Changes to Severe Weather Outlooks

The Storm Prediction Center has instituted several changes to it's severe weather outlook products effective today.

If you are familiar with the convective outlooks that the SPC issues several times per day you know that the outlooks indicate where general thunderstorms are expected, where there is a low probability of severe weather (the "See Text" category), and where there is a Slight, Moderate, or High Risk of severe weather. Effective today there is one change and one additional category being used in the Day 1 through Day 3 outlooks.

There are real numbers behind the determination of each of the former and current risk areas. A 15 percent probability of a tornado (Moderate Risk category) may seem low, but the normal probability of a tornado, for example, during the peak of the season on May 13 in central Oklahoma is only about 1.5 percent. A Moderate Risk in  in this case would indicate about 10 times of the normal probability for a tornado.

The probability for a tornado changes by season.The probability of a tornado in central Oklahoma this week in October is normally about 0.15 percent, ten times lower than in mid-May.

Here are the new/revised categories and what they describe. The probabilities for Day 1 are more detailed than Days 2 and 3.

Day 1
General Thunderstorms
    10 percent or greater probability of non-severe or near severe thunderstorms

Marginal
    - 2% tornado probability, or
    - 5% severe hail or severe wind probability.
 
Slight
    - 5% tornado probability, or
    - 15% severe hail or severe wind probability WITH OR WITHOUT 10% or greater probability
      of hail 2 inches or greater in diameter, or wind gusts 75 mph or greater.
   
Enhanced (the upper end of the former SLIGHT category)
    - 10% tornado probability WITH OR WITHOUT 10% or greater probability
      of an EF2 or greater tornado, or
    - 15% tornado probability, or
    - 30% severe hail or severe wind probability WITH OR WITHOUT 10% or greater probability
      of hail 2 inches or greater in diameter, or wind gusts 75 mph or greater, or
    - 45% probability of severe hail or wind.
 
Moderate
    - 15% tornado probability AND 10% or greater probability
      of an EF2 or greater tornado, or
    - 30% tornado probability, or
    - 45% severe wind probability AND 10% or greater
      probability of a wind gusts 75 mph or greater, or
    - 45% severe hail probability AND 10% or greater
      probability of hail 2 inches or greater in diameter, or
    - 60% severe wind probability, or
    - 60% severe hail probability WITH OR WITHOUT 10% or greater probability
      of hail 2 inches or greater in diameter.
       
High
    - 30% tornado probability AND 10% or greater probability
      of an EF2 or greater tornado, or
    - 45% or greater tornado probability WITH OR WITHOUT 10% or greater probability
      of an EF2 or greater tornado, or
    - 60% severe wind probability AND a 10% or greater
      probability of a wind gust 75 mph or greater.
   
Days 2 and 3
General Thunderstorms
      - 10% or greater probability of non-severe or near severe thunderstorms.

Marginal
      - 5% total severe probability.

Slight
      - 15% total severe probability WITH OR WITHOUT 10% or greater probability
        of significant severe.

Enhanced
      - 30% total severe probability WITH OR WITHOUT 10% or greater probability
        of significant severe, or
      - 45% total severe probability.

Moderate
      - 45% total severe probability AND 10% or greater
        probability significant severe, or
      - 60% total severe probability (Day 2 only).

High (Day 2 only)
      - 60% total severe probability AND 10% or greater
        probability of an EF2 or greater tornado or a wind gust 75 mph or greater.
       

The SPC has a number of examples of the new vs. old  categories on its web site. Here is an example of the difference in outlooks for the day of the Southeast tornado outbreak on April 27, 2011.

The Day 1 convective outlook for April 27, 2011 in the new format (left) and old format (right)

The Storm Prediction Center is also considering changes to the Day 4 to 8 outlook product. This is still in experimental mode and more information can be found here.

Although much of the country is currently enjoying pleasant fall weather, we have entered a ramp-up period to a secondary peak in severe weather season. October through December is a period of increased frequency of severe weather and tornadoes from far eastern Texas through Alabama, an area dubbed "Dixie Alley", with a peak in the frequency of tornadoes in mid to late November.

Mean number of tornadoes for October through December in Dixie Alley.

If you would like to explore the climatology of severe weather the SPC has an interactive web page where you can view the probabilities for tornadoes, significant wind, and significant hail for 52 one-week periods. You can animate the maps to see how the severe weather shifts through the country and how the probabilities change from week to week.