From 1986-2011, the annual national POD increased from ~0.30 to ~0.75 without significant changes in the FAR. Over that time period, the mean annual lead time for tornadoes that had warnings issued prior to the tornado was effectively constant at 18.5 minutes, as was reported by Erickson and Brooks (2006). Large changes took place beginning in 2012. The POD dropped by ~0.15 and the FAR dropped by ~0.06. Lead time for warned tornadoes decreased to approximately 15 minutes. Coupled with this, warning duration and warning area decreased. These changes in performance, at a time when no official change in warning procedures took place, are consistent with a raising of the threshold for the weight of evidence to issue a warning (Brooks 2004). Warning forecasters have been behaving in a manner that seems to emphasize limiting false alarms. Because of the relative rarity of tornadoes, changes in the FAR are associated with changes in the POD that are almost twice as large. Thus, large decreases in the POD are concomitant with small decreases in the FAR.
We have also looked at warning performance as a function of the damage rating and length of time the tornado lasted. In general, performance is better for stronger, longer-lasting tornadoes. Paradoxically, the answer to the question of whether official performance goals are met is dominated by performance associated with weak, short-lived tornadoes.