Does Weather Typically Move Slower In Summer? It's Not Your Imagination – Blame The Jet Stream
We all know the scenario: A slow-moving thunderstorm comes along in the summer months and unleashes intense rainfall that can add up to an inch or more in a short amount of time, leaving you soaked if you're caught outdoors.
Why storms can be so sluggish and drenching this time of year is partly related to how atmospheric winds hit their annual tap of the brake pedal.
Blame The Jet Stream: Summertime is when the jet stream loses its punch, which is the key reason why storms move more slowly 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.
Why It Loses Steam: First, the jet stream retreats northward to an average position near the border between the United States and Canada, meaning it has less of an influence on the U.S. as a whole. The jet stream's average wind speed also slows down, so its ability to steadily push along weather systems is not as pronounced.
(MET 101: What To Know About the Jet Stream)
Here is a look at three ways the jet stream losing its oomph in summer can cause storms to move more slowly:
Fronts Stall More Often In Summer: Cold fronts coming across the northern tier are more likely to get stuck this time of year since 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, triggering serious flash flooding. This pattern can last for days over a region until the front eventually fizzles.
'Pop-Up Storms' Are Often Slow Movers: High humidity combined with heating from the sun often forms isolated to scattered storms that seemingly billow up randomly this time of year, especially in the South. 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.
Tropical Storms And Hurricanes: Slow steering currents due to a lack of the jet stream's influence can cause tropical storms and hurricanes to stall in some situations. Hurricane Harvey (Texas – 2017), Hurricane Florence (Carolinas – 2018) and Tropical Storm Imelda (Texas – 2019) are recent examples of flood disasters created by this scenario.
(MORE: Why Slow Storms Are The Worst)
Summer's Jet Stream Is A Reversal From Winter: The jet stream is stronger during winter's 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.
Winter's Jet Stream Is Also More Wavy: Rather than riding in a flat west-to-east fashion like in the summer, the jet stream tends to undulate north-to-south more often in winter. As a result, weather systems nearly anywhere in the country can move rather briskly unless an atmospheric blocking pattern sets up.
Chris Dolce has been a senior meteorologist with weather.com for 15 years after beginning his career with The Weather Channel in the early 2000s.
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