Thunderstorm Risk Categories: Explained
- May 06, 2019
All about the Storm Prediction Center’s thunderstorm risk categories
What are the chances your area will experience thunderstorms?
The Storm Prediction Center (SPC) tries to answer that question for you with their severe weather risk categories. In this post we’ll cover the five different severe thunderstorm risk categories so you can have a better understanding of what to expect.
First Thing’s First
What is the Storm Prediction Center?
The Storm Prediction Center is a part of the National Weather Service (NWS) and the National Centers for Environmental Prediction (NCEP).
It’s mission is simple and important: To provide timely and accurate forecasts and watches for severe thunderstorms and tornadoes over the contiguous United States.
In order to accomplish this goal, the Storm Prediction Center issues Day 1, Day 2, and Day 3 Convective Outlooks. These outlooks depict non-severe thunderstorm areas and severe thunderstorm threats across the country.
These outlooks include:
- Listings of severe thunderstorm risk areas
- Concise summary of threat type(s) and timing
- Scientific and technical discussion for sophisticated users
The Storm Prediction Center derives the level of categorical risk from probability forecasts of tornadoes, damaging winds, and large hail on Day 1. For Days 2-3 they consider a combined severe weather risk.
What are the Thunderstorm Risk Categories?
There are technically six risk levels ranging from 0 (No severe thunderstorms expected) to 5 (Widespread severe storms expected). We’ll list them in order of least risky to most risky.
Thunderstorms Risks
The first risk category is Thunderstorms or TSTM. The Storm Prediction Center represents this level with a light green color.
This category means that there are general or non-severe thunderstorms. Basically, there shouldn’t be any severe thunderstorms.
It is important to remember that lightning and flooding threats exist within all thunderstorms. So although this is represented by the color light green, it’s not exactly a green light.
If your area is under this risk category you should still pay attention to local storm reports.
Marginal Risk
The second risk category is Marginal risk, or MRGL. This is a level one risk categorized and is represented by the color dark green.
When a Marginal risk is active for your area it means isolated severe thunderstorms are possible. These storms are most likely limited in the following areas:
- Duration
- Coverage
- Intensity
As with the first category, it’s important to be alert and understand where your nearest shelter at all times.
Slight Risk
The third level of risk is called Slight risk. It is also referred to as SLGT or yellow risk in the Convective Outlooks.
While a Slight risk sounds pretty tame, it’s still something to pay attention to.
When a Slight risk is active it means scattered severe storms are possible. They’re only possible because an organized system is forecast but the level of storm severity within the system is varied.
These storms will be most likely be:
- Short-lived
- Not widespread
- Isolated
- Intense
A lot of people hear the term “slight” and think the risk can be overlooked. This is simply not true. Be aware that intense thunderstorms are possible.
Enhanced Risk
The Storm Prediction Center labels Enhanced risk with ENH or the color orange. Enhanced risk means an area of greater severe storm coverage with varying levels of intensity is forecast.
You can also think of enhanced risk as the possibility of numerous severe storms.
Threats to areas in an Enhanced Risk typically include:
- The potential for tornadoes
- Frequent lightning
- Damaging winds between 58-70 mph
- Hail with a diameter between 1-2 inches
When an Enhanced risk area is issued, you should take it very seriously. Its presence means that government forecasters are confident enough that widespread storms will develop. It also means they believe these storms are capable of causing significant damage or even causing fatalities.
For anyone in an Enhanced risk area, it would be wise to keep an eye on the sky during any activities.
Moderate Risk
The penultimate category on the Storm Prediction Center’s severe thunderstorm risk chart is Moderate risk. We shorten Moderate risk to “MDT” and use the color red to signify it.
A Moderate risk means an area where widespread severe weather with several tornadoes and/or numerous intense and/or numerous severe thunderstorms is likely. Some of these could be intense.
Threats to areas in Moderate risk typically include:
- The likelihood of tornadoes
- Frequent lightning
- Damaging winds in excess of 70 mph
- Large hail in excess of 2 inches diameter
When a Moderate risk area is issued, you should take it very seriously. It means government forecasters believe widespread severe storms that are long-lived and intense are likely.
“Moderate” doesn’t sound like a very intense category, but these storms can cause a lot of damage and even death.
High Risk
High risk forecasts are quite rare. On average, the Storm Prediction Center only issues them once or twice a year.
Typically, High risk areas are issued when particularly strong and widespread severe storm or tornado outbreaks are expected.
These correspond to areas that strong tornadoes are likely to develop. Threats to areas in a High risk typically include:
- The likelihood of tornadoes, often strong and/or long-lasting
- Frequent lightning
- Damaging winds in excess of 80 mph and lasting hours at a time
- Likelihood of structural damage from storms
- Large hail in excess of 2 inches
If your area is under a High risk you should take it very seriously. Government forecasters are very confident that widespread storms will develop. They are also confident that these storms will cause significant damage and/or fatalities.
Remember that a High risk category means that government forecasters expect widespread, severe thunderstorms that are particularly intense. This is the highest rating the Storm Prediction Center has.
Storm Safety
Keeping yourself, your loved ones, and your business safe from severe weather like thunderstorms starts with the right warnings. We highly recommend paying attention to the Storm Prediction Center’s Thunderstorm Risk in the Day 1, Day 2, and Day 3 outlooks.
When severe weather is in the forecast, seconds matter. That’s why we also recommend using some kind of real-time alerting tool or set of tools.
You can learn more about real-time alerting tools by clicking the method you think will work best for you below.
