Dorian Growing More Organized as it Heads for Puerto Rico

Tropical Storm Dorian is growing more organized over the Eastern Caribbean as it heads for an encounter with hurricane-weary Puerto Rico on Wednesday night. Dorian is a threat to affect The Bahamas on Friday, and Florida this weekend.

Dorian passed through the Lesser Antilles on Monday night and Tuesday morning with top winds of 50 mph. On Monday night, the center of Dorian passed just south of the south coast of Barbados. Tropical storm conditions occurred at Barbados Grantley Adams Airport, which reported sustained winds of 38 mph (10-minute average), gusting to 55 mph, at 9 pm AST. The airport recorded 0.78” of rain (19.8 mm) from the passage of Dorian as of 8 am EDT Tuesday; rainfall amounts at personal weather stations on the island were generally in the 0.5 – 1.5” range. As of 10 am EDT Tuesday, the heaviest rains from Dorian in the Weather Underground PWS network were 3.7” on Martinique.

On Tuesday morning near 6 am AST, the center of Dorian passed directly over St. Lucia; passage of the storm disrupted the circulation, and no tropical storm-force winds or rains in excess of 1” were recorded at the island’s Hewanorra Airport. As of the 11 am EDT Tuesday NHC advisory, top winds for Dorian were holding at 50 mph and the central pressure remained 1005 mb, based on data from an Air Force hurricane hunter mission.

Figure 1. Data from the Martinique radar at noon EDT August 27, 2019, showed that Dorian had increased in organization, with a growing region of low-level spiral bands. Dorian had closed off a cloud-free center, a sign that the storm may be beginning to build an eyewall.

Latest radar images on Tuesday afternoon from the Martinique radar showed that after passing St. Lucia, the center appeared to be reforming to the west of Martinique--a significant northward jump that will have implications on the storm's future track. Dorian had increased in organization, with a growing region of low-level spiral bands. Curved bands had formed near the center of circulation, a sign that the storm may be ready to build an eyewall. Dry air was still interfering with this process, though, and satellite images showed a lopsided appearance to the storm, no doubt created by dry air intrusions, particularly on the southeast side.

Figure 2. Dorian made one of the closest passes to Barbados by a tropical cyclone in recorded history. According to the NHC Hurricane History website, only one tropical storm (green lines) and two tropical depressions (blue lines) have made a direct landfall on Barbados. The strongest hurricane to affect the island was Hurricane Allen of 1980, which passed just north of the island as a Category 3 hurricane with 125 mph winds (orange line). Image credit: NOAA Historical Hurricane Tracks.

Track forecast for Dorian

The track outlook for Dorian is both classic and concerning. Dorian will head generally northwest over the next several days, a trajectory strongly supported by GFS and European ensemble model runs. Within that motion, the models agree on a gradual multi-day bend later this week around a weak upper-level low that will be moving from The Bahamas toward Cuba.

The upshot is that Dorian is predicted move over Puerto Rico or the eastern Dominican Republic on Wednesday night, then angle subtly toward the left on a path that may take it through The Bahamas on Friday then into Florida by Saturday night. The average path of the 20 GFS and 51 European ensemble members from 0Z Tuesday bring Dorian onto the Florida coast this weekend, as do most of the individual ensemble members. The 0Z Tuesday operational runs of our three top track models—the GFS, Euro, and UKMET—also agree on a Florida landfall, with the UKMET across southern Florida, the GFS across central Florida, and the Euro over northern Florida.

Before any such scenario might materialize, we will have to deal with Dorian’s approach to Puerto Rico and the Dominican Republic. Track models have pointed for several runs to a path that could take Dorian over or near the Mona Passage between easternmost Hispaniola and westernmost Puerto Rico. This would put Puerto Rico on the stronger right-hand side of Dorian. If the storm angles slightly west, it could experience major disruption from the higher terrain of western Hispaniola; if it angles slightly east, it could make landfall in Puerto Rico. The apparent reformation of the center observed on radar early Tuesday afternoon implies an increased threat to Puerto Rico, and a decreased threat to the Dominican Republic.

