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The risk of severe thunderstorms

Here's this week's Science Behind the Forecast.

Every week WAVE 3 meteorologist Tawana Andrew breaks down what we know and what we don't about the climate and weather here in Louisville.

Bill Burton: It's time for us to take a look at the Science Behind the Forecast with WAVE 3 meteorologist Tawana Andrew. Good morning, Tawana.

Tawana Andrew: Good morning. Today we're talking about something that's super important as we continue to see that threat of some severe weather in our forecast.

BB: Yes, we're talking severe thunderstorms today and the varying degrees of risks that come with them. Enlighten us, Tawana.

TA: As a meteorologist, you'll hear us talking about severe weather outlooks, and those come from the National Weather Service's Storm Prediction Center. And they can issue those up to eight days in advance. And there is very specific criteria in place for the National Weather Service. So for a storm to be severe, you have to have a tornado — that one I think a lot of people know — damaging winds over 58 miles per hour, and or hail three-quarters of an inch in diameter or larger.

So the National Weather Service is going to be putting out all of these convective outlooks, usually. We'll have day one through three and then there'll be specific ones for days four through eight. The convective outlooks for day one through three, they're more specific, and they will show the severe threats location. A lot of people do not know that the day one convective outlook is issued five times a day, it's actually valid from the morning until the morning the next day. So let's say around 7 a.m. through 7 a.m. the following day, and it just outlooks where that severe thunderstorms can occur. The day two outlook is actually issued twice a day. And it basically covers the morning of the following day to the morning [of] the day after that. And then for the day three convective outlook, it's a little bit different. Because what the SPC will do is they just kind of show the percentage of a probability of any severe weather hazard over a particular area.

So for day one and two, this is the one where you'll hear us saying their specific categories for a threat, you'll hear us say the marginal risks, slight risks, moderate risk, high risk. Those are the categories I think everybody hears the most, and breaking them all down for a marginal risk, that's usually in that green color, that weird limey green color. That shows where severe thunderstorms would not be organized or last very long. So the severe threat is pretty isolated. It's low, so it's just one or two thunderstorms. They get a little bit strong, and then in a couple of hours, they're gone.

For a slight risk. It's like a lemony yellow situation on the map. And it depicts an area where scattered severe weather is expected. So the storms aren't very organized, the severe risk isn't very widespread, and you can have one or two or three thunderstorms that really blossom [and] get very strong, and then they're gone. For an enhanced risk that's that orange. With that there is a greater potential for numerous severe thunderstorms. So this is where you can end up with severe thunderstorm after another after another and that goes on for a couple of hours. For a moderate risk that shows up in red. We don't have these very often, but that denotes an area where widespread severe weather is expected. Think what we saw in Iowa earlier this week [where] those large, large, long-track tornadoes went through. Now the last category is a high risk. If you see this crazy shade of pink on the map, I hope you are paying attention. That is where meteorologists are incredibly confident that a widespread severe weather outbreak is going to be likely.

This transcript was edited for clarity.

Bill Burton is the Morning Edition host for LPM. Email Bill at bburton@lpm.org.

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