The Storm Prediction Center has used the same risk categories for a large part of most of our meteorological careers. Although they have adjusted the probabilities, percentages, and yes even the colors — the categorical risk areas have remained the same. In Spring 2014, that will change. The Storm Prediction Center currently uses a simple categorical risk system which highlights the potential for severe thunderstorms across the country. The risk areas of Slight, Moderate and High are triggered by percentage chance of a type of severe weather within 25 miles of any point. And while the newer outlooks also highlight the potential for general non-severe thunderstorms across the country, Spring 2014 will mark a dramatic change in the outlooks.
The main change will be the addition of two risk levels, both falling on opposite sides of the “Slight” risk of severe thunderstorms. “Marginal”, a new risk category, will highlight the potential for severe thunderstorms that includes a 5-10″ chance of wind and hail and a 2-5% chance of tornadoes — but does not trigger a slight risk. “Enhanced” falls on the other side of “Slight”; in other words the risk levels are enhanced (30-45% risk) but not organized or widespread enough to trigger a Moderate Risk. Below, the Storm Prediction Center has released a new probability to categorical description table to help understand the changes.
In addition to the added risk categories, the Storm Prediction Center will remove the “See Text” category from their outlooks. Used since the 90’s or earlier, See Text was placed on the graphical outlooks to alert users that a percentage risk area for thunderstorms was triggered, but it was too low to trigger a Slight Risk. So the user could scroll to the text discussion for more information. The new outlooks “Marginal” category does a fair job in replacing the old See Text descriptor.
In general, these changes seem to positively favor the interpretation of the Storm Prediction Center’s outlooks. But for some, the additional categories will just create more noise on the outlook graphics. There will now be more lines, more details, and more for the common user to try and decipher. That being said — are SPC outlooks really designed for the layman? Given the terminology and incredible detail in their outlooks, one could assume not.
Regardless of our individual opinions, we’ll all be getting used to seeing the words “Marginal” and “Enhanced” risk to describe thunderstorm threats in our area by the time Spring 2014 rolls around.