Dr. Jeff Masters ·

Above: A person wades in floodwaters as a van is overtaken by water in a Youngsville, Louisiana neighborhood on Sunday, July 14, 2019, during Tropical Storm Barry. Image credit: Mark Hargrave.

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.”

One study they referenced, Wang et al. (2018), “Quantitative attribution of climate effects on Hurricane Harvey's extreme rainfall in Texas”, used a global climate model to track Hurricane Harvey-like stalled systems. The researchers found that their model predicted an increase in the number and intensity of these stalled storms over southeast Texas through the mid-21st century. They cautioned, though, that the results may be different running other models.

Figure 2. Storm-total rainfall from Hurricane Harvey, August 24 – 31, 2017. Harvey stalled near the coast and dumped over 40” (yellow colors) in Houston, with isolated amounts in excess of 50” (pink colors) south of Houston and northwest of Port Arthur. Harvey lasted 117 hours as a named storm after Texas landfall, shattering the old record for named storm longevity after Texas hurricane landfall set by Fern (1971) at 54 hours. Harvey also broke the tropical cyclone-generated United States rainfall record, with over 60” of rain in Nederland, Texas, breaking the old United States record of 52” in Hawaii set by Hurricane Hiki in 1950. Image credit: NOAA.

We've been (mostly) lucky with Barry

Heavy rains continue to fall from the remnants of Hurricane Barry across the lower and mid-Mississippi Valley. Sewer systems were overwhelmed in coastal Alabama, and flash flooding was in progress Monday over southwest Louisiana, where radar data indicated up to 17 inches of rain has already fallen. A CoCoRaHS observer reported 14.03" near Bagley, LA, as of 9 am CDT Monday.



Even with these impressive totals, Barry was an example of a slow-moving landfalling hurricane that we got very lucky with. Though Barry will likely be responsible for several hundred million dollars in damages, the toll could have been over ten times greater had the original forecasts of widespread 20+” rainfall totals occurred--like the infamous “no-name” Louisiana storm of August 2016, which dumped 20 - 30 inches of rain over a 3-day period as it meandered over Louisiana. More than 30,000 people were rescued from the floodwaters that damaged or destroyed over 50,000 homes, 100,000 vehicles and 20,000 businesses. The flood killed 13 and did $10.6 billion in damage.

We don’t yet know exactly why Barry’s inland rains were later and less intense than expected through Sunday, but one piece of the puzzle is the strong northerly winds that impeded Barry from the outset. The northerly winds created strong wind shear and pushed dry air into Barry’s north side and led to a tilted system, with a broad, complex surface low near the coast and a mid- and upper-level circulation well to the south (further offshore). This somewhat unusual configuration resulted in Barry’s mid-level circulation sitting atop very warm, moist air and the very warm waters of the northern Gulf, boosting instability and spawning extremely intense convection well south of Barry’s surface center. Meanwhile, the atmosphere to the north of Barry’s center remained less moist and less unstable, and it’s possible that outflow from the persistent offshore storms further impeded the growth of inland convection. It’s surprising that even our best computer models failed to pick up on how much this evolution would cut back on potential inland rainfall through Sunday. Tropical meteorology researchers will have much to learn from this fascinating case study.

On the plus side, Barry's evolution from a disturbance in the eastern U.S. was extraordinarily well forecast nearly a week in advance. See the weather.com article for more on the long, strange odyssey of Barry.

Figure 3. Radar-estimated precipitation from Hurricane Barry as of 12:48 pm EDT July 15, 2019. Most of Barry’s extreme rains in excess of 10” (red colors) fell offshore, except for in a band across central Louisiana.

A global slow-down in tropical cyclone translation speed

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.

Figure 4. Global time series of annual-mean tropical cyclone translation speed and their linear trends from 1949 - 2016. Gray shading indicates 95% confidence bounds of the trend. Image credit: Kossin, 2018, A global slowdown of tropical-cyclone translation speed, Nature volume 558, pages 104–107 (2018).

A slowdown in forward speed also affects storm surge

Storm surge modeler Joannes Westerink of Notre Dame pointed out to us that a slowdown in forward speed of landfalling tropical cyclones also has a big impact on the storm surge. A slower-moving storm will:

1. Allow for a larger precursor forerunner surge to develop—an important factor in Hurricane Ike and Hurricane Sandy, for example.

2. Allow for more time to drive inland bay/estuarine penetration of surge, and thus more back bay flooding. However, there will be less coastal flooding along shores exposed to the open ocean compared to a fast-moving storm.

3. Allow more time for surge to spread across wetland systems, thus reducing the effectiveness of surge attenuation by wetlands. For example, slow-moving Category 1 Hurricane Isaac of 2012 inundated the Caernarvon wetlands southeast of New Orleans and overtopped the nearby Mississippi River levees at Braithwaite, while the much stronger Katrina of 2005 (Category 3) and Gustav of 2008 (Category 2) were only able to inundate the Caernarvon wetlands--not overtop the Braithwaite levees.

Related:

Slow-Moving Hurricanes Like Barry Growing More Common, our July 2019 post.

Protective Wind Shear Barrier Against Hurricanes on Southeast U.S. Coast Likely to Weaken in Coming Decades, our June 2019 post.

Highly Unusual Upward Trends in Rapidly Intensifying Atlantic Hurricanes Blamed on Global Warming, our February 2019 post.

Dangerous Rapidly Intensifying Landfalling Hurricanes Like Michael and Harvey May Grow More Common, our October 2018 post.

Extreme Hurricane Rainfall Expected to Increase in a Warmer World, our June 2018 post.

Observed Slowdown in Tropical Cyclone Motion May Portend More Harvey-Like Rainstorms, our June 2018 post

Will Global Warming Make Larger Hurricanes?, our April 2018 post.

Will Global Warming Make Hurricane Forecasting More Difficult?, our January 2017 post.

Top Ten Tropical Cyclone Events of 2016 Potentially Influenced by Climate Change, our December 2016 post.

Hurricane Patricia's 215 mph Winds: A Warning Shot Across Our Bow, our 2016 post.

Katrina-Level Storm Surges Have More Than Doubled Due to Global Warming, our 2013 post.

Damaging Katrina-Level Storm Surges are Twice as Likely in Warm Years, our 2012 post.

Big Money for Hurricane Research, our October 2006 post.

Hurricane scientists Kerry Emanuel, Jim Kossin, Michael Mann and Stephan Rahmstorf wrote an excellent May 30, 2018 realclimate.org post, Does global warming make tropical cyclones stronger?

Bob Henson contributed to this post.