Understanding severe weather risk categories
Every day, the Storm Prediction Center (SPC) comes out with outlooks on the risk of severe weather. Broadcast meteorologists and meteorologists working at the National Weather Center commonly interpret these predictions and inform the public. There are five risk categories: marginal, severe, enhanced, moderate, and high.
What classifies as a severe thunderstorm?
A thunderstorm is classified as severe if it meets one of the following criteria:
- Winds greater than 58 mph
- Hail diameter greater than an inch (quarter-sized)
- Contains a tornado
If an area is placed in any kind of risk, there is a chance for severe weather to occur. Prepare for the possibility of a severe thunderstorm hitting your area when under any risk.
A marginal risk for severe weather means that isolated severe thunderstorms are possible but are limited in coverage, intensity, or duration. Under a marginal risk, damaging winds and large hail are possible, but only in a few spots. An isolated tornado can also occur but the chance is very low. Our area commonly gets placed under this category during severe weather season.
A slight risk for severe weather means that scattered severe storms are possible but typically are short-lived and not widespread. Intense severe thunderstorms are possible under this category but are isolated in nature. Damaging winds and large hail become scattered in this category with tornadoes still low in coverage . In some instances, damaging winds may eclipse 70 mph, and hail could be up to golf ball sized. In our area, these risks are commonly issued a couple times a month during severe weather season.
An enhanced risk for severe weather means numerous severe storms are possible and are more persistent and widespread with a few instances of very intense storms. Under this risk, several spots will likely experience damaging winds and/or large hail. More significant winds and hail become a little more common. In these instances, a few tornadoes can be possible, possibly on the strong side. In our area, this risk is not that common and usually are issued on average once a year. Eastern portions of Virginia will see this risk a few times a year.
A moderate risk for severe weather is significant. Widespread severe weather is likely. Storms can be long-lived and intense. This is when there is a greater chance to see damaging winds and large hail than not in a 25-mile radius. Intense supercells are likely or significant straight-line winds in storms develop. Tornado outbreaks can happen in this category. Moderate risks are only issued a handful of times a year for the entire country, and are rarely issued anywhere in Virginia.
A high risk for severe weather means widespread severe storms are expected. Storms can be very long-lived, widespread, and powerful. This is when you typically see major tornado outbreaks or derechos. High risks for severe weather are issued on a rare occasion. The last time our viewing area saw severe weather at this magnitude was when the 2012 derecho tore through the area.
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