Revisiting the Isochrones

One of the innovations introduced last year was the drawing of isochrones, or lines of equal time. These isochrones indicate the start time of the four-hour period where 95% of the reports at a point were expected to occur - each point has one four-hour period. This year, to aid in the drawing of the isochrones, participants now also draw hourly report coverage areas each hour from 18Z to 03Z. The final product looks something like this forecast from 25 May 2017:

The above image says that the area to the east of the 19Z line will see its most severe weather from 1900 UTC - 2300 UTC, and the areas east of the 23Z line will see the peak severe weather from 2300 UTC - 0300 UTC the following day. Ideally, these lines will be displayed on the 15% threat area (the "slight" risk equivalent) to determine the eastern bound of the final line - currently participants and the NSSL desk lead have this forecast as a background when drawing their isochrones.