As weather-watchers focus on Typhoon Vongfong in the Pacific and Cyclone Hudhud in the Bay of Bengal, a subtropical depression has formed south of Bermuda, prompting a tropical storm watch there.
With maximum winds of 35 miles (56 kilometers) per hour, the system was 590 miles south of Bermuda, according to the U.S. National Hurricane Center in Miami. A storm gets a name when its winds reach 39 mph.
While one potential system may be enough to give pause, there are clues that larger storm-friendly environmental forces such as the Madden-Julian Oscillation and a Kelvin wave are coming together across the basin, said Dan Kottlowski, a hurricane forecaster at AccuWeather Inc.
“That’s why the season lasts until late November,” Kottlowski said.
Fortunately, the pieces are coming together a month after the Atlantic season’s statistical peak, at a time when the heat is draining out of the tropics. An alignment like this a month ago might have provided fuel for a severe outbreak.
The first piece of the puzzle is the Madden-Julian Oscillation, which many researchers believe enhances conditions for tropical storms and hurricanes. It’s entering the Atlantic.
The MJO is a “tropical disturbance” that moves eastward across the globe, according to the U.S. Climate Prediction Center in College Park, Maryland.
Kottlowski said a simple way of thinking about it is to imagine a large swell moving through the atmosphere, similar to the way one would move across an ocean. When the MJO arrives in the Atlantic, it lifts the upper atmosphere, making it easier for thunderstorms to grow. Those are the building blocks of hurricanes.
Along with the MJO, a Kelvin wave will be moving into the basin. It also can lift the atmosphere and spur thunderstorms, and may help the potential storm north of the Virgin Islands get going.
This season’s only major Atlantic hurricane, Edouard, formed last month as another Kelvin wave swept through, Phil Klotzbach, lead author of Colorado State University’s seasonal hurricane forecasts, said in his 14-day outlook yesterday.
The wave and the MJO led Klotzbach to say the potential for storm development has reached average levels across the basin for the next two weeks. That’s an increase from the end of September.
Two computer models are calling for tropical disturbances to develop in the next six to 10 days, in addition to the one being tracked by the hurricane center, Jeff Masters, co-founder of Weather Underground, wrote in his blog today.
On top of all of this, there’s a large cyclonic flow of winds above Central America. Kottlowski said the atmospheric gyre won’t become a storm itself, but it can spin off frontal systems that may grow into tropical systems.
With just over seven weeks left in what has been a lackluster season — one with five named storms, compared with an average of 12 — it looks like there are still a few things that bear watching. Keep an eye on that MJO.