Tropics ease off, but what’s on horizon?
Tumult in the tropical Atlantic abruptly shut down Sunday and may remain idle for the near future following a week where forecasters juggled three hurricanes, a tropical storm and two motivated disturbances.
With Florence and Joyce demoted to depressions, Helene going post-tropical and Isaac reduced to a remnant, atmospheric scientists paused Monday to examine the bustle of activity that is challenging August predictions of a below normal storm season.
“In terms of what made it go so nuts, there’s not a 100 percent slam-dunk answer,” said
hurricane expert Phil Klotzbach, who is lead writer for Colorado State University’s seasonal storm forecast. “That’s what makes science interesting. Not everything is a slam-dunk.”
Through mid-September, the Atlantic basin has been slightly above average with 10 named storms, five hurricanes and an accumulated cyclone energy, or ACE, of 81.4. ACE is a measure of the intensity and lifespan of storms. A normal amount of activity for this time of year is 7.5 named storms, 3.6 hurricanes and an ACE of 63.5.
For the first time since July 31, there are no tropical storms, hurricanes or typhoons on the planet, Klotzbach noted.
“Sometimes the globe just needs a break,” he said.
Contributing to the ramp-up last week in the Atlantic was the Madden-Julien Oscillation, or MJO — an eastward moving grouping of clouds, showers and winds that traverses the planet and returns to its starting point every 30 to 60 days.
A train of tropical waves leaving the west coast of Africa — seeds for strong Cape Verde hurricanes — coincided with the MJO’s visit. Also, a favorable weather pattern of low wind shear and lots of tropical moisture opened up to encourage strengthening.
“This year, despite the recent uptick in activity, the overall activity remains typical of a less active season,” said Gerry Bell, lead seasonal hurricane forecaster for the National Oceanic and Atmospheric Administration’s Climate Prediction Center. “During August through October, all hurricane seasons have at least several named storms, and most have at least three hurricanes.”
This year, three hurricanes — Florence, Helene and Isaac — and two tropical storms — Gordon and Joyce — formed between Sept. 1 and Sept. 12.
The U.S. avoided all of last week’s named storms except Florence, a Cape Verde cyclone that became the season’s first major hurricane. Florence made landfall in North Carolina on Friday as a Category 2 cyclone, killing at least 17 people. In addition to a raging storm surge, nearly 36 inches of rain fell between Thursday and Monday in some parts of the state, contributing to devastating flooding that cut off Wilmington and left 14,000 people in emergency shelters.
For Palm Beach County, Florence trapped an area of high pressure over Florida that lowered rain chances and hiked weekend temperatures to 91 degrees in West Palm Beach — 3 degrees above normal. Monday morning’s temperature dipped only to 79, which is 4 degrees above normal.
If 10-day forecasts for a quieter period in the Atlantic hold true, that may be the end of Cape Verde storms like Florence for 2018.
While hurricane season lasts through Nov. 30, the tropical waves that produce Cape Verde storms tend to diminish by late September, said Bob Henson, a meteorologist for Weather Underground, part of IBM.
“We’ve come past the normal peak, and it was an especially sharp peak this year because everything kind of aligned,” Henson said. “It was just a constellation of ingredients that came together, and if the formula changes just a little bit, the numbers can drop pretty sharply.”
Henson said a massive plume of Saharan dust could help knock down any budding systems in the eastern Atlantic, and there is still up to a 55 percent chance that El Niño will develop in the fall, which would work to thwart late-season storms.
There is a 65 to 70 percent chance a winter El Niño may form.
“The season is not over; that’s an important message,” Henson said. “There is very warm water in the Caribbean, and my concern is if something should develop there, it will have plenty of oceanic jet fuel to draw on.”
According to a NOAA database, 22 tropical storms or hurricanes have come within 100 nautical miles of West Palm Beach during October since 1865.
That includes 2005’s Hurricane Wilma, which cut a path of destruction from Florida’s southwest coast through Jupiter on Oct. 24.
“Certainly we can still see storms,” Klotzbach said. “The conditions in the Atlantic aren’t quite as harsh for them as we thought they might be.”
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‘The season is not over . ... There is very warm water in the Caribbean.’ Bob Henson
Weather Underground meteorologist