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Understanding the Storm Prediction Center’s Convective Outlooks

SPC Outlook for 7/13/2020 SPC Outlook for 7/13/2020 |

Allison Finch
Updated: July 13, 2020 10:37 AM
Created: July 12, 2020 01:11 PM

Our number one goal while forecasting is to keep the public informed and safe. Through late spring all the way to early fall thunderstorms, tornados and severe weather are all a threat to our viewing area. While forecasting these storms, one of the most valuable tools we have access to is the Storm Prediction Center’s convective outlooks. These convective outlooks lay out the chance for thunderstorms, damaging winds, hail and tornados in a way that is easy to explain to the public.

When the Storm Prediction Center (SPC) was first created, its forecasts were never intended for public use. Meaning that they usually remained within government agencies like the National Weather Service (NWS) and public officials. Now that the SPC regularly posts their forecasts online (Twitter, Facebook, etc.) and local meteorologists use them to explain the threat to the public, the five categories that the SPC breaks the convective outlook down into are widely misunderstood. While local forecasters and public officials can easily explain the difference between a marginal risk compared to a slight risk; most people do not understand the difference.  While it is important to know this information, local meteorologists and forecasters will always make sure you’re informed about the possible risks.

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The SPC breaks their convective outlooks into six different categories.

TSTM: General Thunderstorms (Light Green)

No severe thunderstorms are expected, but there is a 10% chance (or higher) that a thunderstorm will affect that region during the forecast period. 

1: MARGINAL (Dark Green)

Isolated severe thunderstorms are possible. While these storms are usually unorganized and have a limited longevity, there is still a threat. Most likely gusty winds and hail will be associated with these storms. While the threat of a tornado cannot be ruled out, the threat is low (usually about 2% chance or less).

2: SLIGHT (Yellow)

Scattered severe storms are possible. These are short-lived storms with low coverage (scattered) within the forecasted area have varying levels of intensity. Stronger gusty winds and hail will be associated with these storms. The threat of tornados is a bit higher in this level with about a 5% chance or less of occurring within 25 miles of your location. While that seems small, tornados are pretty uncommon in most locations. Having a 5% chance of a tornado occurring is 5 times the normal odds of it happening. (For perspective, imagine winning the lottery and then ending up with 5 times the winning amount. Not that that would happen, but in the rare case it did, you would have a lot more money than expected.)

3: ENHANCED (Orange)

Numerous severe storms are possible. More organized concentration of severe thunderstorms that have varying levels of intensity. There is a high chance that with these thunderstorms there will be damaging winds and hail. The chance of a tornado occurring within 25 miles of your location now increases to 10 - 15%, depending on the storm. (That is 10 – 15 times the normal odds of it happening.)

4: MODERATE (Red)

Widespread severe storms likely. Widespread severe weather with several tornadoes and many thunderstorms that could be intense. Damaging winds and big hail occur within these types of storms. The chance of a tornado occurring within 25 miles of your location increases to 15-30%. (That is 15 – 30 times the normal odds of it happening.)

5: HIGH (Pink)

Widespread severe storms are expected. These storms are usually long-lived; include long track tornadoes as well as hurricane force winds that would cause widespread damage.

Now that you know the risk categories, what can you do to keep yourself safe?

When your area is expected to have thunderstorms, it is important to listen to meteorologists and public officials who know the specifics on the current (or upcoming) storm. Some of the safest and easiest practices that you can make sure you follow include: “When thunder roads, head indoors” and “Turn around, don’t drown®!”1 Lightning can strike up to 10 miles away from the storm, if thunder is loud enough for you to hear it, you’re close enough lightning to strike; head indoors! If in the unfortunate event you don’t have access to a building or vehicle, it is important to remember to never go under a tree for shelter. While the tree will help keep you dry, it put you at a higher risk of being struck by lightning. “Turn around, don’t drown®” is a helpful way to remember that when you’re driving to not drive through flooded roadways. While it might seem like a hassle to find an alternative route, the water level is uncertain. Sometimes water levels go up and sometimes they go down, driving through water of an unknown depth poses a risk to your safety.

1: Weather.gov


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