Changes Coming to SPC Severe Weather Forecasts

If you follow severe weather, you know the Storm Prediction Center (SPC) issues daily forecasts that warn the public of possible severe weather. In an attempt to simplify these forecasts, SPC is making some changes to their outlooks starting Tuesday, October 22nd.

Here's a look at an old outlook issued by SPC. The severe threat is broken up into four categories:

1. See Text
2. Slight
3. Moderate
4. High




With the old outlooks, the "See Text" risk doesn't really convey a meaningful severe threat. The "Slight" risk also covers too large of a range when it comes to possible severe weather. SPC is hoping the new outlooks will solve these problems.

The new outlook breaks up the "See Text" risk into non-severe thunderstorms and a "Marginal" risk. The outlook also keeps the "Slight" risk, but adds an "Enhanced Slight" risk to help denote areas with a slightly higher severe threat. The "Moderate" and "High" risks will remain the same.

1. Marginal
2. Slight
3. Enhanced (Short for "Enhanced Slight")
4. Moderate
5. High
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Here's what all the new categories mean statistically for a Day 1 Outlook:

General Thunderstorms
-10% or greater probability of non-severe or near severe thunderstorms.

Marginal
- 2% or greater tornado probability, or
- 5% or greater severe hail or severe wind probability.

Slight
- 5% or greater tornado probability, or
- 15% or greater severe hail or severe wind probability.

Enhanced
- 10% or greater tornado probability, or
- 30% or greater severe hail or severe wind probability.

Moderate
- 15% or greater tornado probability AND 10% or greater probability of an EF2+ tornado, or
- 30% or greater tornado probability, or
- 45% or greater severe wind probability AND 10% or greater probability of a wind gusts 75 mph or greater, or
- 45% or greater severe hail probability AND 10% or greater probability of hail 2 inches or greater in diameter, or - 60% or greater severe wind probability, or
- 60% or greater severe hail probability.

High
- 30% or greater tornado probability AND 10% or greater probability of an EF2+ tornado, or
- 45% or greater tornado probability, or
- 60% or greater severe wind probability AND a 10% or greater probability of a wind gust 75 mph or greater.


For a closer look at how SPC is changing the Day 1-3 Outlooks, click here.
To see how SPC is changing the Day 4-8 Outlooks, click here. -BA