The National Weather Service has announced changes in the way the Storm Prediction Center will publish advanced alerts about the possibility of severe weather. The changes will be effective during the upcoming 2014 severe weather season. The changes do not directly affect the local watches and warnings that are broadcast, such as tornado watches, severe thunderstorm watches, severe thunderstorm warnings and tornado warnings. Instead these categories are often reviewed by meteorologists, storm spotters, and storm chasers in advance of arrival of the severe weather.
The old and the new with “Marginal” and “Enhanced.”
Risk categories will range from marginal to slight to enhanced to moderate to high.
A marginal (new) risk area surrounds a slight risk area, and marks the lowest risk area in an area of severe weather risk.
A slight risk area defines an area where threat exists for scattered severe weather, including scattered wind damage or severe hail and possibly some isolated tornadoes. During the peak severe weather season, most days will have a slight risk somewhere in the US. Isolated significant severe events are possible in some circumstances, but are generally not widespread.
An enhanced (new) risk area surrounds a moderate risk area and is a higher risk than the slight risk area, and a lower risk than the moderate risk area.
A moderate risk area defines an area where more widespread and/or more dangerous severe weather is possible or likely. Numerous tornadoes (including some strong tornadoes), more widespread or severe wind damage and very large/destructive hail could occur. Major events, such as large tornado outbreaks, are sometimes also possible on moderate risk days, but with greater uncertainty. Moderate risk days are not uncommon and typically occur several times a month, especially during peak season. A slight risk area typically surrounds a moderate risk area, where the threat is lower.
A high risk area defines a considerable likelihood of a major tornado outbreak or (much less often) an extreme derecho event. On these days, the potential exists for extremely severe and life-threatening weather, including widespread strong or violent tornadoes and/or very destructive straight-line winds (Hail cannot verify or produce a high risk on its own, although such a day usually involves a threat for widespread very large hail as well). Major events, such as large tornado outbreaks are possible or even likely in these areas. Many of the notorious severe weather days* have been high risk days. Such days are quite rare; a high risk is typically issued only a few times each year. High risk areas are usually surrounded by a larger moderate risk area, where uncertainty is greater or the threat is somewhat lower.
In Spring 2014, the National Weather Service Storm Prediction Center intends to broaden this system to add two new categories to the three that already exist. The additions effectively increase the gradations of risk of severe weather.
In summary the risk increase would be as follows (from lesser risk to greater risk):
The word “enhanced” does not seem intuitive in this gradation scheme. Some in the general public may perceive “enhanced” as being a higher risk than “high.”
*The Washington, Illinois tornado was in “high” category on November 17, 2013 by the old system.
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