September 19, 2019
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Tropical Storm Dorian is an increasing threat to the Southeast U.S.

Ramon Espinosa | AP
Ramon Espinosa | AP
Eduardo Torres, from Puerto Rico, walks with his daughter in the ocean before the arrival of Tropical Storm Dorian in Combate, Puerto Rico, Tuesday, Aug. 27, 2019.

Tropical Storm Dorian is headed for a hit on Puerto Rico and the U.S. Virgin Islands on Wednesday, bringing damaging winds of up to 70 mph and heavy rains that could top eight inches in spots. These rains may cause life-threatening mudslides.

By the weekend, the strengthening storm could approach the Southeast U.S. coast as a major hurricane, although there is considerable uncertainty about its final destination.

“The risk of dangerous storm surge and hurricane-force winds is increasing in the central and northwestern Bahamas and along the Florida east coast, although it is too soon to determine where these hazards will occur,” the National Hurricane Center wrote in its 11 a.m. advisory Wednesday. “Residents in these areas should ensure that they have their hurricane plan in place and not focus on the exact forecast track of Dorian’s center. “

Following overnight computer model runs, it appears probable that Dorian could become a Category 1 hurricane before its closest pass to Puerto Rico. At 11 a.m. Wednesday, the center of circulation 25 miles southeast of St. Croix, where it is likely to make a direct hit, moving northwest at 13 mph. The strengthening storm packed peak winds of 70 mph, an increase of 20 mph since Tuesday night.

Radar imagery from Puerto Rico on Wednesday morning revealed Dorian’s attempt at forming an eye, a sign of intensification. A tropical storm warning is in effect for Puerto Rico while hurricane warnings cover Vieques, Culebra and the United States and British Virgin Islands.

Widespread 30 mph to 50 mph winds with gusts to 60 mph or more are probable over Puerto Rico, especially on the island’s eastern side. The islands of Culebra and Vieques, located to the east of the main island, could experience a direct hit and see gusts approaching 80 mph.

For eastern Puerto Rico, heavy rain and strong winds will arrive by Wednesday afternoon and continue into the evening, with the worst conditions on its east coast.

A flash flood watch covers the entire island, where four to six inches, with localized eight-inch or greater amounts, could be in the offing, except in west and northwest Puerto Rico where rains will be spottier and less intense.

The island’s infrastructure, including housing and the power grid, are still fragile following Hurricane Maria in 2017, and while Dorian pales in comparison to the power of that storm, it is likely to cause power outages and damage to any homes that have not been rebuilt to withstand such weather.

According to the Associated Press, up to 30,000 homes on the island still have blue tarps for roofs.

The National Weather Service in San Juan warns that the rains could cause “rapid rises in rivers and small streams as well as a risk of mudslides.” The heaviest rain totals, exacerbated by mountainous terrain, are likely to fall in the eastern half of the island.

There remains an outside chance the storm will pass far enough to the east of Puerto Rico that rainfall amounts and wind speeds will be lower than currently forecast, especially on the island’s western half.

Meanwhile, St. Croix in the U.S. Virgin Islands was placed under a flash flood warning just after 8 a.m. Wednesday and is likely to catch the brunt of this storm. Wind gusts there could top 70 mph.

St. Thomas and St. John as well as the British Virgin Islands are also likely to deal with tropical storm or even hurricane conditions through Wednesday night with rainfall of 4 to 6 inches (and isolated amounts up to 10 inches, especially in high terrain).

By Thursday morning, Dorian is expected to pull away to the north-northwest. That is when the real forecasting headache begins.

Forecasting the future path of Dorian is not easy, as the storm’s center has consistently been located to the east (or right) of where computer models have projected. Based on its current position, any subtle differences in initial steering flow will have huge implications on where Dorian ends up.

At the moment, landfall anywhere from Florida’s east coast to the Outer Banks of North Carolina is possible, and there is an outside chance the storm will just skirt the coast and recurve out to sea. The Alabama, Mississippi, and Florida Gulf Coast should also monitor the storm as there is a scenario in which it crosses the Florida peninsula, enters the Gulf and then makes a second landfall.

Because Dorian will not interact with the high terrain of the Dominican Republic or Puerto Rico, it will not weaken significantly before emerging near the Bahamas. That gives it a higher starting point for intensification as it enters a basin of warm water and hospitable wind conditions. There is little holding Dorian back at that point, and computer models show it has the potential to rapidly intensify to at least a Category 2 storm, and quite possibly a major, or Category 3 or greater, hurricane.

“Dorian’s unanticipated northward jog on Tuesday changes a few things, all for the worse,” wrote Brian McNoldy, Capital Weather Gang’s tropical weather expert. “It means the eastern Puerto Rico will take a more direct hit, it means Dorian’s circulation will not get disrupted nearly as much as if it went closer to Hispaniola, and it significantly increases the odds of a hurricane landfall in the southeast U.S. later this weekend.”

While storms in the Dorian’s present location have historically tended to favor an earlier recurve out to the open Atlantic, Dorian is far from your run-of-the-mill storm, and has already proved an exceptionally difficult storm to predict.

“Only a small nudge separates Florida from the Carolinas,” McNoldy wrote.

That means Florida through the Carolinas are still in play; the National Hurricane Center’s prediction calls for Dorian to be a Category 3 storm off the northeast coast of Florida by late Sunday night into early Monday morning. However, the center noted that “confidence in the long term track remains low.”

“In addition, users are reminded not to focus on the exact forecast points as the average 5-day track error is around 200 miles,” forecasters wrote.

Uncertainty also abounds in the intensity forecast, which may end up being a bit of an over-performer. Models also show an expanding storm with an increasingly large wind field as it approaches the mainland United States over the weekend. Some modeling indicates a significantly elevated likelihood of rapid intensification, in which the storm’s peak winds could increase substantially in a short time interval (at least 35 mph in 24 hours).

The potential for rapid strengthening is particularly concerning, considering that high tides along the East Coast will be running unusually high as so-called “King Tides” could cause coastal flooding in the absence of any storm, and could exacerbate storm surge impacts from Dorian.

Climate change-related sea level rise also heightens the risk of storm surge flooding by raising the baseline sea levels upon which storm surges build.

The bottom line? Stay tuned to later updates. Dorian is a tricky storm, and the forecast will evolve quickly. Always have a plan and a hurricane kit ready, and heed all forecasts and warnings.

 



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