A “marginal risk” of severe weather is the lowest threat level issued by the Storm Prediction Center (SPC). It indicates a low-end, isolated threat of severe thunderstorms. Despite the mild terminology, “marginal” does not mean “zero risk.” It implies that while widespread severe weather is not expected, localized severe conditions are still possible.
Understanding Severe Weather Risk Categories
The Storm Prediction Center communicates severe weather threats through a five-category tiered system, each representing an increasing risk for phenomena like tornadoes, damaging winds, and large hail. The lowest category is Marginal, followed by Slight, Enhanced, Moderate, and finally, High. This system helps the public understand the potential coverage and intensity of severe weather.
A Marginal risk (Level 1 of 5) suggests isolated severe storms with limited organization or longevity. Moving up, a Slight risk (Level 2 of 5) indicates scattered severe storms that are not widespread but can be short-lived and occasionally intense. An Enhanced risk (Level 3 of 5) implies a greater coverage of severe storms, with varying levels of intensity, and an increased likelihood for more numerous or widespread events.
The Moderate risk (Level 4 of 5) signifies a higher potential for widespread and/or more dangerous severe weather, often including numerous intense storms. The highest category, High risk (Level 5 of 5), is reserved for the most dangerous and widespread severe weather outbreaks, such as major tornado outbreaks or significant derecho events. These categories help convey the overall severe weather environment, aiding preparedness.
Characteristics of Marginal Risk
A marginal risk typically implies severe storms will be isolated, short-lived, and of lower intensity. This means the threat is not widespread and impacts will likely be localized to small areas. For instance, hail associated with a marginal risk is generally small, often around 1 inch in diameter, which is considered the minimum size for severe hail.
Damaging winds under a marginal risk are typically isolated and may reach at least 60 miles per hour. Tornadoes, while possible, are usually weak (EF0 or EF1 on the Enhanced Fujita Scale) and brief. Although the overall threat is low, these isolated events can still pose a danger.
Staying Prepared During a Marginal Risk
Even under a marginal risk, stay weather-aware and monitor local forecasts. Multiple ways to receive severe weather warnings, such as a weather radio or smartphone alerts, can provide crucial lead time if a storm develops quickly. Knowing where to seek safe shelter, whether indoors in a sturdy building or a designated community shelter, is a foundational preparedness step.
Developing a basic emergency plan for your household helps ensure everyone knows what to do if severe weather approaches. While extensive preparations might not be necessary for a marginal risk, a proactive approach to weather safety is always beneficial. Any severe weather warning should be taken seriously, regardless of the initial risk outlook.