The Storm Prediction Center (SPC), the National Weather Service agency responsible for severe weather predictions, plans to unveil several new categories to its severe weather forecasts this spring. These forecasts — called "convective outlooks" — will see "marginal" and "enhanced" added to the four existing categories of severe weather risk.
A severe thunderstorm is officially defined as one that: produces wind damage or winds in excess of 60 MPH; produces hail equal to or greater than the size of quarters (1.00" in diameter); or a tornado.
The SPC currently uses five categories to convey the predicted threat for severe weather to the public, ranging from least severe to most severe:
- "General thunderstorms" — shaded green on its maps — indicate the risk for non-severe thunderstorms.
- "See Text" — overlain on top of the general thunderstorm category, indicates a marginal risk for severe weather but not enough to warrant a slight risk.
- Slight risk for severe thunderstorms — shaded yellow on its maps
- Moderate risk for severe thunderstorms — shaded red on its maps
- High risk for severe thunderstorms — shaded purple/magenta on its maps
The highest risk for severe weather was centered in the "high risk" zone from east-central Mississippi up through extreme southeastern Tennessee. And, as we know now, that is where the worst tornadoes occurred that day.
The risk categories are based on the probability for a certain type of severe weather to occur based on climatological norms. These probabilities can be confusing for both the general public as well as some weather geeks, which is why the SPC condenses them into categories to begin with.
Here are the tornado probabilities for the same outbreak pictured above on April 27, 2011.
The 45% risk zone over Alabama and Mississippi indicates that there is a 45% higher-than-normal chance of seeing at least one tornado within 25 miles of any point in the shaded area.
Normally, a 2% higher-than-normal tornado risk warrants mention, and 5% warrants concern. For hail and wind, a 15% higher-than-normal risk warrants concern.
The black hatching on the tornado outlook map indicates the risk for significant tornadoes EF-2 or larger. The same on a wind outlook indicates the risk for winds in excess of 75 MPH, and on a hail outlook it indicates the risk for hail larger than the size of golf balls.
Certain combinations of probabilities correspond with different types of risk. For example, a 5% higher-than-normal risk for tornadoes on day one (today) would warrant the issuance of a slight risk. Here is a chart from the SPC explaining the current probabilities and how they correspond to the categorical outlooks.
The new categories to be rolled out by the SPC include "marginal" and "enhanced."
A marginal risk for severe thunderstorms will replace "see text" on the convective outlooks. An enhanced risk for severe thunderstorms will occupy the slot in between slight and moderate risk to provide forecasters an additional step to more accurately convey the risk for severe weather.
According to a post on the Washington Post's Capital Weather Gang this past January, the two new categories will change the color scheme on the SPC's maps. The risk will range from cool to hot — cooler colors indicating a lower risk, warmer colors higher.
- General thunderstorms will appear blue on the map.
- Marginal risk for severe storms will appear green on the map.
- Slight risk for severe storms will appear yellow on the map.
- Enhanced risk for severe storms will appear orange on the map.
- Moderate risk for severe storms will appear red on the map.
- High risk for severe storms will appear purple/magenta on the map.
Theoretically, the addition of these two new categories will help forecasters more accurately convey the threat for severe weather to the public, but as Jason Samenow noted in the previously linked Capital Weather Gang post, adding more terminology could also serve to confuse the public. Public reaction to the updates will make or break the SPC's decision to update their forecasts.
The revamp will likely go live either later this month or next.