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• Apr 22, 2008 11:04 am US/Central
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  What Does "Slight Risk For Severe Weather" Mean?

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What Does "Slight Risk For Severe Weather" Mean?

FORT WORTH (CBS 11 News) ― You may have seen your favorite CBS 11 Storm Team meteorologist use the term "Slight Risk" before. We often say that, "The Storm Prediction Center has placed North Texas in a Slight Risk for severe weather."

But what does that really mean?

Well, the Storm Prediction Center (SPC) is in Norman, Oklahoma. It is a weather office that is responsible for forecasting severe weather throughout the entire country. This is the office that issues Severe Thunderstorm Watches and Tornado Watches. This office also forecasts the possibility of severe weather happening in a specific area over the next couple of days.

They use three categories to define areas that might experience severe weather. Those categories are: Slight Risk, Moderate Risk and High Risk. There is no specific definition for each risk, but they are roughly based on the probability of severe weather happening in a specific area.

Click here to see an example of the SPC placing parts of Oklahoma, Kansas and Missouri under a slight risk for severe weather. Note also that the coastal area of North Carolina is also under a slight risk for severe weather.

As mentioned, there is no specific definition for each Risk ranking, but it should be recognized that a High Risk carries a higher probability of severe weather occurring in the area outlined.

Allow me to quote from a research paper on this topic.

In principle, these definitions are associated with areal coverage of severe weather events ... and, as such, have some relationship to probabilities. However, as these categories are used, some influence comes from the perceived intensity and type of the anticipated severe weather. A day that is expected to produce a major tornado outbreak is likely to get a higher risk category than a day with an expectation of comparable areal coverage of primarily non-tornadic severe weather. Thus, the single word descriptor is being made to serve several different purposes in actual practice.
-- Probabilistic Forecasting II; Chuck Doswell and Harold Brooks

To state it another way, if an area goes under a "High Risk," then the probability is higher for a significant severe weather outbreak likely involving torandoes. If there is a "Slight Risk" issued for an area, there is still the risk for tornadoes, but the severe risk is not as high as it would be with a "High Risk."

Unfortunately, there is no "30 percent chance of severe weather" associated with these Slight/Moderate/High Risk rankings. So, we can't give a percentage chance of severe weather happening. We, of course, do give rain chance forecasts, but we can't give a percentage chance for severe weather.

It can also be a bit deceiving when the word "slight" is used. This should not diminish the threat of severe weather. In fact, if a Slight Risk is issued, it should alert you to a higher than normal probability of severe weather happening.

For a more in-depth explanation of the Slight/Moderate/High Risk categories, please visit this website provided by the Storm Prediction Center.

(© MMVIII, CBS Broadcasting Inc. All Rights Reserved.)

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