Storm-Based Warnings: A Local Case Study

Posted October 15, 2008 12:51 p.m. EDT
Updated November 2, 2009 9:23 a.m. EST

A tornadic thunderstorm on August 27.  The storm prompted a new "storm-based" warning that covered a very small area, compared to how large an area would have been warned under the "county-based" scheme.

On the afternoon of August 27, 2008, a number of strong and severe thunderstorms rumbled across WRAL-TV viewing area.  One storm in particular makes a great example of why storm-based warnings can help prevent over-warning people and the "cry wolf" effect.


The storm in question developed early in the afternoon in northern Johnston county, just north of Selma.  As it was moving northeast, it began to show signs of rotation that suggested a tornado was developing.  To alert people to the danger, the National Weather Service issued a tornado warning for that storm and the area most likely to be affected.  Since the storm was small and moving fairly slowly, the area likely to be affected by that tornado was relatively small as well.


With "storm-based warnings", the National Weather Service is able to mark off just that small area, warning only those in the direct path of that dangerous storm.  In this case, the warned area encompassed only about 177 square miles and affected about 10,000 people.

Under the old, "county-based" warning scheme, that same storm would have resulted in at least four, if not five counties being warned.  Since the storm was near where Wake, Franklin, Johnston, Nash, and Wilson counties all come together -- they don't call it "Five County Stadium" for nothing! -- the warning would have had to have been issued for all five of those counties.  That covers 3,000 square miles and would have affected nearly a million people!


Storm-Based vs.County-Based Warning

Warning TypeArea affectedPeople Affected
County-based~ 10,000 mi2950,000
Storm-based177 mi210,000

In this example, if we were using the "county-based" method, we would have warned more than 900,000 people for a storm we knew from the outset would not affect them.  For those folks, it was a guaranteed miss, a false alarm.  We call that area the "false alarm area", and it's those folks that storm-based warnings were created to help.  Instead of being needlessly warned, they aren't alerted at all because they weren't going to be affected.


It's in everyone's interest to warn only those affected by a severe thunderstorm or tornado.  When warnings are issued, some things are mandated to happen -- school children file into their safe places, factories shut down, and so on.  Each of those has an impact, not the least of which is economic.  A recent study suggested that a 75% reduction in the size of warning area could save our economy $100 million per year in lost business, wages, and productivity.