Over the past few weeks, severe storms have impacted the region. Leading up to these weather events, you have likely heard the Storm Team 27 Weather Team break down the severe threat into five risk levels (marginal, slight, enhanced, moderate and high). In the days leading up to a severe weather event, who decides what category the area should be in, and what exactly do these different categories mean?
What is severe weather?
While we see thunderstorms at all times of the year, they are not always severe. For a thunderstorm to be severe, it has to be capable of producing either a tornado, damaging winds of at least 58 mph, or hail of 1-inch diameter or larger.
Who issues severe weather outlooks?
When communicating the threat of severe weather, broadcast meteorologists work closely with NOAA’s Storm Prediction Center (SPC) in Norman, Oklahoma.
Meteorologists at the SPC monitor the threat of severe weather and release a daily Convective Outlook to highlight what areas across the country have the greatest potential for severe thunderstorms. These outlooks include five risk levels, including marginal, which is the area that has the lowest potential, and high, which is the area where severe weather is likely.
The image below shows an example of the convective outlook from March 25, when the area had a slight risk of severe weather. On this day, severe storms produced damaging wind gusts across the region.
What do the severe weather risk levels mean?
Now that you know who issues the five risk levels for severe weather, let’s break down what they mean.
Marginal Risk (Level 1 of 5): When a marginal risk is issued, it means there is a small threat of severe weather across that area. The marginal risk covers an area where isolated severe thunderstorms will be possible. If severe thunderstorms develop in an area covered by a marginal risk, they are usually limited in coverage, duration and intensity. The SPC uses dark green to highlight where there is a marginal risk of severe weather.
Slight Risk (Level 2 of 5): The next threat level is a slight risk of severe weather. The Slight risk covers an area where organized severe thunderstorms will be possible, with widespread severe thunderstorms not expected. Severe thunderstorms with varying levels of intensity will be possible. The SPC uses yellow to highlight where there is a Slight risk of severe weather.
Enhanced Risk (Level 3 of 5): When an enhanced risk is issued, numerous or widespread severe thunderstorms will be possible. These storms will vary in levels of intensity. Part of the Valley had an enhanced risk of severe weather on April 1, when severe storms rolled through the valley. The SPC uses orange to highlight where there is an enhanced risk of severe weather.
Moderate Risk (Level 4 of 5): When a moderate risk is issued, several tornadoes and/or numerous severe thunderstorms are likely, with some intense storms likely as well. The SPC usually reserves this risk for days when several supercells capable of producing intense tornadoes and very large hail are likely, or for days with squall lines producing widespread damaging winds are expected. The SPC uses red to highlight a moderate risk of severe weather.
High Risk (Level 5 of 5): When a high risk is issued for an area, a severe weather outbreak is expected from either numerous intense and long-tracked tornadoes or a long-lived derecho-producing thunderstorm complex that produces hurricane-force wind gusts and widespread damage. This risk is rarely used and is reserved for days when there is high confidence that an area will be impacted by widespread coverage of severe weather. The SPC uses magenta to highlight where there is a high risk of severe weather.
In our area, severe thunderstorms are possible at all times of the year, and you should always be prepared and have a plan in place for when severe thunderstorms impact the region.