Tuesday, July 16, 2019

Slow-Moving Hurricanes Like Barry Growing More Common


Dr. Jeff Masters · July 15, 2019, 12:32 PM EDT

As Tropical Storm Barry intensified into a hurricane on its three-day trek along the Gulf Coast, the storm moved at an excruciatingly slow pace—between three and nine miles per hour. The very slow motion allowed Barry to generate a larger storm surge and dump heavier rains than a faster-moving storm would have (though most of those rains happened to fall offshore this time because of Barry's unusual structure). A study published on June 3, 2019 by scientists from NASA and NOAA found that North Atlantic hurricanes like Barry have been stalling near the coast with increasing frequency in recent decades, resulting in an increase in dangerous heavy rainfall.

The scientists said that there was not a clear mechanism explaining the observed tropical cyclone speed reduction, and that natural variability and/or human-caused climate change could be to blame. “There is some evidence that those large-scale wind patterns are slowing down in the tropics, where Atlantic storms usually start,” said Hall. “The storms are not being pushed as hard by the current that moves them along. That’s a climate change signal.”


The results of the new study agree with those of a June 2018 study by University of Wisconsin hurricane scientist Jim Kossin, A global slowdown of tropical-cyclone translation speed, which found that the forward speed of tropical cyclones has decreased globally by about 10% since 1949. As a result of their slower forward motion, these storms are now more likely to drop heavier rains, increasing their flood risk. Most significantly, the study reported a 20% slow-down in storm translation speed over land for Atlantic storms, a 30% slow-down over land for Northwest Pacific storms, and a 19% slow-down over land for storms affecting the Australia region. A storm moving 20% slower over land has the opportunity to dump up to 20% more rain atop a given point over land, increasing the flood risk for flood defense systems designed for a 20th-century climate with less extreme precipitation events. The paper concluded that “these trends have almost certainly increased local rainfall totals in these regions.” Another increased hazard slower storms bring is increased wind damage, due to an increase in the duration of damaging winds structures are exposed to.


tags: extreme weather, severe weather

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