Notice the Weather Moving Slower? Here’s Why That Happens Heading Into Summer

Chris Dolce

Published: May 31, 2021

Late spring and summer is when the atmosphere does its annual tap of the brake pedal, which means the storms you experience are about to move slower because of the jet stream's shrinking influence on the nation's weather.

The jet stream is a narrow ribbon of strong winds about 30,000 feet high in the atmosphere. It can steer and influence the weather conditions where you live.

Fall, winter and early spring are when the jet stream has its greatest influence on weather conditions in the United States. As you can see in the image above, the jet stream is stronger during those colder months, which means storm systems can move faster as they travel from west to east. There is also a greater variation in temperatures across the country.

An example of a wintertime jet stream setup.

But in late spring and summer, the jet stream typically does two things:

1. It retreats northward to an average position near the border between the U.S. and Canada.

2. The average wind speed of the jet stream slows down because of a decreased temperature contrast between the Earth's higher and middle latitudes.

Because of those two factors, rain and thunderstorms, on average, move slower this time of year.

An example of a summertime jet stream setup.

One reason is that frontal systems, which help form rain and thunderstorms, move slower or even stall because the jet stream has lost its ability to shove them away.

That also allows for building heat and humidity to form south of the jet stream. When that happens, we can see hot domes of high pressure build, resulting in multiday heat waves.

The high humidity is also why the South often sees isolated to scattered "pop-up" afternoon thunderstorms daily. In minutes, those sluggish storms can unload an inch of rain in one area since the jet stream isn't around to move them along at a faster speed.

Of course, the slow-moving nature of thunderstorms this time of year also means there is a higher occurrence of flash flooding.

(MORE: Late Spring and Summer Are Prime Time for Flash Flooding)

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.