|Above: Enhanced infrared satellite image of Tropical Storm Dorian at 1655Z (12:55 pm EDT) Monday, August 26, 2019. Image credit: tropicaltidbits.com.|
Watches and warnings have been hoisted for much of the Windward Islands and Puerto Rico in advance of Tropical Storm Dorian, which is gradually organizing over the warm waters of the western Atlantic deep tropics and posing concerns for the Greater Antilles and beyond. Update: As of 5 pm EDT, Dorian was located about 60 miles southeast of Barbados and about 165 miles east-southeast of St. Lucia, heading west-northwest at 14 mph. Top sustained winds were holding at 60 mph. Tropical storm warnings were in effect for Barbados, Martinique, St. Lucia, St. Vincent and the Grenadines, and St. Lucia, with a hurricane watch also in effect for St. Lucia. A tropical storm watch has been issued for Puerto Rico.
Dorian is predicted by the National Hurricane Center to be near hurricane strength by the time it moves from the Windwards into the eastern Caribbean early Tuesday. Some further strengthening is quite possible, although there are big uncertainties in Dorian’s intensity outlook (see below).
|Figure 1. Natural-color satellite image of Tropical Storm Dorian at 1655Z (12:55 pm EDT) Monday, August 26, 2019. Image credit: tropicaltidbits.com.|
Dorian’s shield of showers and thunderstorms (convection) expanded notably on Sunday night. This expansion increases the odds that Dorian will maintain an inner core resilient enough to fend off a vast pool of mid-level dry air surrounding the storm. Outflow is improving toward the north, west, and south of Dorian, another sign of intensification. The Barbados radar was out of service on Monday, but Martinique radar showed the outermost bands of Dorian approaching the Windward Islands.
Track outlook for Dorian
There have been some subtle but important shifts in model guidance on Dorian’s track. As predicted for several days, the storm is expected to move across the east and northeast Caribbean toward Hispaniola and Puerto Rico. The latest NHC forecast has Dorian moving near westernmost Puerto Rico and the far eastern Dominican Republic on Wednesday night as a Category 1 hurricane. Such a track would put Puerto Rico on Dorian’s more intense right-hand side. Model ensembles are in strong agreement on this general track, though only a slight shift west or east could bring the core of Dorian into the Dominican Republic or Puerto Rico. We should gain more confidence by Tuesday on the exact track through this region. It is crucial to note that Dorian is a compact system and is not expected to be remotely as strong as catastrophic Hurricane Maria, though any impacts would obviously carry a heavy psychological toll on the battered island.
|Figure 2. WU depiction of forecast track for Tropical Storm Dorian as of 11 am EDT Monday, August 25, 2019.|
The GFS and European model ensembles on Sunday night (0Z) were in close agreement on a track toward Puerto Rico and the Dominican Republic, but with considerable divergence from that point on. The key factor is an elongated upper low extending from the northwest Atlantic into the central Caribbean. It appears that the southern end of this low may detach and cut off around Cuba later this week. If so, this will tend to pull Dorian on a west-northwest-angling track toward Florida. This scenario is reflected in the 12Z Monday run of the recently retired “legacy GFS”, and in the majority of GFS ensemble members from 0Z Monday. The Euro ensemble is similar, but with a split late next week in The Bahamas, with some members angling Dorian toward the Gulf and others taking it further toward the Southeast U.S. coast.
Especially given the upcoming Labor Day weekend, we will need to keep a close eye on Dorian’s evolution.
[#NerdTweet] While #Dorian may encounter higher shear in Caribbean, there is some subtlety. You can see the PV max split off in the Bahamas and back west of the TC. If Dorian isn't fast enough to run head-long into this upper low, this could result in a weak shear environment. pic.twitter.com/r0WY6Ie00f— Levi Cowan (@TropicalTidbits) August 26, 2019
Intensity outlook for Dorian
Dorian’s intensity changes should be gradual for the next day or so, with the bigger question marks located down the line. The storm is likely to be near or just above minimal hurricane strength as it moves through the Windwards on Monday night and early Tuesday.
Several of the factors that can support rapid intensification of a hurricane are in place for Dorian. This includes light wind shear (only around 5 knots through at least Tuesday, and 5 -10 knots thereafter) and very warm sea surface temperatures, or SSTs, of around 29°C (84°F). The warmth extends to some depth, and Dorian will be passing over increasingly high oceanic heat content, a very important factor in rapid strengthening.
|Figure 3. A vast pool of dry Saharan air extended around Tropical Storm Dorian on Monday, August 26, 2019. Image credit: CIMSS/SSEC/UW-Madison.|
The biggest short-term nemesis for Dorian’s intensification is the expansive pool of very dry mid-level air surrounding the storm. Ordinarily, a tropical cyclone would be killed off quickly by so much dry air, but in this case Dorian appears to have walled off just enough moisture in its inner core to allow convection to blossom, which in turn enables the storm to strengthen. Thus far, Dorian has done an impressive job of keeping dry air at bay.
As long as its inner core remains intact, and wind shear remains light, Dorian has the potential to embark on a period of rapid strengthening at some point. The rapid intensification index in the 12Z Monday SHIPS model showed a 41% chance of Dorian approaching major hurricane strength (95 knots, the top end of Category 2) by late Tuesday. The newer DTOPS algorithm shows a 0% chance of this scenario, so there is uncertainty to say the least. Dynamical model guidance is also of limited use with Dorian, given the small size of its inner core. Even the high-resolution HWRF and HMON models have been struggling with Dorian’s gradual intensification over the last couple of days. We can't rule out Dorian weakening in the northeast Caribbean—a notorious "hurricane graveyard"–but it is possible that the huge field of dry upper-level air is prompting these models to keep Dorian weak despite the factors mentioned above.
Dorian’s intensity forecast gets even more muddled, if that's possible, later this week, as the outcome will hinge on how the storm moves through the Greater Antilles (assuming it does). If the center of this compact storm stays mostly over the channel between Hispaniola and Puerto Rico, or passes over the relatively low terrain of the easternmost Dominican Republic, it may experience less disruption than usual for a tropical cyclone in this area. Any more substantial encounter with land could lead to very rapid weakening. Needless to say, Dorian’s strength from that point onward cannot be predicted with any confidence, though conditions would likely favor strengthening in The Bahamas if Dorian makes it that far.
|Figure 4. Natural-color satellite image of Invest 98L at 1742Z (1:42 pm EDT) Monday, August 26, 2019. Image credit: tropicaltidbits.com.|
Tropical or subtropical cyclone likely to form by Tuesday off U.S. East Coast
Although it’s much closer to the United States than Dorian, the system known as Invest 98L is of much less concern right now. As of midday Monday, 98L consisted of a tight low-level swirl evident on visible satellite imagery about 300 miles southeast of Cape Hatteras, North Carolina, together with a much larger, 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 system that may take on some more symmetric characteristics.
In its 2 pm EDT tropical weather outlook, NHC assigned 90% odds that this system will become a tropical or subtropical cyclone by Wednesday, and most likely sometime before Tuesday. Wind shear is moderate to strong (10-20 knots), but SSTs are warm enough (28-29°C or 82-84°F) to support development. About 75% of GFS model ensemble members from 0Z Monday, and about 25% of Euro members, bring 98L to tropical or subtropical storm strength as it races northeastward. The ensembles are in strong agreement on a track that could take it over or near the Canadian Maritimes late this week. The next name in the Atlantic list is Erin.
We’ll have an update on Dorian later tonight.