The National Weather Service has changed how it warns Illinoisians about tornadoes.
The U.S. was ravaged by tornadoes two years ago. Record numbers of tornadoes - including several deadly ones - occurred in many areas of the country. More than 550 people were killed by tornadoes, including 158 in Joplin, Mo., alone.
Nearly all of the deadly 2011 tornadoes were preceded by NWS tornado warnings - some with 20 to 30 minutes of advance notice or more. However, the death tolls were extremely high, despite warnings being issued through multiple communications methods.
To address various questions and findings in the wake of the Joplin tornado, the NWS offices in Kansas and Missouri experimented with enhancing severe weather warnings in 2012.
The 2012 experiment yielded some successful results; however, because of widespread drought there were a limited number of cases. The decision was made to expand this experiment to all NWS offices in the central U.S., including the NWS office in Lincoln.
The "enhanced" severe weather warnings - or Impact Based Warnings - are intended to give the public, emergency managers who activate outdoor warning sirens and the broadcast media, more information about the severe weather threat.
NWS tornado and severe thunderstorm warnings will look similar; however, the information will be streamlined including the addition of a new "Impacts" section.
The Impacts portion of the warning is intended to describe what type of damage can be expected from the warned storm.
The intended outcomes of providing extra information in the Impact Based Warnings are to improve NWS communication of critical information, to make warnings easier to read and identify the most important information, to highlight storms that are particularly dangerous, to provide different levels of potential storm impacts within the same product, and enable people to prioritize warnings in or near their areas of interest.
The Impact Based Tornado Warnings will be sent from the NWS in the same manner as in the past, so no changes are needed to weather alert radios or computers/mobile devices programmed to receive these messages. The information within the warning, though, will look and sound a bit different.
With respect to tornado warnings, there are three possible impacts that will be communicated:
Tornado damage is possible within the area of the warning. The duration of the tornado is generally expected to be short-lived. Based on the tornado climatology of central and eastern Illinois, nearly 75 to 80 percent of Illinois tornado warnings will be like this.
Credible evidence from trained storm spotters and radar indicates that considerable tornado damage is imminent or on-going, and the tornado duration is expected to be long lived. These are fairly rare in central and eastern Illinois, occurring with about 20 percent of the tornadoes.
Catastrophic damage from a tornado is occurring and there is a severe threat to human life, and the tornado duration is expected to be long lived. This will be "exceedingly rare," the NWS said, and only used when reliable sources confirm a violent tornado. This type of tornado has only occurred eight times in central and eastern Illinois the past 63 years.
In addition, severe thunderstorm warnings will be enhanced in the Impact Based Warnings Experiment. With many severe thunderstorm warnings, the primary threat is damaging straight-line wind, severe wind from a downburst, and/or very large hail.
However, sometimes, short-lived tornadoes can rapidly develop and cause enhanced damage within an area of high wind. In this case, the severe thunderstorm warning will indicate that there is some potential for a short-lived tornado by stating a tornado is "possible."
This will be used when the available radar and storm spotter information does not indicate a widespread, long track tornado threat, and that a brief tornado touchdown may occur. For more information about Impact Based Warnings, visit www.crh.noaa.gov/crh/?n=2013_ibw_info.