When the forecast calls for a 30% chance of rain, it's reasonable to think that the chances of seeing rain are 3 out of 10. But when the forecast calls for a 30% chance of tornadoes, the risk is much higher and much more dangerous. Why is there such a discrepancy between severe weather and rain forecasts?

Categories

As I've previously covered, starting in September the Storm Prediction Center will rate the risk for severe weather on any given day on a categorical scale ranging from "marginal" to "high." Forecasters arrive at these rankings based on the probability for damaging winds, large hail, or tornadoes. The higher the probability for severe weather, the higher the category.

When the two new categories — "marginal" and "enhanced" — are placed into service in September, forecasters will use the above chart to determine which category to issue and where. For example, if there is a 10% probability of tornadoes in Atlanta, forecasters would include the city in an "enhanced risk" for severe weather.

These probabilities are different from the chance of rain you hear in your local forecast. When you hear that there's a 30% risk for tornadoes, they're not just telling you that a tornado will occur 3 out of 10 times. They're telling you two things:

  1. There is a 30% chance of seeing a tornado within 25 miles of any point in the risk area.
  2. The risk of seeing a tornado today could be hundreds of times higher than normal.

The second point is what's important. A 30% chance sounds relatively low, but since tornadoes are relatively uncommon, the risk is much higher than it appears. We can see how many more times likely than normal tornadoes are on a certain date by looking at severe weather climatology maps.

Let's use two examples from this past year, starting with the tornado outbreak back on April 28.

Tornadoes

On April 28, meteorologists issued a 30% risk for tornadoes across parts of Mississippi and Alabama, meaning that there was a 30% chance for tornadoes within 25 miles of any point within the shaded area.

It also meant that tornadoes were 50 times more likely than normal. How did we figure that out?

The Storm Prediction Center publishes maps showing the overall probability for tornadoes, damaging winds, and hail based on weather over the 30-year period between 1982 and 2011. We use these maps to come up with the climatological risks.

Over most of the 30% risk zone on April 28, a tornado occurred within 25 miles of anywhere in the area of concern over the 30-year period 0.60% of the time. In other words, the climatological risk of seeing a tornado on April 29, based on 30-year averages, is 0.60%.

Since the forecast risk of tornadoes is 30% and the climatological risk is 0.60%, this means that tornadoes are 50 times more likely than normal.

But wait! It gets even worse.

Do you see the black hatching over the 30% risk area? That signifies the potential for significant tornadoes, or those rated EF-2 or stronger. The climatological probability for significant tornadoes is even smaller, sitting at 0.20% across the risk area. This means that the risk for significant tornadoes is 150 times higher than normal. That's an astronomically high risk, and a far cry from the math behind a 30% chance of rain.

Damaging Winds

Let's look at another high risk day, this one from the beginning of last month.

The risk for damaging winds across parts of Nebraska and Iowa was a top-of-the-scale 60% on June 3, signalling forecasters' concerns that an intense derecho would form. This means that there was a 60% chance of seeing wind damage (or winds recorded above 58 MPH) within 25 miles of any point in the shaded area.

As we did with the tornadoes, let's look at the severe weather climatology for June 3.

The climatological probability of seeing severe winds across the risk area is 2.00% on June 3, putting the area's overall risk for seeing wind damage 30 times higher than normal.

Again, as with the tornadoes, the black hatching indicates the risk for significant severe winds, which are defined as 75 MPH or higher.

Here's the map showing the climatological probability of seeing significant severe winds on June 3:

Values range from 0.30% to 0.40%, which would equate to the risk of seeing winds greater than 75 MPH clocking in around 150 to 200 times higher than normal.

When should we be concerned?

For future reference, here are the risks generally considered to warrant concern and special attention:

  • 15% risk for damaging winds
  • 15% risk for hail
  • 5% risk for tornadoes

Not to say that lower probabilities are anything to shrug off — "marginal" doesn't mean "not gonna happen," after all — but once the values start ticking up to these thresholds, it's time to start paying extra attention.

When viewed in the context of severe weather occurring x amount of times out of ten, it's easy to brush off a 30% risk of tornadoes as nothing much. But when viewed properly through the spectrum of climatology, forecasters are really trying to tell you that the risk for severe weather is much, much higher than normal, and that you should take appropriate action to stay safe.

[Top image via AP, all others by the SPC]