Tropical Storm Dorian bears down on Barbados on Monday following an overnight strengthening. The organized system is beginning to appear more symmetrical on satellite. (NOAA/TropicalTidbits.com)

Tropical Storm Dorian marched steadily westward Monday, maintaining its strength, on its way to the Windward Islands and then into the Caribbean.

Maximum sustained winds were around 60 mph Monday evening, with a west-northwest movement of 14 mph. Tropical storm warnings are in effect for Barbados, St. Lucia, Martinique and St. Vincent, and St. Lucia is also under a hurricane watch. Tropical storm watches are in effect for Dominica, Grenada, and Puerto Rico.

The storm is expected to enter the Caribbean early Tuesday and may approach Puerto Rico on Tuesday night and the Dominican Republic on Wednesday, potentially as a Category 1 hurricane.

The National Hurricane Center describes Dorian as a “compact” storm, meaning impacts of tropical storm-force winds extend outward less than 45 miles from the center.

Small tropical cyclones are notoriously difficult to predict, especially when it comes to strength. The smaller a storm, the more difficult it is for models to resolve the finer details and understand the storm’s inner workings. Moreover, subtle wobbles of only a few miles can have drastic differences both on the impacts received on any land masses and on the storm’s progression in general. The latter will potentially play a big role in compounding uncertainty later in the forecast.

First in line for a visit from Dorian is Barbados, with a population of 285,000. Tropical storm-force winds are forecast to arrive there Monday night. The weather will deteriorate overnight into early Tuesday morning for Martinique, St. Lucia, and St. Vincent and the Grenadines. Strong wind gusts and several inches of rain are possible during Dorian’s roughly 24-hour passage. Then attention turns to Puerto Rico.

The most possible storm track brings the center of Dorian just southwest of Puerto Rico; how close of an approach it makes is not known. By then, the National Hurricane Center is forecasting Dorian to have achieved hurricane strength. The main time frame to watch in Puerto Rico is Tuesday night through Wednesday. Due in part to Dorian’s size, however, it is not clear how the tightly wound storm will respond to minute fluctuations in the surrounding environment.

“There are two major sources of uncertainty in intensity within the next three days,” said Brian McNoldy, Capital Weather Gang’s tropical weather expert. “Vertical wind shear and interaction with land.”

Wind shear — or a change in wind speed/direction with height — can tear apart a fledgling storm or knock its stacked circulation off-kilter. The less shear, the more a hurricane can grow in intensity.

“While the wind shear is quite low now, there has not been a lot of run-to-run consistency on what the wind shear will be once Dorian gets into the eastern and central Caribbean,” according to McNoldy. “If it remains low, Dorian can easily become a hurricane. But if it’s high enough, Dorian will struggle to maintain tropical storm status.”

The most likely forecast calls for Puerto Rico, especially its southwestern zones, to be brushed by Dorian’s circulation. Wind gusts of 30 to 45 mph or higher and three to five inches of rain would accompany this scenario. However, more of a direct hit or a more distant miss are all on the table.

After its closest approach to Puerto Rico, Dorian is then forecast to hit or at least graze Hispaniola, potentially crossing over the Dominican Republic.

“That is a large mountainous island that can totally disrupt a storm’s circulation,” McNoldy said. “Some storms never recover from that encounter.”


Group of simulations from American (blue) and European (red) computer models from Monday for Tropical Storm Dorian. Each color strand represents a different model simulation with slightly altered input data. Note that the strands are clustered together where the forecast track is most confident, but they diverge where the course of the storm is less certain. The bold red line is the average of all of the European model simulations, while the blue is the average of all the American model simulations. Please note that this does not indicate strength, but instead solely where Dorian's circulation, in whatever strength/form, may go. (StormVistaWxModels.com)

When encountering Hispaniola some models suggest that Dorian’s circulation will be shredded into a more innocuous, disorganized clumping of remnant showers and thunderstorms. That would spell the end of its life cycle.

However, there’s some chance that Dorian threads the needle between the Dominican Republic and Puerto Rico. If that happens, it could emerge into the West Indies and Bahamas, tapping into plentiful warm water to grow stronger. If this were the case, the southeastern United States would have to carefully watch. Keep in mind that this part of the forecast is almost a week away, and a lot will change.

Elsewhere in the tropics, a disturbance 295 miles southeast of Cape Hatteras, N.C., was declared a tropical depression Monday evening. While it is forecast to become a tropical storm Monday night (named Erin), it will remain out to sea, its main effect kicking up the surf and generating rip currents along the Eastern Seaboard.