Outlook Scale Definitions
The Storm Prediction Center uses a five-level system for their severe weather outlooks. Ranging from one to five – or marginal to high – this is the nationally recognized system for for severe weather outlooks. A category 1 (marginal) risk is the lowest on the scale, and the most frequently issued. On the other side of the spectrum, a category 5 (high) risk is rarely issued and only used when confidence is high on a major outbreak of severe weather. The number of storms, severity of storms, and overall confidence of a forecast play a role in the risk level of a severe weather outlook. We’ve seen violent, killer tornadoes occur in category 2 and category 3 risk days (May 15, 2013 and December 26, 2015).
While not included in our custom-made graphics and our scale, there is a light green shading called Thunder. That is not an official severe weather risk area, but where general thunderstorm are possible. An example would be most summer afternoons with popup storms.
Once we’re within the day of an event the SPC issues probability-specific graphics depicting the risk of tornadoes, damaging wind gusts, and large hail. The image above shows where those specific hazards fall in the overall risk scheme. The probabilities are for the overall chance of experiencing a specific hazard within 25 miles of your location. A 15% chance of damaging winds or large hail within 25 miles of your location is a category 2 risk, while a 15% risk of damaging tornadoes is a category 4. Obviously hail is much more common than tornadoes, hence the difference in risk levels for the ‘same’ probability.