Dorian Heading toward Lesser Antilles

A tropical storm warning is in effect for Barbados in advance of Tropical Storm Dorian, a compact system that has the potential to strengthen quickly. Tropical storm watches have been hoisted for St. Lucia, St. Vincent, and the Grenadines.

As of 11 am EDT, Dorian was centered about 465 miles east-southeast of Barbados, heading just north of due west at 14 mph. Dorian’s top sustained winds remained at minimal tropical-storm strength (40 mph), and those winds extended out only about 25 miles from Dorian’s center.

Dorian has struggled to maintain a core of convection (showers and thunderstorms) amid a vast field of dry mid-level air (relative humidity only around 40%). The intermittent convection hasn’t been enough to allow Dorian to take advantage of conditions that are otherwise quite favorable, including light wind shear (only about 5 knots) and warm sea-surface temperatures of 28-29°C (82-84°F).

Intensity forecast for Dorian

The key to Dorian’s future is whether it can wall off a central core of moisture that nurtures convection and allows the storm to strengthen. Wind shear affecting Dorian will remain at only around 5 knots into Monday. Thunderstorms continued to pulse around Dorian’s center at midday Sunday, a sign of potential intensification given that this is a time of day when convection is typically at its weakest.

Watching today to see how well #Dorian fights the dry air around it. Currently, convective bursts near the center are rather healthy, with cold tops of -80C (they're tall).

The key to watch is whether they can become more sustained than they have been, instead of periodic. pic.twitter.com/Aqc0j0KOcv

— Levi Cowan (@TropicalTidbits) August 25, 2019

The usual nighttime maximum in convection on Sunday night may give Dorian its best shot at more robust intensification. If Dorian is still a weak tropical storm on Monday, it may have real trouble strengthening much more than that. Wind shear along Dorian’s path will be gradually increasing from Tuesday onward, and the atmosphere will remain fairly dry (RH below 50%).

A NOAA Gulfstream IV aircraft was on its way to the Lesser Antilles Sunday afternoon to sample the environment around Dorian.

Mission en route to Tropical Storm #Dorian today with a landing in St. Croix.

Hopefully we clear up some model uncertainty. pic.twitter.com/MdvqwoOj8b

— Nick Underwood (@TheAstroNick) August 25, 2019

Track forecast for Dorian

Well-established steering currents around the Bermuda High in the northwest Atlantic will tend to carry Dorian on a fairly typical path for this location and time of year: toward the west-northwest, gradually angling northwest over time. Models are in close agreement that Dorian will pass through the Lesser Antilles on Tuesday and enter the east-central Caribbean Sea.

Figure 1. WU depiction of forecast track for Tropical Storm Dorian as of 11 am EDT Sunday, August 25, 2019.

The specifics of Dorian’s track later this week will be shaped in part by its strength. If Dorian remains a small, weak tropical storm, its track will lean leftward, as low-level easterly trade winds will have a bigger influence. On such a track, Dorian could stay in the Caribbean longer and avoid or postpone any landfall on the Greater Antilles. It could also easily dissipate in such a scenario, as westerly wind shear will be ramping up ahead of an upper low. If Dorian intensifies, its track will lean more rightward, and the odds of impacts on Hispañiola and Puerto Rico will increase.

The 0Z Sunday suite of ensemble runs from the GFS and European models is distinctly bearish on Dorian’s future. About 90-95% of the GFS and European runs keep Dorian as a weak tropical storm and kill it off in the northeast Caribbean. If Dorian somehow beats these very long odds and makes it past the northeast Caribbean without being torn to shreds by the Greater Antilles, it could conceivably move onward to affect the U.S. more than a week from now, but the chances of that appear slim.

Figure 2. Infrared image of Invest 98L at 1716Z (1:16 pm EDT) Sunday, August 25, 2019. Image credit: NASA/MSFC Earth Science Branch.

98L may still become a tropical storm

A complex of thunderstorms well east of Florida may still consolidate into a tropical cyclone over the next couple of days. Dubbed Invest 98L, the disturbance has an elongated structure consistent with convergence along a frontal zone. Increasing southwesterly winds at upper levels will be hauling the moisture and storms northeast as the frontal zone sharpens.

About 40% of European ensemble runs and about two-thirds of GFS runs bring this disturbance to tropical-storm strength as it parallels the U.S. coast well offshore. In its 2 pm EDT tropical weather outlook, NHC gave this system a 70% chance of becoming at least a tropical depression by Tuesday and a 80% chance by Friday. Given the frontal characteristics already present, any development may well be of a hybrid or subtropical nature. A hurricane-hunter reconnaissance flight tentatively set for Sunday was cancelled, but another flight is on the books for Monday if needed.

Beyond Florida, this system should remain far enough east to keep any associated heavy rains away from the U.S. East Coast. There is some potential for this system to affect the Canadian Maritimes and/or Newfoundland and Labrador toward the end of the week.

NHC is no longer tracking short-lived Invest 90L, a low-level feature that has moved onshore across the northwest Gulf coast ahead of an upper-level low. Heavy showers and thunderstorms are scattered across southeast Texas and southern Louisiana and more concentrated offshore. Widespread 2-5” rains can be expected in these areas, with pockets of 5-10” possible. The heavy rain will work its way northeast toward Mississippi and Alabama early this week as an upper-level trough and surface front approach.

Tropical Depression Ivo decaying in Northeast Pacific

As predicted, former Tropical Storm Ivo has met its match in the cool waters off the Mexican coast. Ivo’s top sustained winds were down to 30 mph at 11 am EDT Sunday, when the depression was centered about 500 miles west-northwest of Cabo San Lucas. Ivo is predicted to degenerate into a remnant low by Sunday night. There are no other systems of immediate concern in the Northeast Pacific.

Storm-centered Morphed Integrated Microwave Imagery at CIMSS (MIMIC) maps TS #Bailu's trek across southern #Taiwan and second landfall in #China. pic.twitter.com/xGXTvWxrsP

— UW-Madison CIMSS (@UWCIMSS) August 25, 2019

Bailu moves onshore

In the Northwest Pacific, Tropical Storm Bailu moved onto the China coast south of Xiamen on Saturday night after dousing southern Taiwan with rains of up to 23 inches. The storm led to at least three deaths and nine injuries in Taiwan and the Philippines. See the weather.com article for more on Bailu’s impacts.

The next concern in the Northwest Pacific is Invest 99W, which may become a tropical storm before reaching the Philippines by midweek. 99W could intensify sharply in the South China later this week, posing a late-week threat for the Vietnam or south China coast.