This transcript was edited for clarity.
Bill Burton: It's time for us to take a look at the Science Behind the Forecast as I'm joined by WAVE 3 meteorologist, Tawana Andrew. Good morning, Tawana.
Tawana Andrew: Good morning. Today's topic is about a low that makes me feel pretty low.
BB: This is not fun at all. They can do some really weird, and I mean really weird things, cut-off lows. That's our topic today. What do we need to know about cut-off lows?
TA: So cut-off lows are defined by the National Weather Service in a great way.
BB: Of course.
TA: The actual definition for a cut-off low is a low-pressure area with a distinct center of cyclonic circulation, which can be completely encircled by one or more isobars or height contour lines.
BB: Oh, yeah, that just rolls right off the tongue.
TA: For everybody else who's trying to understand what language that was, basically, it is an area of low pressure that becomes detached from the main atmospheric flow. So it's no longer under the influence of the westerly. So across the U.S., we see prevailing winds going from west to east across the country, or it becomes disconnected from the jet stream. And these are not large systems at all. They're usually only a few hundred miles wide, so they're kind of tiny on the global scale, and they can be very irritating to a forecast. They are, literally, one of the things I do not like forecasting.
So for a cut-off or a closed low, these typically occur when a short wave, which is a little pocket of energy in the atmosphere, moves southward along a trough, so one of the big dips in our jet stream. So once that moves south, the momentum disconnects that part of the trough from the main atmospheric flow, and that's what creates this area of closed low pressure. And it just kind of hangs out by itself for a while. And as they move over a location, they can cause some pretty dreary conditions. So underneath the closed low you get a lot of clouds, you'll get a lot of consistent rain, or if in the winter, you'll get a lot of consistent snow in the forecast as they pass by. And they are pretty challenging to predict, because, of course, they're no longer underneath the influence of that main atmospheric flow, so they'll kind of just drift or linger over an area. It could be for several days. It could be, in some cases, for several weeks.
BB: They're the guest that has no idea it's time to go.
TA: Basically, you're just like, “Come on, get out of here.” But sometimes they try to make things even more interesting, and they will retrograde. So we'll see them moving backwards, so from east to west across the country sometimes.
BB: Oh, that's fun.
TA: And imagine trying to explain that to people on television.
BB: No thank you.
TA: It gets really weird. And then computer models, they try to do their best to forecast these lows, and usually they're pretty good at figuring out when they will happen. But the thing is, they're not good at all at forecasting when that low will rejoin the main flow. Usually they think it's going to happen faster than what actually occurs. So for me as a meteorologist, there is a struggle to figure out if this thing is going to last for a couple of hours or linger for a couple of days, and that is where the forecast gets a lot more difficult. And these lows can occur anywhere in the world at any time of the year.
BB: You’ve got to increase the degree of difficulty, right?
TA: Exactly. So we're not the only ones dealing with it. It's everyone that can see it. And usually if you have a cut-off low, just be nice to your meteorologist, because they're probably going through it at that point.
BB: If you hear about a cut-off low, give your meteorologist a wide berth. There's a lot to handle there, but we understand cut-off lows now, thanks to this edition of Science Behind the Forecast with WAVE 3 meteorologist Tawana Andrew. Thanks for the knowledge, Tawana.
TA: Of course.
