What is Zonal Flow?

When meteorologist are putting together a forecast, one of the places we start is by looking at the jet stream. That’s the flow of winds in the upper part of the atmosphere. It’s these winds that drive our weather. The winds in the jet stream literally push storm systems along and across the country, generally from the West Coast to the East Coast.

The jet stream also gives us a good look at where storms will develop and how strong they’ll get. When we’re looking for storms, we often look for bends in the jet stream. When you get a dip in the flow of those upper level winds, we call it a trough. As the winds curve around those bends in the air flow, they create spin in the atmosphere, and it’s that spin that can help create and strengthen storms here at the surface.

But, when the air in the jet stream is flowing pretty much directly west to east, there are no bends in the jet stream. We call a situation like this “zonal flow.” The map above is the output of the GFS model at the 250mb level for Saturday Evening. This is showing us the strength and flow of winds in the upper atmosphere and is a depiction of the jet stream at that time. Notice everything is pretty much flowing directly from the west to the east. There aren’t any big turns or bends anywhere in the upper atmosphere across the Continental U.S. This is a pretty good example of “zonal flow”

OK. That’s nerdy, but so what? What does it mean for the weather this weekend? Well, take a look below…

This is the GFS precipitation output map for the same time Saturday evening. Notice there is some rain in the Pacific Northwest and a few showers in the Gulf of Mexico and off the Southeast Atlantic Coast. But, the rest of the country is pretty much in the clear. That’s the effect of zonal flow. When we see zonal flow in the upper atmosphere it means we won’t be seeing any Big Bad Storms anytime soon.

So, enjoy your weekend. It looks like it’s going to be storm free from coast to coast.