Dorian Slicing through Windward Islands; Future Intensity a Big Question

Tropical Storm Dorian was closing in on Barbados Monday evening, on a track destined to take its strongest winds and heaviest rains across the island overnight and close to St. Lucia early Tuesday morning. Both islands are under tropical storm warnings, along with Martinique, St. Vincent, and the Grenadines. A hurricane watch is up for St. Lucia. Top winds were holding at 60 mph as of 8 pm EDT Monday. A direct landfall on Barbados is possible somewhere around 10 PM EDT Monday.

Dorian is fighting to preserve itself amid a vast shield of dry air extending across most of the Caribbean and western Atlantic (see our post from Monday morning). Against the odds, the storm has managed to build a robust low-level circulation and a shield of showers and thunderstorms (convection) that’s fought off the dry air thus far. Wind shear is on the light side (5 – 10 knots), making it easier for Dorian to hold its own. Any localized increase in wind shear could compromise the circulation fairly quickly and lead to a drop in strength.

Apart from the dry air, the raw materials are in place for Dorian to embark on a period of sustained strengthening. Sea surface temperatures (SSTs) of 28-29°C (82-84°F) are more than adequate, and there is substantial oceanic heat content beneath the surface.

Figure 1. Oceanic heat content (OHC) along Dorian's path is at a level considered favorable for rapid intensification, ranging from 60 - 90 kilojoules per square centimeter for at least the next several days. Even higher values are in western parts of The Bahamas, where Dorian could arrive by Saturday. When unusually warm ocean waters extend to great depth, a hurricane’s churning winds simply stir up more warm water, allowing dangerous rapid intensification to occur if wind shear is low. Thus, total oceanic heat content is a key metric used to determine the potential for hurricane rapid intensification. Image credit: NOAA/AOML.

Guidance from the SHIPS statistical model continues to send mixed messages in the short term. At 18Z Monday, the SHIPS rapid intensification index gives a 36% chance of Dorian reaching the threshold of major hurricane strength (95 knots, at the top end of Category 2) by Tuesday night. The more recently developed DTOPS model within SHIPS shows a 0% chance.

As a smaller tropical storm, Dorian could both strengthen and weaken more quickly than usual. Because of Dorian’s compact size, and its counterintuitive survival in the midst of widespread dry air, the standard global and high-resolution models should be taken with an especially large grain of salt for intensity guidance. The official National Hurricane Center (NHC) forecast as of 5 pm EDT Monday calls for Dorian to intensify slightly, reaching Category 1 strength on Tuesday before it approaches the Dominican Republic and Puerto Rico on Wednesday.

The Monday morning (12Z) suite of ensemble models is quite consistent in taking Dorian near the channel between Hispaniola and Puerto Rico, as reflected in the NHC forecast. This track would put Puerto Rico on the stronger right-hand side of Dorian, which implies that winds of at least tropical-storm strength could affect much of the island. Such winds could be enough to bring down vulnerable trees and power lines, so the territory will need to take Dorian seriously. Heavy rains are also expected, with widespread 2-4” totals and localized amounts of 6” or more are quite possible. A state of emergency has been declared in Puerto Rico; see the weather.com article for more on Dorian’s impacts.

There has been a dramatic shift in the #ECMWF ensemble guidance for #TC #Dorian.

36hrs ago, only 4/51 members (8%) had Dorian surviving past #Hispaniola. Now the vast majority (>75%) track Dorian past near the Bahamas.

Odds are increasing Dorian may be a threat for mainland USA pic.twitter.com/KbhcixgWKv

— Philippe Papin (@pppapin) August 26, 2019

Dorian could weaken dramatically if its center passes over land, but if it emerges from the Greater Antilles with its low-level circulation more or less intact, conditions will likely support strengthening as the system moves northwest into the Bahamas. Models are consistent in showing a slight leftward bend in Dorian’s track at this point, and the ensemble average for both the European and GFS models would bring Dorian onshore in Florida this weekend, possibly as a hurricane. The model average hides a great deal of variation, though. Some tracks take Dorian into the Gulf of Mexico, while others recurve it along or near the U.S. East Coast. The outcome will depend on small-scale features that are impossible to anticipate with certainty this far in advance.

To cite just one potential scenario, the 12Z Monday run of the UKMET model predicts that Dorian may be a rare triple threat to the U.S., with three landfalls in the nation—first in Puerto Rico, then in Florida, followed by a third landfall along the Gulf Coast, after the storm crosses the Florida Peninsula.

Figure 2. Infrared satellite image of Tropical Depression 6 at 1655Z (12:55 pm EDT) Monday, August 26, 2019. Image credit: tropicaltidbits.com.

Tropical Depression 6 forms off southeast U.S. coast

We will likely be dealing with two named storms in the Atlantic on Tuesday, as newly formed Tropical Depression 6 is expected to become Tropical Storm Erin.

TD 6 was nearly stationary, as of 5 pm EDT Monday, drifting east at just 2 mph. The depression consisted of a tight low-level swirl evident on visible satellite imagery about 300 miles southeast of Cape Hatteras, North Carolina, together with a large, elongated field of convection along a frontal zone just to its south and east. The front is predicted to weaken as it pushes to the northeast, leaving behind a slow-moving system that is likely to take on a more symmetric aspect on Tuesday.

Wind shear above TD 6 is moderate to strong (10-20 knots), but SSTs are warm enough (28-29°C or 82-84°F) to support development. There is very strong agreement in the Monday morning GFS and European ensembles for TD 6 to become a tropical or subtropical storm that will move slowly through Wednesday, then head northeast at an accelerating pace, perhaps bringing gale-force winds and heavy rains to Nova Scotia by Thursday or Friday. Increasing wind shear should prevent TD 6 from reaching hurricane strength.

Dr. Jeff Masters contributed to this post.