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Capital Weather Gang

Seemingly overnight, the oceans are exploding with tropical cyclone activity

September 12, 2018 at 1:04 PM

Wind speed and streamlines around the global tropics highlighting what are now six active tropical cyclones. Tropical Depression Paul was downgraded to a remnant low. (

The Northern Hemisphere is facing an onslaught of hurricanes and typhoons, seemingly overnight. With three storms spinning in the North Atlantic — Hurricane Florence one of them — the tropics have exploded to life at the peak of the annual season. At the same, in the tropical Pacific, Super Typhoon Mangkhut is the most intense tropical cyclone in the world, packing 170 mph winds.

Why the remarkable uptick in activity? In the Atlantic, in particular, it's all thanks to a sudden alignment of the two things that fuel hurricanes: energy and wind.

If the winds aloft in the atmosphere are too strong, they can shear apart a developing storm. It’s ironic but true — calm winds are needed to brew a hurricane. The amount of shear in the Atlantic has reached its seasonal minimum, kindling any fledgling storm and fostering its growth.

What also changed in the past two weeks is how much instability — or “juice” — storms had to work with. Until two weeks ago, we were well below average. Then all of a sudden, a switch flipped, and the results were explosive.

Category 4 Hurricane Florence is on a crash course with the coast of the Carolinas. As steering currents in the atmosphere slacken this weekend, concern is growing that Florence may stall — prolonging its barrage and producing catastrophic rainfall.

Related: Hurricane Florence charges toward Carolinas with ‘potential for unbelievable damage’

Florence has company in the Atlantic. Helene is a Category 1 west of Cabo Verde, boasting 90 mph winds. While the storm is impressive on satellite, it looks likely to remain over the open ocean. The storm will probably become swept up in the jet stream over the weekend, possibly dousing parts of Europe with some heavy rainfall toward the middle of next week.

Isaac is also out there spinning around. The tropical storm will buffet the Lesser Antilles with 60 mph winds before passing well south of Cuba and Puerto Rico in the Caribbean.

Overwhelmed yet? But wait — there’s more.

We’re also watching not one, but two additional systems in the Atlantic. A disturbance posted just offshore of the Yucatan Peninsula is likely to become a tropical depression by the weekend. The National Weather Service may dispatch an Air Force reconnaissance plane to probe the system Wednesday. The National Hurricane Center is advising residents along the coasts of Texas and Louisiana to monitor the system.

An additional wave of low pressure several hundred miles southwest of the Azores may also develop tropical or subtropical characteristics in the next couple of days, but it remains no immediate threat to any land.

If the other two systems in the Atlantic develop into tropical storms, there could be five cyclones simultaneously. That’s only ever happened once — between Sept. 10 and 12, 1971.

Into the Pacific, Super Typhoon Mangkhut is producing winds sustained to 170 mph and enormous waves as it treks about 200 miles west of Guam. The monster storm is expected to clip the northern Philippines on Friday as a the equivalent of a strong Category 5 hurricane.

That’s not the only tempest lurking off the coast of China. Tropical Storm Barijat will pass south of Hong Kong Wednesday. Hong Kong will also have a close call with Mangkhut, which is predicted to pass near the city of 7 million over the weekend, probably as a low-end Category 1 equivalent.

Hawaii is dealing with its own Pacific threat. Tropical Storm Olivia is deluging the archipelago with up to 15 inches of rain in spots. Barely two weeks ago, Hawaii set a state record for all-time storm total rainfall — an astonishing 52.02 inches from Hurricane Lane. With the tropical paradise under the gun again, it’s no surprise that climate change may favor more Hawaii storms in the years ahead.

There is one additional system to watch west of Mexico, but it looks as if it will remain tame.

So combining all of this tropical storm action together, how does it compare with what's normal?

The Accumulated Cyclone Energy, or ACE, is a metric that combines the duration and intensity of storms. Across the Northern Hemisphere, the year is at 159 percent of average for the date. The largest contribution to the anomaly comes from the East Pacific (245 percent of average), followed by the West Pacific (124 percent of average) — the Atlantic and Indian oceans are both slightly above their climatological averages.

This surge of activity in the middle of September is not a big surprise, though. If we look at the timeline of historical activity in each of the major basins, this time of year is no stranger to action:

So right now, tropical storm activity is typically elevated in all of these basins.

Annual time series of tropical cyclone activity across the three major Northern Hemisphere basins: West Pacific (top), East Pacific (middle), Atlantic (bottom). (NOAA/HRD)

But even at this active time, this year is even more busy than normal. Capital Weather Gang contributor Phil Klotzbach, a hurricane researcher at Colorado State University, maintains a website for tracking this sort of activity. Using the data available there, when we can break things down by basin, we see that every ocean basin is featuring near normal to above normal tropical activity in 2018:

A comparison of ACE values in the four major basins around the Northern Hemisphere: West Pacific (upper left), East/Central Pacific (upper right), Atlantic (lower left), and North Indian (lower right). The climatological value is the black line, and the 2018 value is the blue line. (Colorado State University) (CSU/CSU)

What does this all mean in terms of climate? The answer is unclear. There is research to suggest that the number of storms that develop will not change significantly as a result of climate change. Instead, there is a likelihood that those that develop could become increasingly strong. Regardless, one thing is clear: What’s happening in the tropics worldwide right now is out of the ordinary.

Brian McNoldy works in cyclone research at the University of Miami’s world-renowned Rosenstiel School of Marine and Atmospheric Science (RSMAS). His website hosted at RSMAS is also quite popular during hurricane season.

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