Storm Prediction Center Risk Levels - What Do They Mean?
If you've lived in the South, especially the Tennessee Valley, for any length of time, you surely have lived through a handful of severe weather events. In the days leading up to them, you may hear us on-air discussing the "risk level" of severe weather in the area. This is known as the Categorical Outlook, a daily product issued by the Storm Prediction Center in Norman, Oklahoma. We typically to refer to them as "levels", such as "level 1 out of 5", and so on. But what specifically do these threats entail, in general? Let's take a look.
The outlook scheme uses 5 categories to highlight the risk of severe weather, and 1 general label to outlook the chance of scatter thunderstorms. Breaking it down -
The level 1 risk, known as a MARGINAL RISK, indicates the chance of isolated, low-end severe thunderstorms. Generally, the threats are quarter sized hail, and wind gusts up to 60mph at times. Typically, tornadoes aren't the main threat with these risks. They are very common, and can happen frequently in the summer with pop-up convection.
The level 2 risk, known as a SLIGHT RISK, indicates the chanced for some more intense, but still not very widespread severe thunderstorms across an area. Some isolated areas of winds in excess of 60mph can be expected, as well as the chance for one or two tornadoes in the stronger storms.
The level 3 risk, known as an ENHANCED RISK, begins to indicate the beginning of what we typically consider a more notable threat. More widespread tornado threats can begin in this bracket, one or two of which may be on the stronger side. Very widespread wind damage is possible, as well as hail larger than an inch or even two at times.
The level 4 risk, (known as a MODERATE RISK), begins to indicate higher possibilities of a severe weather outbreak. Multiple more intense tornadoes can occur during these sorts of days, which happen only a handful of times during a given tornado season, and widespread damaging wind is all but a guarantee. Hail in excess of 2 inches can occur during days like this as well. Some memorable severe weather days have been level 4 out of 5 risks here, and we all know they are nothing to shrug off.
The level 5 risk, (known as a HIGH RISK), is the highest possible risk category, and only occurs once every year or two. Widespread, intense tornadoes are likely with such a risk, and large hail in excess of 2 inches can be widespread and just as deadly at times. Derechos, or long-lived intense lines of severe force winds, can also warrant this sort of risk, especially intense ones - they can, at times, host hurricane force winds and can do widespread, extremely intense damage. Some notable level 5 risks include April 27, 2011, March 25, 2021, and March 17, 2021. All of these featured intense and violent tornadoes, and I don't think anyone needs reminding as to the horrific nature of April 27th. With all this in mind, hopefully you are equipped with some basic knowledge that helps you know what to expect when severe weather strikes in the Tennessee Valley. Stay tuned to us, and we'll break the threats down to the city!