A hell of high water

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When Hurricane Harvey stalled over Houston, the storm inundated the city with 40 inches of rain.

When Hurricane Harvey stalled over Houston, the storm inundated the city with 40 inches of rain.

Forecasters were not surprised by slow activity early in the 2018 hurricane season, but some have a different concern as they look ahead to the peak of the season, which starts around Aug. 1. Traditionally, the biggest concern has been winds. But every year since 2015, there has been at least one storm that stalls, resulting in excessive rainfall and flooding.

Last year, Harvey stalled over Houston, dropping 40 inches of water on the city. “The [storm] category tells me nothing about rain and storm surge,” says Hal Needham, president of Marine Weather & Climate, which specializes in flood-risk analysis and climatology. “They don’t have a high category number, but when you get 40 inches of rain in your community, it becomes a problem.”

Hal Needham

Hal Needham

Jim Kossin, an atmospheric research scientist with the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information, says Harvey was not unique in that respect. “There’s a general slowdown globally of tropical cyclones by 10 percent over the last 70 years or so,” he says. “There’s a lot of evidence to suggest that the circulation would slow down with global warming.”

Temperature differences between the Arctic and the equator create the winds that drive storms. The Arctic is warming faster than the equator, experts say, which means winds haven’t pushed hurricanes as hard. Joaquin in 2015 and Harvey in 2017 both stalled because of weak driving winds. As global temperatures continue to rise, they say, rainfall from the storms is expected to increase.

“Because tropical cyclones are carried along within their ambient environmental wind, there is a plausible a priori expectation that the translation speed of tropical cyclones has slowed with warming,” Kossin wrote in the scientific journal Nature. “In addition to circulation changes, anthropogenic warming causes increases in atmospheric water-vapor capacity, which are generally expected to increase precipitation rates. Rain rates near the centers of tropical cyclones are also expected to increase with increasing global temperatures.”

Needham says a phenomenon called the albedo effect can be used to describe what is going on in the Arctic. When the sun shines on ice, it reflects, rather than being absorbed. Because more of the polar ice is melting, there’s more bare ground exposed, and more of the sun’s heat is being absorbed. “When you look at a satellite view, it looks very dark, and the solar energy is being absorbed, which melts the ice even more,” Needham says. “That’s one reason the Arctic is warming faster than the tropics.”

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Quiet for now

One of the most critical elements that forecasters study is ocean temperatures in the Pacific and Atlantic. Ken Kunkel, a research professor at North Carolina State University, says there were signs that an El Niño could develop during the summer and into the fall, creating wind shear in the Atlantic. Water temperatures close to the U.S. coastline in the Gulf of Mexico and along the Atlantic coast are slightly above normal, but temperatures are lower than average near Africa, in the “main development region.”

“Those two factors could result in a less-than-normal season,” Kunkel says. Because of the warmer water near Florida, however, he says a storm that makes it to the coast would have “plenty of energy to develop.”

In May, NOAA’s Climate Prediction Center forecasted a 75 percent chance that the 2018 Atlantic hurricane season would have near- or above-normal activity. Forecasters predicted a 35 percent chance of an above-normal season, a 40 percent chance of a near-normal season and a 25 percent chance of a below-normal season.

Gerry Bell

Gerry Bell

“We typically only see one or two storms during June and July,” says Gerry Bell, lead seasonal forecaster at NOAA’s Climate Prediction Center. “The lack of activity so far is typical. NOAA issues an update to the May hurricane season outlook in August each year. This updated outlook coincides with the start of the August-to-October period, which is when 90-plus percent of Atlantic hurricanes form.”

NOAA forecasters predicted a 70 percent likelihood of 10 to 16 named storms (with winds of 39 mph or higher). Five to nine of those storms could become hurricanes (winds of 74 mph or higher), including one to four with a Category 3, 4 or 5 rating (winds at least 111 mph). An average season produces 12 named storms, with six becoming hurricanes and three reaching major status.

Bell says studies have shown that the main cause of recent activity is the Atlantic Multidecadal Oscillation, a mode of natural variability that affects sea surface temperatures. The AMO has been producing warmer Atlantic surface readings and an enhanced West African monsoon, which together decrease vertical wind shear and produce wind patterns that energize storms moving westward from Africa.  

“Historically, opposing phases of the AMO have lasted about 25 to 40 years,” Bell says. “They are known to be a major factor behind the alternating 25- to 40-year periods of increased, then decreased, Atlantic hurricane activity that we have seen well back into the 1800s.”

Conditions similar to today’s contributed to high activity for Atlantic hurricanes from the 1940s to 1960s, Bell says. “Opposite conditions contributed to the low-activity era for Atlantic hurricanes during 1971 to 1994 and, prior to that, during 1900 to 1925.”

Needham says that last year, a La Niña episode in the Pacific contributed to the busy season. This year, things have moved into what’s called an El Niño Southern Oscillation neutral, which typically would result in reduced activity.

Hurricane Harvey stalled because of weak driving winds, a trend that experts say will continue as global temperatures rise.

Hurricane Harvey stalled because of weak driving winds, a trend that experts say will continue as global temperatures rise.

Storms of the century

The term “100-year storm” has been used a lot the past few years, but these are localized. “When we talk about a 100-year storm, it’s for a neighborhood,” Needham says. “There are 3,007 counties in the U.S., and you can expect three counties to have their 100-year storm in a given year.”

Bell says there have been more and stronger hurricanes in the Atlantic Basin since 1995. “Such changes are not being observed in any of the other hurricane regions around the globe,” he says. “A main cause of this increased activity in the Atlantic Basin is the multidecadal signal, which accounts for the set of wind, pressure and ocean temperature patterns that have been favoring more hurricanes. These patterns are distinctly different from those expected from anthropogenic greenhouse gas warming.

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“Also, based on a critical survey of the published scientific literature to date, an international expert team recently concluded that ‘it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes,” Bell adds.To help improve its predictions, NOAA is using the GOES-17 weather satellite that launched in March. It works with the GOES-16 (now known at GOES-East) to contribute what NOAA calls a “comprehensive picture of weather throughout the Western Hemisphere, allowing forecasters to observe storms as they develop.” A new polar-orbiting satellite, NOAA-20, will gather high-resolution data from around the globe, driving three- to seven-day forecasts that are critical to storm preparedness and evacuations.

Also for 2018, the National Weather Service will run a version of the Global Forecast System called FV3 GFS with a new core alongside the current model, often referred to as the American version. This marks the first core upgrade to NOAA’s flagship weather model in more than 35 years, which should enable more sophisticated tools, including graphics that show arrival times for tropical storm-force winds.

This article originally appeared in the August 2018 issue.

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