Analysis of "Re-Alert" Areas Resulting from Free Form Polygon-Based Convective Warnings

As the Meteorological Service of Canada progresses down the path of introducing free form polygon (FFP) based warnings to its summertime convective warning program, authoritatively informing the public of the potential threat is crucial. With the new FFP-based warning system, the public will be notified when a warning moves over their location and when all warnings of the same type are ended for their location (i.e. all clear). Since the size and orientation of the warning polygons are designed to be proportional to the velocity vector of the storm, variations in the warning shape are expected as the storm evolves. Undesirably, with these natural variations, areas near the edges of an active warning polygon are prone to repetitive warning notifications being sent to the public as the warning gets updated. These areas are undesirable since sending a series of alternating notifications indicating a person is either in or out of an alert has the potential to create confusion. We refer to areas where the alternating alerting notifications occur as “Re-Alert” areas. If the public is subjected to re-alerting frequently enough, the public’s trust in the agency’s ability to accurately forecast severe threats may be diminished.

Since FFP-based warnings that move with the storm threat are a new concept in Canada, the rate of occurrence of re-alert areas is unknown. This talk will describe the causes of re-alert areas and preliminary findings from the analysis of FFP-based warnings. In addition, we will present potential impacts on various dissemination platforms and strategies being explored to mitigate them.