Tornadoes Are More Powerful Than We Realize

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We are heading into the time of year when bouts of thunderstorms often move at a slower pace, sometimes unleashing torrential rainfall in a short amount of time because atmospheric winds hit their annual tap of the brake pedal.

Blame the jet stream. Late-spring and summertime is when the jet stream loses its punch, which is the key reason why weather systems move slower this time of year. The jet stream is a narrow ribbon of strong winds about 30,000 feet high in the atmosphere, and it usually has a bigger influence on our weather from later in fall through winter and early spring.

Two factors cause the jet stream to have less of an influence in hotter months. First, it retreats northward to an average position near the border between the U.S. and Canada. The average wind speed of the jet stream also slows down because of a decreased temperature contrast between the Earth's higher and middle latitudes.

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

There are multiple sources of heavy rain this time of year. First, 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. Sometimes thunderstorms move sluggishly, or "train," along these boundaries and drop several inches of rain in just a few hours.

Heat and humidity pools south of the jet stream. When that happens, we can see hot domes of high pressure build, resulting in multi-day heat waves. Occasionally, thunderstorm complexes called mesoscale convective systems track along the periphery of those heat domes and unleash torrential rain.

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.

And, of course, we've seen several recent examples of tropical storms and hurricanes in slow steering currents creating flood disasters.

(​MORE: It's The Peak Time For Flash Flooding)

H​ere's how this summertime pattern is different than winter: As you can see in the image below, the jet stream is stronger during those colder months since there is a greater difference in temperatures across the country. That means storm systems can move faster as they travel from west to east through the stronger upper-level wind flow.

The jet stream also dives farther south, meaning weather systems just about anywhere in the country can move rather briskly unless an atmospheric blocking pattern sets up.

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Chris Dolce has been a senior meteorologist with weather.com for over 10 years after beginning his career with The Weather Channel in the early 2000s.