Hurricane watch up for Puerto Rico as Dorian churns west; Florida eyeing potential weekend impact

A hurricane watch has been issued for Puerto Rico ahead of Tropical Storm Dorian, where conditions are forecast to begin deteriorating as early as Wednesday morning. The island is already under a tropical storm warning, with the threat of heavy rainfall and potential flash flooding and mudslides constituting the biggest concerns for the storm-weary island of 3.2 million.

“Our greatest concern is for the rain,” said Matthew Brewer, a meteorologist at the National Weather Service in San Juan. “We’re expecting between two and six inches, with local eight-inch amounts. We have some tight topography gradients, so flash flooding could be a problem.”

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The island could see strong winds, too, although the winds will be only 10% to 20% of what the island experienced during Hurricane Maria, the Category 4 hurricane that devastated the island in 2017.

“The southwest part of the island will experience tropical storm force sustained winds,” explained Brewer. “The metro, where most people live, will only see those winds as gusts.” Tropical storm force winds fall in the 39-to-73-mph range. These winds will accompany squally weather beginning overnight Tuesday night into the predawn hours Wednesday.

As of Tuesday early afternoon, Tropical Storm Dorian was centered about 350 miles southeast of Ponce, Puerto Rico, after moving directly over the island of St. Lucia. The system continued to fight dry air that is surrounding the storm, prompting the National Hurricane Center to lower the estimated intensity of the storm’s sustained winds to 50 mph.

A pulse of deep convection – shower and thunderstorm activity – accompanied a flare-up Monday night and Tuesday morning, and Dorian has looked much healthier since the sun rose Tuesday morning. Evidence of better outflow ― air exiting the top of the storm in multiple directions – suggests Dorian will more efficiently ingest and expel air, allowing for further intensification.

However, radar data and hurricane hunter aircraft observations show that the storm has not yet closed off a consolidated center of circulation, which is holding back its intensification. Tropical Storm Dorian’s center of circulation reformed to the north of the previous center, the Hurricane Center stated in a 2 p.m. update. This is an indication that the storm continues to try to intensify, but it’s struggling with ingesting dry air that inhibits thunderstorm activity near its core.

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“It looks ragged on radar,” wrote tropical weather expert Brian McNoldy. “But the satellite presentation is the classic ‘CDO,’ or Central Dense Overcast, with fairly symmetric outflow and obvious banding. That is usually a predictor of intensification, and it wouldn’t be surprising to see an eye form later today.”

With their 11 a.m. briefing package, the National Hurricane Center forecast Dorian to remain just below the 74 mph threshold for hurricane strength as it passes just southwest of Puerto Rico.

“Slow strengthening is forecast during the next 48 hours, and Dorian is forecast to be near hurricane strength when it moves close to Puerto Rico and eastern Hispaniola,” the center wrote.

It’s important to note that, because of Dorian’s compact size, any subtle shift in track will have significant forecast implications. The National Weather Service in San Juan wrote that since “Tropical Storm Dorian is a small system, any change in the forecast track could affect the impacts.”

In Puerto Rico, communities are still recovering from Hurricane Maria, and the infrastructure on the island is still vulnerable, particularly the power grid. The biggest concerns with Dorian involve heavy rain, mudslides, and power outages.

According to The Associated Press, up to 30,000 homes still have blue tarps on them. On Monday, Gov. Wanda Vázquez declared a state of emergency for Puerto Rico. Vázquez is new to the office after former Gov. Ricardo Rosselló resigned on July 26 under intense public pressure following a scandal involving leaked text messages between him and his political allies.

The Mona Passage wild card

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By Wednesday evening, Dorian looks to meander close to or through the Mona Passage between Hispaniola and Puerto Rico. This is a big wild card in the forecast. If Dorian tracks just a tad farther to the northeast, it could have more direct impact to western Puerto Rico. If it jogs a bit to the west, it would likely be shredded by the mountains of the Dominican Republic, which rise to about 10,000 feet and can disrupt a fledgling storm’s circulation.

But if it threads the needle and Dorian’s center slips through the Mona Passage, that would open the doors to eventual intensification near the Bahamas and a track that may take it ominously close to Florida. Model support for that scenario is increasing.

“People are on alert here in Miami, but just cautiously watching,” McNoldy wrote. “With Hispaniola still a wildcard in its future, anything beyond Thursday is extremely uncertain.”

Two of the main global computer models forecasters use – the American GFS and the European ECMWF – have trended stronger with their intensity estimates since Monday. The National Hurricane Center has followed suit. On Monday, they were forecasting a 35 mph depression off the Florida coast Saturday night into Sunday morning. That forecast has increased to predicting a storm on the cusp of becoming a Category 1 hurricane off the Florida coast. Considering that NHC’s five-day mean forecast error is 16 mph, it’s conceivable that Dorian could be either a weaker tropical storm or a more formidable hurricane.

“The [models] have . . . increased the odds of Dorian crossing Florida and entering the Gulf,” McNoldy wrote.

It’s way too early to know of any specific impact to the mainland United States. We’ll have a much better idea by Thursday as to the ultimate impact of Dorian, pending its movement near Puerto Rico and the Dominican Republic.

It should be noted that if the storm survives its passage toward the Bahamas, statistical models suggest it could significantly intensify prior to arriving in Florida. However, other models do not indicate such a scenario, which is causing Hurricane Center forecasters to warn of unusually high uncertainty with this system.

For those with interests in the Florida peninsula, the best advice is to be aware of the latest forecasts and buy any needed supplies to complete your hurricane preparedness kit should the storm head that way.

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The Washington Post’s Andrew Freedman contributed to this report.