As many of you know, the current scale the SPC uses for severe thunderstorm forecasts follows four general categories:
-General Thunderstorms (green shading)
-Slight Risk (yellow shading)
-Moderate Risk (red shading)
-High Risk (purple shading)
Common sense dictates that a high risk for severe weather has a much higher potential to be devastating than a slight risk for severe weather. The SPC is adding two more categories into the mix with "marginal risk" and "enhanced risk," which will respectively fall on either side of the "slight risk" zones. The scale now runs from 1 to 5, with a 5 denoting a high risk.
Here's the new scale:
0: General Thunderstorms (light green shading)
1: Marginal Risk (dark green shading)
2: Slight Risk (yellow shading)
3: Enhanced Risk (orange shading)
4: Moderate Risk (red shading)
5: High Risk (purple shading)
The update is meant to convey more information about the risk for severe weather on any given day, preventing people from being lulled into a false sense of security.
Here's an example from the infamous tornado outbreak of April 27, 2011. This is what the categorical severe weather outlook looked like that afternoon as the massive tornadoes were starting to tear through Mississippi and Alabama:
And here is what the same forecast would have looked like with the new scale:
You can see that Memphis is under an enhanced risk for severe weather, which is a 3 on a scale from 1 to 5. Birmingham has a high risk for severe weather, putting it at a 5 out of 5 on the severe weather scale.
Here's another example from a severe weather outbreak along the I-95 corridor on June 1, 2011. This is how the old outlook appeared:
And this is what the new scale would look like on the same map:
You can see that the new outlooks provide a much more detailed look at the threat for severe weather across the slight risk area, while also showing that there's a marginal risk for severe weather across much of the Midwest and Florida in areas that were previously labeled as "general, non-severe thunderstorms."
The new outlook has the potential to greatly impact how the public receives severe weather forecasts, but it could also confuse people: several people pointed out to me today that the terms "enhanced" and "moderate" are subjective, and that enhanced sounds worse than moderate. That might be an issue that the SPC needs to work out.
What do you think of the update?
I write regularly for The Vane, which is a new weather website I launched with Gawker. I'm also on Facebook and Twitter.