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 an example of the changes in the outlooks using an example in the Northeast US from 2011. You can visualize the new outlook area with an “ENH” or Enhanced risk running throughout the majority of our forecast area. Previously, the outlooks would only show this area as “Slight”despite there being an enhanced percentage of severe weather within that slight risk.
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.
The Storm Prediction Center will also issue new, zoomed in outlooks during widespread/significant severe weather events. When issuing a PWO (Public Weather Outlook), the graphics will zoom in on the area of enhanced severe weather allowing media and public viewers to better understand where the greater risk for severe weather is occurring on a more local level. An example is included below.
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. The new PWO graphics are likely a tremendous product that has been missing for years — and a welcome addition to the already impressive suite of products.
Regardless of our individual opinions, we’ll likely all be getting used to seeing the words “Marginal” and “Enhanced” risk to describe thunderstorm threats in our area by the time September 2014 rolls around. You can read more in an official release from the Storm Prediction Center right here.