The movement of the weak upper low steering Dorian later this week will have a hugely important influence on Dorian’s ultimate track. It is too soon to know exactly how that path will unfold, but it is certainly one with potential to cause trouble in the Southeast U.S., particularly Florida, over the Labor Day weekend. It is even possible that Dorian could perform a rare triple feat—three U.S. landfalls, one in Puerto Rico on Wednesday, one in Florida over the weekend, and one on the Gulf Coast early next week.

So ironic - 4 years ago tonight, TS Erika was about same intensity as #Dorian is now, about to affect NE Caribbean, and exact 5-day forecast was cat 1 near Florida. Latest #Dorian forecast is similar. Will it dissipate near Hispaniola like Erika or will it survive? Both possible. pic.twitter.com/mBOpP9UBnG

— Dr. Rick Knabb (@DrRickKnabb) August 27, 2019

Intensity forecast for Dorian

Dorian has been remarkably resilient in the face of massive amounts of dry air that have periodically impinged on its inner core. The storm’s convective core has waxed and waned with the repeated infusions of dry air, but Dorian’s circulation has been robust enough to keep the storm going in a roughly steady state.

There are signs that Dorian could undergo more sustained intensification between now and its approach to Puerto Rico and the Dominican Republic late Wednesday. Wind shear will be light to moderate (5 – 10 knots), and Dorian will be traveling over very warm water (around 29°C or 84°F) extending to some depth. The 12Z Tuesday run of the SHIPS model Rapid Intensification Index has a 35% chance of Dorian becoming a 80 mph Category 1 hurricane by Wednesday morning. Dynamical models like the GFS and European model do not show such an increase, though, including the HWRF high-resolution model--typically the most reliable for intensity.

Dorian’s strength by Thursday will hinge on the amount of land disruption it experiences from Puerto Rico and Hispaniola. If there is substantial land interaction, Dorian may emerge quite weak, but if it stays mostly over the Mona Passage, it could remain a potent tropical storm. From that point on, conditions will increasingly favor strengthening, and there is a chance of rapid intensification depending on Dorian’s state of organization at that point. The atmosphere surrounding the storm will be considerably more moist (midlevel relative humidity around 60%, vs. 45% on Tuesday). Wind shear will continue light to moderate, and Dorian will be passing over some of the warmest waters of the Atlantic basin (water temperatures 29-30°C or 84-86°F), with very high values of oceanic heat content.

The potential certainly exists for Dorian to be a hurricane in The Bahamas and for Florida, and residents there should prepare now for that possibility. The intensity forecasts for Dorian from the 6Z Tuesday runs of our top intensity models for Saturday, when the storm is expected to be nearing the east coast of Florida, ranged from nothing (the HMON model, which predicted dissipation) to Category 3 strength (the COAMPS model).

Figure 3. Natural-color image of TD 6 at 1550Z (11:50 am EDT) Tuesday, August 27, 2019. The depression's exposed low-level center can be seen well to the north of the associated showers and thunderstorms. Image credit: RAMMB/CIRA/CSU.

Tropical Depression 6 meandering off southeast U.S. coast

We will likely be dealing with two named storms in the Atlantic by Wednesday, as Tropical Depression 6, which formed in the waters about 300 miles southeast of Cape Hatteras, North Carolina on Tuesday afternoon, is expected to become Tropical Storm Erin by Wednesday morning.

TD 6 was nearly stationary, as of 11 am EDT Tuesday, drifting north at just 2 mph. The depression consisted of a tight low-level swirl evident on visible satellite imagery, with a small area of heavy thunderstorms displaced to the southeast of the center by high wind shear around 20 knots. SSTs are warm enough (28-29°C or 82-84°F) to support development, and wind shear is predicted to drop, and it is likely that TD 6 will become Tropical Storm Erin by Wednesday morning as it drifts slowly north or northeast.

By Wednesday evening, a trough of low pressure is expected to capture TD 6/Erin, forcing the storm to head northeast at an accelerating pace. The system is likely to bring sustained gale-force winds near 40 mph and heavy rains to eastern Nova Scotia and Newfoundland on Friday, when the storm is expected to be extratropical. Increasing wind shear should prevent TD 6/Erin from reaching hurricane strength, with NHC forecasting the system to top out with 45 mph winds on Thursday.

Bob Henson co-wrote his post.