EF scale guidelines revised by NTP
The NTP has made a number of minor revisions to the Canadian implementation of the EF scale. These minor changes were required because of recurring issues with the use of some damage indicators - issues that had been preventing those DIs to be used reliably. The changes are noted in the revised document using a red font.One of the key changes is the inclusion of the Scalable Box Method for determining the Degree of Damage for the Trees damage indicator (C-T). This method has been used by the NTP for a number of years, and has been applied to historical events as well, but the method was not described within the EF scale document itself. The method now appears in Appendix A. Note that this method was designed to approximate the past practice for the determination of an F/EF-scale rating from tree damage, so there is not expected to be a significant step change in the historical record.
Also noteworthy is that the NTP has specified the conditions under which in-situ wind observations can be used to determine an EF scale rating. Wind observations of tornadoes are extremely rare, but have been used in the past to rate an event, or support a rating based on other damage indicators. However, there were no guidelines in place setting out the best practices for doing so. This is addressed in Appendix B of the document.
The revised document describing the Canadian implementation of the EF scale can be found here.
