What makes hurricanes stall, and why is that so hard to forecast?

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<span class="caption">When Hurricane Dorian, seen here from the International Space Station, stalled over the Bahamas in September 2019, its winds, rain and storm surge devastated the islands.</span> <span class="attribution"><a class="link rapid-noclick-resp" href="https://earthobservatory.nasa.gov/images/145559/a-devastating-stall-by-hurricane-dorian" rel="nofollow noopener" target="_blank" data-ylk="slk:NASA">NASA</a></span>
When Hurricane Dorian, seen here from the International Space Station, stalled over the Bahamas in September 2019, its winds, rain and storm surge devastated the islands. NASA

A lot can go wrong when hurricanes stall. Their destructive winds last longer. The storm surge can stay high. And the rain keeps falling.

During Hurricane Sally, Naval Air Station Pensacola reported more than 24 inches of rain as the storm’s forward movement slowed to walking speed along the coast. We saw similar effects when the decaying Hurricane Harvey sat over Houston for four days in 2017 and dropped up to 60 inches of rain in some areas – that’s 5 feet! Hurricane Dorian slowed to 1 mile per hour in 2019 as its winds and rain battered the Bahamas for two days.

Post-Tropical Storm Beta was the latest stalling storm, flooding streets in Houston as it slowly crept up the Texas coast and eventually moved into Louisiana.

Research shows that stalling has become more common for tropical cyclones in the North Atlantic since the mid-20th century and that their average forward speed has also slowed.

So, why does this happen? Here are answers to some questions I hear as a meteorologist about how storm systems move and why they sometimes slow to a crawl.

Why do some storms move fast and others slow?

Hurricanes are steered by the winds around them. We call this the atmospheric flow. If those winds are moving fast, they’ll move the storm fast. You can picture it as a leaf floating on a stream. If the stream moves slower, the leaf moves slower. When the flow turns, the leaf turns.

What the atmospheric flow is doing in a given location on a day-to-day basis can be pretty variable. How quickly a given storm will move depends on