Tropical lull following Maria, Lee may not last through early October
By Kristina Pydynowski, AccuWeather senior meteorologist
September 27, 2017, 2:11:20 PM EDT
While no new threats are lurking behind Maria, residents of the Caribbean and United States should not let their guard down as tropical season is far from over.
Maria will continue to churn up dangerous seas from the Bahamas to the East Coast of the U.S. before heading out to sea later in the week.
Hurricane Lee came back to life last Friday evening. Lee will remain over the open waters of the central Atlantic Ocean and not impact any land.
Behind Maria and Lee, there are no new storms on the immediate horizon in the Atlantic Basin.
While the onslaught of devastating hurricanes Harvey, Irma and Maria came right on cue with the typical peak of hurricane season (September 10), the lack of new development is also following climatology.
“Historically, there is a lull in activity from late September to early October,” AccuWeather Hurricane Expert Dan Kottlowski said. “That is because the conditions conducive for development collapse.”
“This time of year, the train of tropical waves streaming off of Africa lessens.”
The number of tropical waves tracking westward from Africa has diminished, compared to a couple of weeks ago.
“Wind shear also increases over the central and eastern Atlantic,” Kottlowski said.
Wind shear is the changing of speed and direction of winds at different layers of the atmosphere. Strong wind shear can prevent tropical development or shred apart mature tropical storms or hurricanes.
Strong wind shear is covering a large part of the Atlantic Basin and is expected to increase over the eastern Atlantic.
“If a system comes off of Africa and faces strong wind shear, it will struggle to develop,” Kottlowski said.
While wind shear is currently low over the southern Caribbean and may lessen over the western Gulf of Mexico, any system cruising these waters is expected to remain too weak to organize into a depression or storm through this weekend.
Maria brushes North Carolina with gusty winds, coastal flooding before heading out to sea Wednesday night
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The lull in the tropics will allow the areas devastated by Harvey, Irma and Maria to continue the long road to recovery.
However, tropical season is far from over.
“As features work into the Caribbean and Gulf of Mexico in early October, wind shear is typically low and waters are at their warmest,” Kottlowski said.
In the short term, an area of disturbed weather near western Cuba and the Florida Keys bears watching. This batch of showers and thunderstorms may slowly brew as it tracks northeastward across the Florida Peninsula into this weekend.
Other areas bear watching in the long term.
"There are indications that a large circular area of disturbed weather may develop from the western Caribbean to the southwestern Gulf of Mexico during the first couple of weeks of October," according to AccuWeather Senior Meteorologist Alex Sosnowski.
A setup like this in the past has produced near-shore tropical systems.
There is another parameter meteorologists monitor for tropical development in the long term.
"There is a pulse of tropical activity that circulates around the globe," Sosnowski said.
That pulse is known as the Madden-Julian Oscillation or MJO.
"The MJO may shift to the Atlantic basin for a time during the early or middle part of October," Sosnowski said.
It is too early to say exactly what areas are at risk if the threat in early October escalates. However, this is still ample time for residents and visitors along the U.S. coast and Caribbean to review what preparations would need to be taken if a hurricane threatens.
Hurricane season does not officially end until Nov. 30, and more tropical storms and hurricanes are expected.
“I think we will have four more named storms this year, after Maria,” Kottlowski said. “Of these, three may be hurricanes and one may be a major hurricane."
The numbers include the risk of one to two additional landfalls in the U.S.
“The one thing to note is that westerly winds high above the surface start sinking into the Gulf of Mexico after Oct. 17, so it is very unusual for something to approach Texas after that time,” Kottlowski said.
“While past hurricanes in October have no bearing on what will happen this season, there have been some damaging hurricanes during the middle of the autumn in the eastern and southern U.S.,” Sosnowski said.
“These include Hazel in 1954, Wilma in 2005, Sandy in 2012 and Matthew in 2016.”
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