TS Dorian Forms and Approaches the Caribbean Sea; Invest 98L Likely to Develop off the U.S. East Coast

After a long period of slumber across the tropics, activity has finally started to light up once more as we approach the peak of the season. Earlier this week, Tropical Storm Chantal formed south of the Canadian Maritimes, but was weak, short-lived and far from land areas. There are multiple areas to monitor now across the basin, including Tropical Storm Dorian, which formed from Invest 99L and is moving towards the Lesser Antilles, Invest 98L, which has a 90% chance of forming off the SE US Coast, and Invest 90L, which is located just off the Texas/Louisiana border, and therefore has a low chance of formation.

NHC 5-day Tropical Weather Outlook, featuring all of the areas worth monitoring (Dorian and Invests 98L and 90L. (NHC)

The first system we’ll discuss today is Tropical Storm Dorian, the first system to form in the tropical Atlantic this year. It’s located at a fairly low latitude, as it developed along the ITCZ and disconnected last night. Maximum sustained winds are near 40mph at the moment. The low latitude of the system is the main factor that allowed it to develop, as trades are slower and Saharan dust concentrations are less. However, the system is currently suffering from southeasterly wind shear from an upper-level low to the northwest, but is starting to lighten up. More recently, thunderstorm activity has begun to flare up near the center, which is indicative of strengthening.

Visible imagery as the sun sets over Dorian, showing an organizing system. (MSFC)

Dorian will continue its westward movement over the next several days, but it’ll also likely gain latitude as it does so as a result of beta drift and the weakening of the subtropical ridge. Given that conditions are expected to become more favorable as it approaches the Lesser Antilles, as shear is lowering and moisture is increasing, it’ll likely become a tropical storm by tonight or tomorrow. Most models do not take Dorian past tropical storm strength as it approaches the Lesser Antilles, although the NHC track suggests so. Once it enters the northeastern Caribbean Sea, that’s when the intensity, and therefore track, forecast becomes fuzzier. The weak upper-anticyclone that develops over Dorian might be broken down as a strong upper-level low crosses the Greater Antilles ahead of the storm. At this point, the strength of the system will determine where it’s located, as a stronger storm would be pulled further north than a weaker one that relies more on the trade winds. If the ULL is weak enough or far enough west and/or the storm is stronger than forecast, then it’ll likely continue to track towards the northwest, possibly as a hurricane, towards Puerto Rico or Hispaniola. But if the storm is weaker and/or the ULL is stronger, than increased shear would weaken it and land interaction would ultimately kill it. After it interacts with the Greater Antilles, the track forecasts take it generally towards the Bahamas, although uncertainly is much higher at this point.

EPS ensemble tracks for Dorian, taking the tracks towards the NE Caribbean with varying intensity (WeatherNerds)

While the NHC currently forecasts a hurricane, this intensity forecast has lower confidence than usual, so be prepared for potential swings in the forecast. Impacts will be dependent on track, but heavy rainfall, rough seas, and potentially damaging winds are likely coming towards the central Lesser Antilles from Barbados to Guadeloupe. The storm should move through on late Tuesday, so watches may be needed by late tomorrow. As for Puerto Rico and Hispaniola, impacts will depend on how strong Dorian is at the time and that should impact where it is. A stronger storm would more likely track towards Puerto Rico, where recovery efforts from Hurricane Maria are still ongoing 2 years later. At this point, the forecast shows a landfall in the eastern Dominican Republic, although this also has high uncertainty. Any interests in this part of the basin should keep a close eye on this system.

cone graphic
NHC 5-day forecast for Dorian, showing an intensifying storm approaching the NE Caribbean. (NHC)

In addition to Dorian, another tropical depression is likely to form in the basin, and the culprit is Invest 98L off the east coast of Florida and north of the Bahamas. This system originated from an area of disturbed weather associated with a tropical wave. There is a surface low present, although it appears elongated at this time. This means that development may be more gradual as it moves northeast, but convective coverage has also sustained itself quite well, environmental conditions are conducive for development.

Visible imagery of Invest 98L off the coast of Florida. (COD NEXLAB)

The environment around 98L is expected to remain favorable for further strengthening over the next several days, and the NHC is giving it a very high, 90%, chance of developing in the next 5 days. The system has been prevented from travelling north for a while due to a large frontal mass moving southward across the U.S. Mid-Atlantic. However, the parent trough is in the process of breaking down as a much larger and stronger trough is expected to traverse across the Northern Plains and Great Lakes through the end of the week. The current cause for its northward movement is a weak upper-level ridge to its southeast, which is allowing both for enhanced outflow and less land interaction, as it has moved off the Florida Coast, and a weak trough over the Florida Peninsula. 98L will feel the presence of it and move northeast just off the U.S. Eastern Seaboard. It’s likely to follow just along the Gulf Stream as it travels northeast, which will allow for strengthening of the system. Further strengthening may occur east of New England as it interacts with a jet streak, which could allow for enhancement of baroclinic instability.

EURO 500mb GPH forecast, showing both 98L and the large trough as the main steering current. (Tropical Tidbits).

Because the system is forecast to remain offshore of the U.S., direct impacts are not expected. Rough seas are still likely up and down the Eastern Seaboard regardless, which will cause large swells in coastal areas. In the long-term, an impact to Atlantic Canada is possible in a week, although the storm will likely be undergoing extratropical transition by that point. And even then, most models keep the core of the storm south of Nova Scotia and even Newfoundland. Future impacts will become more certain as the storm develops. Interests in these regions should watch this regardless of the forecast.

Dynamical/Statistical blended track guidance for 98L, showing decent confidence in a track away from the U.S. (UWM Models)

Lastly, Invest 90L was tagged today as an area of disturbed weather off the Texas and Louisiana coastlines. This came from a long-tracked AEW that entered the western Gulf this week, and is now just off the coast. Chances of development are quite low, 10%, as the system is moving onshore near Port Arthur, TX, as I type. Surface observations suggest the low center is ill-defined and winds are rather weak. While convection is plentiful, there’s also some dry air over the Brazos valley and increasing westerly shear, which will further limit chances of formation. Locally heavy rainfall is expected across eastern Texas and Louisiana as this system moves onshore this weekend, but tropical development is unlikely.

Visible imagery of 90L over the Northwestern Gulf. (MSFC)

Stay tuned to your local government officials and news outlets for the latest information regarding all 3 of these systems. The next name on the list is “Erin”, followed by “Fernand”. More articles will be posted as needed, although the efficiency and frequency may be reduced due to obligations related to college (Gig ’em).

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