Used with permission, Jeff Koterba/Omaha World-Herald.

Does your mother know the difference between the damage that can be done by 58 mph winds and 91 mph winds? I didn’t think so. Everyone knows that 91 mph is worse than 58 mph, but Mom probably doesn’t know how much worse. She’s not alone. And that’s part of the reason so many people were surprised by how much damage the June 29, 2012 derecho did.

When a thunderstorm produces winds of at least 58 mph the National Weather Service (NWS) issues a severe thunderstorm warning. The June 29 derecho produced a measured wind gust at Fort Wayne International Airport of 91 mph. What does NWS do when thunderstorm winds reach 91 mph? It issues … a severe thunderstorm warning.

Beginning April 1, 2013, the NWS Northern Indiana weather forecast office will join all other offices in the NWS central region in an experiment to test of a new type of storm warning. “Impact-based warnings” are designed to help people understand the difference between barely severe storms and storms like the one we had June 29, by indicating what the storms might do.

For example, at 3:09 p.m. June 29, NWS issued a severe thunderstorm warning for several northwestern Ohio counties (Example 1). It included the following text: “This storm has a history of producing destructive winds in excess of 80 mph. Seek shelter now inside a sturdy structure and stay away from windows!” Notice that the warning mentioned a wind speed and action to take but said nothing about what that wind might do.

Had impact-based warnings been in effect that day, the same warning would have included language like this: “Impact … damage to vehicles…buildings…roofs and windows. Trees uprooted and large branches up to 9 inches in diameter down.” (Example 2). Impact-based warnings will also tell us whether a storm is indicated by radar or actually observed by humans.

The impact-based warnings experiment includes three forms of tornado warnings, based on the storm’s damage threat. Tornadoes with “considerable” and “catastrophic” damage threats will have new “tags” at the very bottom of the warnings, for example, “tornado damage threat … considerable” and “tornado damage threat … catastrophic.” Note that NWS has been including tags at the bottoms of warnings for the past three to five years (they’re highlighted at the bottoms of the example warnings above). The impact-based warnings experiment expands on these tags to enable users and automated systems to glean information more quickly.

A basic tornado warning will have no damage threat tag. It will include impact language such as, “Significant house and building damage possible.” The vast majority of tornado warnings issued by the Northern Indiana NWS office will look like this.

A “damage threat considerable” tornado warning will include impact language such as, “Major house and building damage likely and complete destruction possible.”

A “damage threat catastrophic” tornado warning will include impact language such as, “This is a life threatening situation. You could be killed if not underground or in a tornado shelter.” The NWS will rarely use the catastrophic language, saving it for storms like the 2011 Joplin, Missouri tornado.

More information about impact-based warnings, including example tornado warnings and a map of NWS offices participating in the experiment, is available on the NWS website.

When a warning is issued, it will be important to access the entire text of the warning. You can do so via NOAA Weather Radio, the Web and some email services. Make sure your friends and family also know how to get the full text of warnings, so they’ll benefit from the impact statements.

Impact-based warnings will make detailed, timely reports from SKYWARN storm spotters even more important. More than ever, NWS will need to know what storms are doing near you; exactly what they’re doing. Did tree limbs come down? How big? Did you see structure damage? Was it just a few shingles blown off or part of a wall blown down? And NWS meteorologists will need that information immediately, especially when a storm system moves as fast as the June 29 derecho did (60 mph). The tornado damage threat tags are especially dependent on spotter reports. Radar data alone is insufficient for NWS meteorologists to determine appropriate damage threat tags.

If we provide the information NWS needs, when it needs it, impact-based warnings could lead to fewer comments like one often heard after the June 29 derecho: “I had no idea it would be that bad.”

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