GEOFF BENNETT: Texas is entering its third week of a record-breaking heat wave.

The heat index could top 120 degrees.

And the sweltering weather is not limited to Texas.

At least 45 million Americans across the Southern U.S. were under heat advisories over the weekend into today.

For a closer look, we're joined by Jeff Berardelli, chief meteorologist and climate specialist for WFLA-TV in Tampa, Florida.

Jeff, thanks so much for being with us.

So, this brutal heat wave, as we said, is entering week three.

What's driving it?

JEFF BERARDELLI, Chief Meteorologist, WFLA-TV: Well, we have a very weird jet stream across the Northern Hemisphere, and especially right across North America.

Things have been stuck for weeks and weeks.

And this heat dome has been stuck across especially Mexico.

And, also, Texas has been on its northern end.

And because of that, we're seeing what is an extreme heat wave, one of the worst heat waves ever in Mexico, and a very bad one also in the southern part of Texas, especially.

And that's going to be spreading to the north.

GEOFF BENNETT: So there's heat.

We have also seen tornadoes and other sorts of extreme weather across the South.

Is all of that connected?

JEFF BERARDELLI: Yes, when we have these heat domes, we call them rings of fire, because, on the north side of these rings of fire, we have the energy, the contrast between really hot and just warm and, at the same time, stronger winds in the upper atmosphere that tend to generate these big thunderstorms.

And then they follow that ring of fire, and they can produce tornadoes and also very, very strong squall lines that have extremely strong gusts of wind as well.

GEOFF BENNETT: Jeff, help us understand the ways in which this heat wave is connected to climate change.

JEFF BERARDELLI: Well, every heat wave basically is connected nowadays to climate change.

Remember that the climate has warmed around two degrees Fahrenheit since around 1900.

Now, that is when you consider both the land temperatures and the surface of the water.

But land itself has increased by about double the average of the Earth's increase.

In Texas specifically, we have seen an increase of around three to five degrees during the summertime.

So, the baseline of these heat waves starts a lot higher.

It's a lot easier to achieve heat waves nowadays.

Also, heat domes aggregate heat really, really well.

So we tend to emphasize the extremes even more than the averages.

So, the averages go up by a couple to a few degrees.

The extremes go up by several degrees.

In addition to the intensity of these heat waves, we're also seeing them last longer and occur more frequently.

The connection between climate change and heat waves is a direct A-to-B connection.

There's a lot of evidence for it.

In fact, Climate Central is doing rapid attribution these type events and says that this particular event in Texas is at least five times more likely to be because of climate change.

And what's happened in Mexico may be very well the worst heat wave we have seen in modern history, because it's been going on for so long, not just it's intensity.

GEOFF BENNETT: So, Jeff, this persistent heat becomes increasingly dangerous, even potentially deadly, as we head into the July 4 holiday week.

How should people in the affected areas prepare?

JEFF BERARDELLI: They're going to have to spend less time outside, frequent breaks, drink lots of water.

I mean, this heat wave has been confined mainly to Texas, and especially Mexico, but it's going to be expanding north into the Southern Plains states, into the Mississippi Valley, all across the Deep South, and even to Florida.

We're going to see temperatures that are going to affect a lot of people.

In fact, 100-degree temperatures will probably impact about 25 million people over the coming week, and 90-degree temperatures or greater will impact about 100 million people.

Then you add to it all the humidity from the Gulf of Mexico, because a lot of this heat will be along the Gulf Coast, and you're talking about heat index, feels-like temperatures 115 to 120 in spots.

And that, of course, can be very dangerous.

GEOFF BENNETT: So, when might this let up?

When can people start to expect some relief?

JEFF BERARDELLI: It's going to be a little while.

The worst of this heat wave is not done yet.

In fact, we're expecting the worst of it to be the middle to the end of this week.

It's going to let up a little bit as we head into the weekend.

But, even over the weekend, we're still going to see some record highs.

I think the worst is going to be probably Wednesday, Thursday into Friday.

So it's going to take a while for this to go away.

But the bottom line is, in a warmed climate because of climate change, we're going to see more of these heat waves in the future, they're going to be more intense, and they're going to last for a longer period of time.

GEOFF BENNETT: Jeff Berardelli is chief meteorologist and climate specialist for WFLA-TV in Tampa, Florida.

Thanks so much for being with us.

JEFF BERARDELLI: You're welcome.