TS Dorian Passes Lesser Antilles while Forecast Uncertainties Remain; TD Six to be Short-Lived

The tropics remain busy today, with two tropical cyclones currently active. Tropical Storm Dorian is located in the eastern Caribbean Sea with winds of 50mph and has a minimum pressure of 1005mb. Dorian is expected to impact the eastern Greater Antilles and potentially the Bahamas and Florida in the long-range. Tropical Depression 06L formed yesterday as well from Invest 98L off the North American coast with winds of 35mph and a minimum pressure of 1008mb. It should remain weak as it accelerates northeast towards Canada.

A basin-wide true-color image of the Atlantic, showing Dorian west of the Lesser Antilles and 06L off of the Carolinas. (College of DuPage Satellite Imagery)

I’ll briefly discuss TD 06L first. The storm is currently very lopsided, as convection is being sheared off towards the south from an upper-level ridge extending off the Georgia and South Carolina coastlines. This has caused the surface center to be exposed well to the north of the main convection. Scatterometer data has been unable to detect gales within the system, so it has stayed at tropical depression intensity. It is also in a region of weak steering currents for the moment, which is why it has not moved very much, only drifting slowly southeast chasing the sheared convection. By tonight, 06L should turn northward as a strong mid-latitude trough progresses across north America. By this point, the depression should gain some instability from a remnant frontal boundary to its north, which may cause the storm to take on a more hybrid appearance and structure. The system will continue accelerating northeast and become post-tropical by Thursday south of Atlantic Canada, who may need to watch out for potential impacts from this system. Regardless, the storm is expected to remain quite weak, and the NHC only forecasts the system to be a weak tropical or subtropical storm at peak on Wednesday night.

Visible imagery of TD 06L off the North Carolina coastline, exhibiting a classic sheared structure. (MSFC)
NHC track for 06L, showing a short-lived system dissipating before it reaches Canada

The main story in the tropics recently has been Tropical Storm Dorian, currently located to the west of the central Lesser Antilles. Last night, Dorian’s center passed over the islands of Barbados and St. Lucia, but fortunately, only minor impacts have been reported thus far. After passing over those islands, aircraft reconnaissance discovered that the center was quite ill-defined and weakening, while radar imagery from Martinique revealed that the center was relocating towards the north a few hours ago. During that time, the structure began to lose definition and it briefly appeared as if Dorian was opening up back into a tropical wave, but since then, the new center has become better-defined and the convective structure is doing the same. While the pressure remains quite high still, the storm as a whole has improved. The radial outflow is quite healthy and frequent bursts of centralized convection have kept the system alive.

Dorian to the west of the Lesser Antilles with an improving structure. (WeatherNerds.org)

Because the center of Dorian has shifted farther north, it’s beginning to follow north of the consensus track, which continues to bring it northwest. The upper-anticyclone above the storm, which is allowing for such impressive outflow, is forcing upper-level lows away and weakening the one helping Dorian gain latitude. However, these upper lows will also cause the ridging to lag behind Dorian’s forward movement, which will replace the Caribbean low as the upper-level steering feature, but may also induce some northerly shear onto the storm. By tomorrow night or Thursday, the track will be either into the eastern Dominican Republic or threading the Mona Passage. It now appears more likely that it’ll stay further from Hispaniola, an island notorious for its high mountains that can shred TCs apart, and closer to the Mona Passage or Puerto Rico. While Puerto Rico is also relatively mountainous, the peaks are lower and it is smaller in size compared to Hispaniola. Therefore, the chances of it surviving its passage out of the Caribbean have increased.

GFS 200mb wind forecast, showing the features at play steering Dorian. (Tropical Tidbits)

Past Thursday, the forecast confidence remains abnormally low. Most of the forecast depends on whether or not Dorian survives the passage out of the Caribbean. Because the storm is now more likely to do so, there are concerns about both the Bahamas and southeastern U.S., particularly Florida, for late this week and this weekend. Dorian should continue progressing northwestward as a weak upper low to its south gets forced around its upper anticyclone. By Friday and Saturday, a mid-level ridge will build to the north of the system near Bermuda and extend west towards the Carolinas. At this point, the storm should halt its northward motion and instead track more west. In addition, conditions for intensification become more favorable, which should result in an intensifying storm. This would bring it mostly north of the Bahamas, with the potential exception of the northwest Bahamas. However, the track would then take it into the east coast of central Florida by Sunday. Afterwards, the system should curve north into the southeastern U.S. as another trough dips into the eastern U.S. this weekend. This all is assuming that it survives the passage past Hispaniola or Puerto Rico.

12z EPS ensemble tracks for Dorian, and it should be noted that these members are plotted too far south given the northward relocation and that track/intensity confidence still remains low. (WeatherNerds.org)

It is important to restate once more that the confidence in this forecast remains low. However, impacts are still coming to both Hispaniola and Puerto Rico, as well as lingering effects in the Lesser Antilles from heavy rainfall. While the rainfall forecasts aren’t particularly high, if a training band develops either from shearing instability or orthographic lift, potentially flooding rains could fall on either island from Dorian. Hurricane watches and Tropical Storm Warnings remain in effect for all of Puerto Rico and the eastern Dominican Republic. The NHC forecasts a landfall on the southwestern portion of Puerto Rico at this point, which would bring gusty winds and possibly minor storm surge as well to the southern coast. As for the Bahamas, impacts should remain minor in the main island chain, but rough seas and potentially squally weather will make for an unpleasant weekend, especially for Great Abaco and Grand Bahama. For those in Florida, keep a very close eye on Dorian, as it is becoming increasingly likely that it’ll have some sort of impact on the state during the weekend, which is also Labor Day Weekend and there will be more tourists about. Dorian could put a damper on that. The entire peninsula is in the cone, and even though it is unusual for a hurricane to make landfall in central or north Florida, it is not impossible. Impacts depend on both track and intensity, which are uncertain, but heavy rains, high winds, and storm surge are all a possibility. Make sure that you have a hurricane plan ready, as there is the potential for a hurricane this weekend.

NHC 5-day cone for Dorian, showing a path across Puerto Rico, north of the Bahamas, and ending near the east coast of Florida.

For anyone threatened by Dorian, please stay tuned to your local officials for the latest information regarding the storm as forecast confidence continues to increase. Another article will be produced sometime between tomorrow and Friday.

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