Are there too many severe thunderstorm and tornado warnings? This has been a big question in recent days. It is a complex issue that encompasses many issues. There are many variables that relate to technology, data and societal response.
Between 1986 and 1993, the National Weather Service in Birmingham issued an average of 31 tornado warnings and 183 severe thunderstorm warnings each year, or 214 total. Between 1994 and 1997, those numbers jumped dramatically. During that four year period, the average number of tornado warnings issued by the Birmingham office rose to 92 and the average number of severe thunderstorm warnings rose to 401 each year.
What led to this jump? Mostly likely, it was the implementation of the WSR-88D Doppler radar network. The Maxwell radar was commissioned in February 1993, the Birmingham radar started operating in May of 1994, and the Hytop radar went into service in July of 1997. Doppler makes it easier to see mesocyclones and tornado signatures, hence more warnings. One would expect there to be more false alarms, but actually the number trended downward from about 88 to 86 percent. The probability of detection or the percentage of tornadoes that occurred within a tornado warning, rose from 56 to 62 percent.
Since 1998, the numbers have skyrocketed even more. During that period, Birmingham issued an average of 141 tornado warnings each year and 463 severe thunderstorm warnings, an average annual total of 606 warnings.
The false alarm ratio (FAR) trend has continued to drop from 86 to 82%. Meanwhile, the probability of detection (POD) trend has risen from 62 to 80%. That means that 80% of the tornadoes occur within a tornado warning.
So there are more warnings. But there also more tornadoes. A lot more of them. The average number of tornadoes in Alabama each year has risen from around 17 to around 55 during the past eleven years. One reason is that Doppler radar data makes it easier for NWS meteorologists to know where to look for potential tornado damage after the storms pass.
There are other factors, like the tremendous rise in interest in spotting and chasing. Technology such as cell phones, digital cameras and video and the internet makes reporting storms easier. While there are more tornadoes overall, the number of significant (F2 or greater) tornadoes has remained about the same.
In 2008, the Birmingham office’s FAR was 81%, which was above the NWS goal of 74%. So that percentage was high. But the POD for BMX was 76%, well above the national goal of 67%. The story is that the Birmingham office does have a high false alarm rate, but they don’t miss many tornadoes.
So it comes down to philosophy. Would you rather have fewer warnings if it means missing some of the smaller tornadoes?
The Birmingham NWS says no. Under Meteorologist in Charge Jim Stefkovich, the staff at BMX takes the approach that they want a higher warning accuracy at the expense of having a higher false alarm rate. Sure they want to lower the false alarm rate, and they have posted some impressive numbers so far in 2009.
More training, research programs like VORTEX2, phased array radar and enhanced computer algorithms for the WSR-88D will help BMX and all NWS office lower the FAR and increase the POD. Probabilistic warnings and other strategies will help people make better personal decisions. I applaud Jim and his team. I think they are on the right track.
When it comes to warnings, err on the side of caution. There is a lot more fussing when a tornado hits and there was no warning.
That’s my opinion…what’s yours?
