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What does a "slight" risk of severe t-storms mean?

9:05 PM, Feb 19, 2014   |    comments
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You often hear us mention a "slight," "moderate," or "high" risk of severe weather.  What exactly does that mean?

The Storm Prediction Center issues what it calls "Convective Outlooks" whenever there is a threat for severe weather.  The outlooks are issued for Day 1, Day 2, Day 3, and an outlook for day 4 through 8.  These maps will show an area colored in to match with slight, moderate, or high risk.  The SPC factors in a lot of ingredients in the atmosphere and at the surface to determine these areas that are colored in on the maps.  This is a way to easily show the areas most likely to see severe weather.

I've had numerous questions over the years asking what those slight, moderate and high risks mean.  A "slight" chance is actually a pretty good chance that you could see severe weather in the outlook area.

Here's how the SPC explains the differences:

 "A SLGT risk implies that well-organized severe thunderstorms are expected but in relatively small numbers/coverage, or a small chance of a more significant severe event. Not all severe storm events will be covered with a SLGT risk, especially during the summer when short-lived, "pulse-type" severe storms are relatively common during the afternoon.

A MDT risk implies a greater concentration of severe thunderstorms, and in most situations, greater magnitude of severe weather and greater forecaster confidence compared to a SLGT risk. A MDT risk is usually reserved for days with substantial severe storm coverage, or an enhanced chance for a significant severe storm outbreak. Typical MDT risk days include multiple tornadic supercells with very large hail, or intense squall lines with widespread damaging winds.

The HIGH risk implies that a major severe weather outbreak is expected, with large coverage of severe weather and the likelihood of extreme severe (i.e., violent tornadoes or very damaging convective wind events). The HIGH risk category is reserved for the most extreme events with the least forecast uncertainty, and is only used a few times each year."

I also showed the map earlier that breaks down the risk areas.  It also has different colors with probabilities in percentages.  I had a question on Facebook asking if the 15% chance in our area means that there is only a 15% chance of severe storms.  Here's how the SPC explains those probabilities:

"The probability values represent the chance of severe weather within about 25 miles of a point, which is about the size of a major metropolitan area. Though severe storms tend to receive a large amount of media coverage, severe weather is uncommon at any one location. Your chance of getting a tornado on any random day are very small, climatologically speaking. Put in that context, even a 10% chance of a tornado within 25 miles of a point means a much bigger threat than usual, and should be taken seriously. Think of how often tornadoes normally happen close to you on any given day, and those small-looking probabilities start to seem large by comparison!"

I hope this explains it a little more.  We will be seeing these maps a lot over the next few months as we enter the peak of storm season in March, April and May.

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