Slow-Moving Atlantic Storms Like Imelda and Dorian are Growing More Common

https://www.wunderground.com/cat6/Slow-Moving-Atlantic-Storms-Imelda-and-Dorian-are-Growing-More-Common?cm_ven=hp-slot-3

Dr. Jeff Masters · September 24, 2019, 4:58 PM EDT

Recovery efforts are underway in southeast Texas after the devastating rains unleashed last week by Tropical Storm Imelda, the fifth-wettest tropical cyclone in continental U.S. history. Imelda made landfall as a minimal tropical storm with 40 mph winds southwest of Galveston, Texas, on September 17, less than two hours after getting named. At landfall, Imelda was traveling northward at just 5 mph, and it maintained a generally northward motion at between 3 and 7 mph for the next 48 hours, gradually weakening. This excruciatingly slow pace allowed Imelda to dump rains of up to 43.39” over southeast Texas, causing catastrophic flooding that killed five. Imelda’s price tag will undoubtedly be in the billions.

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Imelda, Dorian, Florence, Harvey, and Idai are examples of storms we have been seeing more often in recent decades: ones that move more slowly over land, resulting in increased flooding and damage. The forward speed of tropical cyclones (which includes all hurricanes, tropical storms, and tropical depressions) has decreased globally by about 10% since 1949, according to a 2018 paper in the journal Nature by NOAA hurricane scientist Dr. Jim Kossin. 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. (See my June 2018 post, Observed Slowdown in Tropical Cyclone Motion May Portend More Harvey-Like Rainstorms.)

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In a June 2019 reply, Dr. Kossin responded to their concerns, acknowledging that they might be valid over ocean areas, but not over land, where positions are measured directly—not by satellite. He presented new data showing that the highest quality and longest-running storm speed dataset that we have for tropical cyclones—for ones over land in the continental U.S. beginning in 1900—has showed a significant slowdown, which should be independent of the introduction of satellite measurements. Kossin's analysis of a 118-year data set over the continental U.S. from 1900 to 2017 found a 17% slowdown in the forward speed of tropical cyclones over land.

A June 2019 paper by Timothy Hall and James Kossin (open access) found that in addition to moving slower over land, North Atlantic tropical cyclones have become increasingly likely to “stall” near the coast, spending many hours in confined regions: at least 48 hours inside of a circle 400 km in diameter. In an email, Dr. Hall affirmed that Imelda fit well within their definition of a stalling tropical cyclone, since it spent 52 hours inside of a circle 200 km in diameter; Harvey and Florence also were stalling storms by their definition.

The 2018 study by Kossin and 2019 study by Hall and Kossin did not attempt to attribute the slowdown and stalling behavior to human-caused climate change, saying: “there is not at present a clear mechanism explaining the observed tropical cyclone speed reduction.” However, in an email, Dr. Kossin said, “the vast majority of the models consistently predict a slowing of the tropical circulation due to human-caused global warming. So, in my opinion, given that tropical cyclones are somewhat passively carried along in these winds, a reasonable hypothesis is that they are slowing down with warming. This is fairly compelling evidence for a human fingerprint on the slowing we're observing.”

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Texas experienced its second-hottest August on record last month, and this near-record heat heated up the waters of the western Gulf of Mexico, helping fuel Tropical Storm Imelda’s rains. Water temperatures in the western Gulf of Mexico on September 16, prior to Imelda’s formation, were near 30°C (86°F)—about 1°C (1.8°F) warmer than average. For each degree Celsius of ocean warming, about 7% more water vapor can evaporate into the air, and hurricanes can act to concentrate this moisture to generate much heavier rains than a simple linear 7% increase in rainfall per degree C of ocean warming. Human-caused global warming made near-record hot temperatures like Texas and the western Gulf of Mexico experienced in August more likely to occur.

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