By Bob Henson Aug 27 2019 7:07 PM EDT From Weather Channel

Nudged on all sides by a grab bag of influences, Tropical Storm Dorian has held its own, working its way into the eastern Caribbean after passing near Barbados and over St. Lucia. Dorian is likely to move across Puerto Rico as a tropical storm on Wednesday afternoon and evening, and longer-range models point toward a track that could angle across The Bahamas and westward into Florida. Dorian’s strength by later this week remains a big question mark, but a hurricane on Florida’s East Coast over Labor Day weekend is a distinct possibility.

As of 8 pm EDT Tuesday, Dorian was centered about 300 miles southeast of Ponce, Puerto Rico, moving west-northwest at 13 mph. Dorian’s small core was disrupted when it passed directly over the high terrain of St. Lucia, and a new center formed about 60 miles to the north on Tuesday afternoon. As of 7 pm EDT Tuesday, the heaviest rains from Dorian in the Weather Underground PWS network were 4.1” onMartinique. The heavy rains led to widespread flash flooding on the island, as reported by weather.com.

Dorian continues to fight off the effects of dry air, with mid-level relative humidity around the storm at only about 45%. Periodic intrusions of this dry air have at times chipped away at the storms’ core of convection (showers and thunderstorms). The convection was looking especially disheveled on Tuesday evening onsatellite imagery. Yet Dorian’s overall circulation has remained solid enough to keep the storm going. Dorian has excellent upper-level outflow, especially on its western half, as seen in the dramatic feathering of cirrus clouds streaming away from the storm on Tuesday. A hurricane hunter flight late Tuesday found a central pressure of around 1005 mb,

#Dorian still at a standoff with + vs. – atmospheric factors for intensity. That outflow w/transverse cirrus bands tho.. pic.twitter.com/Ve3832evPs— Stu Ostro (@StuOstro) August 27, 2019

Dorian’s impacts on Puerto Rico

The relocated center of Dorian means that the storm’s projected track has shifted slightly north and east, enough to raise the odds that the storm will make landfall in Puerto Rico. The NHC forecast has Dorian passing across the central and western parts of the island late Wednesday. Widespread rains of 4-6” with pockets of 8” can be expected, especially on south-facing slopes. Mudslides will be a threat, especially where soils have been destabilized by the loss of trees in Hurricane Maria. Sustained tropical-storm force winds will most likely be limited to a small area near and just east of Dorian’s center, but these could be enough to bring down trees and power lines.

Heavy rains may also pose a threat into Thursday from the eastern Dominican Republic across the U.S. Virgin Islands.

Dorian’s future beyond the Lesser Antilles

Long-range computer models continue to highlight the risk of a late-week tropical storm or hurricane in or near The Bahamas and a potential hurricane in Florida on Labor Day weekend. A great deal will depend on Dorian’s state as it emerges from the Greater Antilles on Thursday. Some disruption from the islands is likely, and it is possible Dorian will be scrambled enough to inhibit any major recovery. However, the storm’s resiliency thus far suggests it will be able to regroup and strengthen north of the islands.

If and when Dorian can build a robust inner core and eyewall, there will be a markedly greater chance of rapid intensification. Conditions will be very supportive in and near The Bahamas, with modest wind shear (5 – 10 knots), very warm sea surface temperatures (29-30°C or 84-86°F), high oceanic heat content, and more mid-level moisture (relative humidity increasing to 60-70%).

Dynamical models like the HWRF, GFS, and European model are now coming into closer agreement with statistical models on the potential for Dorian to reach hurricane strength as it approaches Florida (see Figure 2).

Figure 2. Predicted wind speeds (colors) and pressure (black lines) for Dorian at 11 am EDT Sunday, September 1, 2019, from the 12Z Tuesday, August 27, 2019 run of the HWRF model. This model, one of our top three performing intensity models at long ranges last year, was predicting that Dorian would make landfall as a Category 1 hurricane with 80 mph winds on the central coast of Florida. Image credit: tropicaltidbits.com.

As a weak upper-level low moves from The Bahamas toward Cuba, models agree that Dorian is likely to arc toward the west or west-northwest, perhaps reaching Florida by Sunday. Update: The official NHC forecast at 11 pm EDT Tuesday placed Dorian near Melbourne, Florida, as a Category 1 hurricane on Sunday evening. Some individual model runs have been much stronger, such as the 18Z Tuesday runs of the HWRF model and the recently retired legacy GFS model. (That version of the GFS was last year’s top-performing model for intensity at 120 hours, as we discussed in a July post.)

We can expect some wild swings in Dorian’s model-depicted intensity for Labor Day weekend until we find out how the storm is structured after it moves past Puerto Rico. There is also a great deal of track uncertainty from this weekend onward, after Dorian takes its predicted arc toward the west. Of the ensemble runs from the 12Z Tuesday suite of models, about a third of European members and about 40% of GFS members make Dorian a hurricane. Most of the ensemble tracks take Dorian into Florida or south Georgia this weekend, but the location spread becomes enormous by next week, literally ranging from South Texas to the Canadian Maritimes. There is also a chance that steering currents will weaken enough to allow Dorian to stall or move very slowly close to the Southeast U.S. coast, raising the prospect of torrential rains.

Tropical Storm Erin forms in Northwest Atlantic

Update (11 pm EDT): Still centered about 300 miles southeast of Cape Hatteras, TD 6 became Tropical Storm Erin at 11 pm EDT Tuesday. Erin consisted of a low-level swirl with a convective shield located mostly to its southeast. The center was exposed for much of Tuesday, but convection had shifted partially over the center by late Tuesday, causing the system’s satellite signature to reach minimal tropical-storm levels. Erin has only a short window to intensify a bit further before it gets shunted northeast and takes on non-tropical characteristics amid increasing wind shear. As a post-tropical storm, Erin could bring heavy rain and gusty winds to the Canadian Maritimes. 

Dr. Jeff Masters contributed to this post.

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