Atmospheric Gridlock: What It Means For Your Forecast

It may only be Tuesday, but we can already say this week is pretty slow and sluggish (at least as far as the weather forecast is concerned). You can thank two rounds of a weather pattern known as a Rex block for that!

Here’s what that means.

A “Rex block” is a weather pattern characterized by an upper-level high-pressure system located north of a closed low.

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Extratropical low-pressure systems like to follow upper-level wind patterns and typically move eastward in the latitudes occupied by much of the U.S. and Canada. But upper-level high-pressure systems can become a roadblock in that motion.

The squiggly shape in the jet stream that accompanies a Rex block pattern halts much of the forward movement of the low, keeping it trapped in roughly the same area for days, sometimes even longer than a week.

What this means for the forecast: Upper-level low-pressure systems produce upward vertical motion, contributing to cloud cover, snow, rain and thunderstorms when they overlap with sufficient moisture.

So when that low doesn’t go anywhere, it means days of that weather.

Upper-level lows are also accompanied by pockets of cold air that have been cut off from the north by the upper-level high and high-pressure ridge, so there ends up being a blob of cooler air stuck in one spot.

(192-hours: Further beef up your forecast with our detailed, hour-by-hour breakdown for the next 8 days – only available on our Premium Pro experience.)

Mother Nature provides us with two prime examples: There are actually two Rex block patterns in this week’s forecast, leading to a lot of slow-moving weather makers.

The first has been over the eastern U.S., causing multiple days of storms in the Southeast over the early half of the week.

The second is forming over the Central U.S., and it’s going to cause the latter half of the week in the region to be pretty dreary and relatively cool, so maybe not the best for people hoping for one last taste of summer.

Sara Tonks is a content meteorologist with weather.com and has a bachelor’s and a master’s degree from Georgia Tech in Earth and Atmospheric Sciences along with a master’s degree from Unity Environmental University in Marine Science.