, at least over western Canada, suggests that the bulk of the CSI will occur at levels above the height of the terrain where the geostrophic approximation applies. However, a second set of charts, for the layer 850-700 mb, is also produced to account for the possibility of CSI at lower levels, particularly over areas where the terrain is lower than in the West.

According to Moore and Lambert (1993), “the release of CSI requires a near-saturated environment with a lifting mechanism.” As a result, the CMC EPV charts include, in pale blue, regions where the model forecasts EPV < 0 and upward vertical motion. A third colour, pale green, defines those regions where the first two criteria are satisfied, and RH > 80% (the instability is conditional on adequate humidity). These fields are presented in an attempt to define where CSI has the possibility of being released.

As discussed in the background section of this document, regions where EPV < 0 can contain areas of ordinary conditional instability (possibly leading to vertical convection), as well as areas of CSI. Where the two types of instability coexist, the ordinary conditional instability and vertical convection will predominate. The CMC EPV charts present, in yellow, areas where EPV < 0 and where the GEM regional model has produced convective (as opposed to stratiform) precipitation in the three hours leading up to the forecast time (except for the 00 hr prog, in which the three hours following that initial time are used). The yellow region is therefore a first attempt to define areas of vertical conditional instability, where CSI will not be the primary precipitation mechanism. The forecaster must, of course, be aware of possible problems in the model’s forecast of convection and compensate as necessary.

Western Canadian meteorologists who have some experience with CSI suggested two further fields for the CMC chart: areas where the EPV < 0 layer is thicker than 50 mb (outlined with a black contour) and those where the EPV < 0 layer is thicker than 100 mb (outlined with a red contour). These layers attempt to define where CSI may exist in over sufficiently thick layers to produce precipitation.

All these fields are briefly described in the chart labels. The charts are produced at the end of the GEM regional run, and should be available at roughly 1530Z (day) and 0330Z (night). The charts are considered experimental, and execute in a personal account at CMC. They are not part of the GEM regional run, and are not supported operationally, so they may be missing at times, though recent experience suggests they will almost always be available.

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