Understanding the new severe weather outlook categories

After years of debate, misuse, and misunderstanding, the Storm Prediction Center has tweaked their severe weather outlook to help all of us have a better understanding of our risk when it comes to severe weather. For more than a decade, the National Weather Service has used a 3 risk category assessing the daily weather patterns for the potential for severe storms; slight, moderate, & high. 90% of the time, when any risk was issued, it was just a slight risk. About 9% of the time, a moderate risk was issued for the potential for not only more widespread severe weather in the outlook area, but also for the potential for outbreak type scenarios. The final 1% were saved for the worst of days, a high risk, when forecaster confidence and Mother Nature would come together to produce some of the most violent weather on the planet.

 

Example of old risk system with last Thursday's outlook

But unfortunately, these 3 categories are still pretty vague… especially slight risk. Areas that deal with severe weather all the time would end up with 30, 40, or even 50 slight risk days a year. Some of these days produced just a couple severe weather reports, while others produced hundreds. It was such a broad based category that had little meaning to most of us. But the SPC is improving this system this year adding two additional categories to give more meaning to your risk if you are placed into one. The tier is as follows; marginal, slight, enhanced, moderate, & high. This gives better definition to what your risk would be. For example, the lowest category is now marginal. This means that thunderstorms are possible, some of which could be stronger, but quite likely all of them will stay under severe thunderstorm criteria, meaning wind gusts will be less than 58mph, hail will be less than 1" in diameter, and there is no tornado risk. Now this doesn't mean that storms can't pack a punch with say 50 mph winds, penny size hail, and torrential rains. It also doesn't mean that a stray storm can't go severe, especially if there is a slight risk issued nearby. But it lends itself to a better understanding because in previous years, this same area would be listed under a slight risk like the picture above. But now, it looks like the picture below…

 

Example of new risk system with last Thursday's outlook

This is the severe weather outlook for Thursday with a marginal, slight, and enhanced risk from parts of Texas all the way up to the Chicago area. But because of the additional categories, this gives you an idea of where the better opportunity for severe thunderstorms are located. In previous years, all 3 categories would be consolidated into the slight risk wording like the first picture at the top of the page.

Still confused?? The infographic courtesy of the National Weather Service office in Kansas City can be seen below with a pretty simplistic explanation on the overall risk for each category…

 


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