No warning was issued before the Parkland tornado hit last week. Why?

Last week, a thunderstorm spawned a tornado that touched down in west Louisville, but there was no warning. So LPM spoke with WAVE 3 meteorologist Tawana Andrew about what's needed for the National Weather Service to issue a tornado warning.

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

Tawana Andrew: Good morning. Usually we're talking about kind of broad topics on this segment, but today we're kind of breaking down a little bit more what happened back on July 4,

BB: There was a pretty ugly storm that came through the area that day and spawned a tornado that hit the Parkland neighborhood, but there was no warning of that tornado from the National Weather Service. So that's our topic today, we're talking about what goes into issuing a tornado warning. What do we need to know?

TA: So just breaking down what happened that day. On July 4, we had a line of thunderstorms moving towards the Louisville metro area. A severe thunderstorm warning with a tornado possible tag that was issued at 12:40 p.m. eastern time for that storm [as it] moved into Harrison County, Indiana.

There was about a 20-minute lead time before an EF0 tornado with 85 mile per hour winds touched down just north of Corydon around 1:05 p.m. So that tornado possible tag was on the severe thunderstorm warning as that tornado was moving through the Corydon area. Now the actual tornado warning for that storm wasn't issued until three minutes later, at 1:08 p.m. as it continued towards Floyd County.

That same storm started to weaken as it got closer to the Ohio River. So the tornado warning was allowed to expire at 1:23 p.m. The same thunderstorm, though, really strengthened pretty quickly, and then dropped a tornado in Parkland neighborhood around 1:27. So there's a four-minute window between when the tornado warning was allowed to expire and when that tornado dropped in the Parkland neighborhood.

That was a EF1 tornado with 95 mile per hour winds — and it was only on the ground for about a minute. So that was very quick. It moved in, caused all that damage and then it was gone. So of course [what] everybody wanted to know afterwards is why wasn't there a tornado warning for this particular storm?

The National Weather Service has specific criteria for severe thunderstorm warnings, and tornado warnings. For a severe thunderstorm warning what you need, wind gusts over 58 miles per hour and/or 1-inch hail or reports of damage. So those are the three things you need for severe thunderstorm warning.

BB: And none of that was happening?

TA: And none of that was happening at that time.

For a tornado warning, you need a thunderstorm with radar indication of significant low level rotation and a favorable environment. That part is important, significant low level rotation or a reported tornado.

Neither of those things were happening at that time. So a tornado warning was not issued.

Keep in mind that radars complete their full volume scans every four to six minutes. Keep in mind, there was four minutes in between the tornado warning ending and that tornado touching down in the Parkland neighborhood. From the time that the circulation on radar was over Floyd County in Indiana, and then it diminished, crossing over into the Ohio River and into West Louisville was three to six minutes.

So the circulation tightened right as it crossed over the river. And that was one scan. And then it was gone in the next scan.

These spin-ups are very tough, they don't last very long and they're very difficult to forecast. So the radar can have a gap about three minutes in between the best scanning situation. And if a tornado like that happens [in] only a minute [and] develops in between scans, they can't see it.

The National Weather Service office in Louisville is excellent at reviewing severe weather events. And this is one severe weather event that they are going to review as well, especially since they technically did miss a tornado. They're going to be using this as a learning tool to make sure that doesn't happen again, and they can be a little bit better at relating to the public what's going on weather wise in an active weather situation.

This transcript was edited for clarity and brevity.