Meteorologists categorize potential severe weather risks to help the public understand the threat level. These categories represent a tiered system, providing a framework for assessing the potential for severe thunderstorms, tornadoes, damaging winds, and large hail. The primary goal of this categorization is to inform and prepare the public without causing undue alarm, allowing individuals to make informed decisions based on the forecast.
Understanding Severe Weather Risk Levels
Forecasters at the Storm Prediction Center (SPC) use a spectrum of categories to communicate the threat of severe weather, ranging from general thunderstorm activity to the highest risk levels. The lowest category is “General Thunderstorms,” indicating that thunderstorms are expected but are not anticipated to reach severe limits. Moving up the scale, the next level is “Marginal Risk,” which signifies the lowest severe weather threat category. This is the first level where severe weather, such as tornadoes, damaging winds, or large hail, is considered possible.
Beyond marginal, the categories escalate to “Slight Risk,” where more numerous or widespread severe storms are possible, though typically short-lived. “Enhanced Risk” indicates an even greater concentration and intensity of severe weather, with longer-lived or more widespread events. The highest two categories are “Moderate Risk” and “High Risk,” reserved for situations where widespread and extremely dangerous severe weather, including significant tornadoes or widespread damaging winds, is expected.
Characteristics of Marginal Risk
A “Marginal Risk” forecast indicates that isolated, short-lived, or low-intensity severe weather is possible across a specific area. Such storms might produce brief tornadoes, scattered instances of damaging winds, or small hail, generally around one inch in diameter. The severe weather events within a marginal risk area are not expected to be widespread or particularly strong.
From a probabilistic standpoint, a marginal risk often corresponds to a 5% chance of severe weather occurring within 25 miles of any given point within the forecast area. This low probability highlights that most locations within the marginal risk zone will not experience severe weather. Despite the low probability, the potential for isolated severe storms means that the threat is not zero. Therefore, even a marginal risk warrants a degree of awareness, as localized hazards can still emerge.
Preparing for Marginal Risk
When a Marginal Risk of severe weather is forecast, it signals a need for increased awareness rather than extensive preparation or panic. The most effective action is to stay informed about the weather conditions throughout the day. Monitoring reliable sources, such as the National Weather Service (NWS) and local meteorologists, provides the most current information. These sources will update forecasts and issue any watches or warnings if conditions escalate.
It is advisable to have a basic safety plan in mind, even for a marginal risk. This includes knowing where to take shelter quickly if a severe thunderstorm warning is issued for your location. Identifying a sturdy interior room on the lowest floor of your home or a designated storm shelter can be beneficial. Understanding the difference between a forecast risk, a “watch,” and a “warning” is also important. A marginal risk is a forecast of potential severe weather, while a “watch” means conditions are favorable for severe weather to develop, and a “warning” means severe weather is imminent or already occurring.