Atmospheric blocks complicate the process of creating a weather forecast. Meteorologist Tawana Andrew explains in the latest Science Behind the Forecast with LPM News’ Bill Burton.
This transcript was edited for clarity and length.
Tawana Andrew: Good morning. Today, we are talking about when things tend to stop in weather.
Bill Burton: There are times when things stop yet there's still a lot going on. We're talking about atmospheric blocks today. What do we need to know?
TA: In weather, we're always talking about things moving across the country, moving across the world. But in the case of an atmospheric block, things get pretty stagnant, and you get the same type of weather pattern happening over location for days or even in some cases, weeks.
With an atmospheric block, you have an area of high pressure or low pressure settling over a region, preventing other weather systems from moving through, or in some cases even forcing them to go around a certain area. They're typically associated with highs because high pressure systems tend to move a little bit more slowly than low pressure systems, and highs tend to cover a larger area.
There's five different types of atmospheric blocks. There's the Omega, Rex, ring of fire, the split flow, and the cut off low.
BB: I'm writing all these down.
TA: With the Omega block, when you look at it on a weather map, it looks like the Omega, the Greek letter symbol. It occurs when you have high pressure basically sitting directly over the center of the country. And you'll have troughs located on both coasts. With this type of configuration, you have air going from the southwestern United States, all the way northward into Canada, swinging around the high and then going right back south into the southeastern United States. Under that area of high pressure, you end up with dry conditions and light winds, while the areas underneath the lows, underneath the troughs, end up with cloudy conditions and rain. And these just tend to sit over the U.S. for weeks at a time, sometimes the western side of the block, the West Coast, will typically see above normal temperatures. Then the eastern side, the East Coast, will usually end up with below normal temperatures.
In most instances, we are underneath the high here in Indiana and Kentucky, so we're just kind of stuck with prolonged dry weather, light winds and usually a lot of sunshine.
BB: Poor us.
TA: I know! That sounds awful, doesn't it? Then sometimes with a blocking high, you end up with that usually in the summer, and these are responsible for significant heat waves. In fact, with a specific kind of blocking high, you end up with what's called the ring of fire. That's when the high pressure sits over the southeastern U.S. You get that ring of fire terminology from the location of the thunderstorms in relation to high pressure. With a high in the center of it, you end up with pretty stable, stagnant air. So the air is sinking, and what that's doing, it's warming the lower atmosphere as it sinks, but it's also trapping heat rising from the Earth's surface. That's why you end up with those prolonged heat waves underneath a high pressure with the ring of fire.
BB: And now I have Johnny Cash playing in my head.
TA: Now that's gonna be in my head all day, too!
Where we got the active weather, where Johnny Cash is kind of running around, will be on the outer edges of the high because that is where the atmosphere is a little bit more unstable. Just unstable enough for, usually, afternoon thunderstorms to pop up, literally creating a ring around the area of high pressure. And it's really cool to see.
BB: Atmospheric blocks. They are not fun. They cause all sorts of grief for meteorologists, but at least now we have a better understanding of them, thanks to this edition of Science Behind the Forecast. Thanks for the knowledge, Tawana.
TA: Of course.
