The National Hurricane Center's forecast for Tropical Storm Dorian at 5PM on August 27, 2019.
Dennis Mersereau
Tropical Storm Dorian will bring gusty winds and flooding rains to the Greater Antilles on Wednesday and Thursday as the storm moves through the Caribbean Sea. While Dorian is an imminent hazard for Puerto Rico and the Dominican Republic, current forecasts bring the storm to the Florida coast by this weekend. Five days is an eternity when it comes to tropical weather. The long slog between now and Sunday is time best spent paying attention to trustworthy forecasts and preparing just in case you have to deal with wind, flooding, or power outages.
1) The Greater Antilles Are Next In Line
Regardless of what happens with Tropical Storm Dorian in the future, it’s certain that the Greater Antilles will have to deal with this storm on Wednesday and Thursday. Puerto Rico and the Dominican Republic will likely take the brunt of Dorian’s wind and rain.
The National Hurricane Center expects four to six inches of rain, with isolated higher totals, in both Puerto Rico and the Dominican Republic as the storm passes through the region later this week. Strong winds could also bring down trees and lead to power outages in the hardest-hit areas.
2) The Forecast Is Uncertain Right Now
An infrared satellite view of Tropical Storm Dorian on August 27, 2019.
Tropical Tidbits
The 5:00 PM EDT advisory from the NHC shows Tropical Storm Dorian making a direct landfall on Puerto Rico on Wednesday with maximum sustained winds of 65 MPH, which would likely subject the island territory—which still hasn’t recovered from Hurricane Maria in 2017—to the strongest winds and heaviest rain.
However, Tropical Storm Dorian is struggling right now, and a struggling storm leads to uncertainty.
A weak storm with a fragile structure is difficult to forecast because it’s susceptible to tiny disruptions that can affect both track and strength. Tropical Storm Dorian has struggled against wind shear and dry air for the past couple of days. The storm’s center of circulation reformed this afternoon farther north than it had been in the morning—which happens frequently in developing storms—which changes the potential track of the storm.
3) Mountains Are The Biggest Hurdle In The Current Forecast
Despite all of its current structural woes, the biggest question mark in Tropical Storm Dorian’s future right now is how well it survives crossing the mountains of Puerto Rico and Hispaniola. The mountains that make up these two islands are tall enough to disrupt the structure of a tropical cyclone, shredding a storm apart if it’s not organized enough to survive the trek over land and regroup once it moves back over water.
The latest forecast from the NHC shows Tropical Storm Dorian surviving to see the other side of the islands, but it’s still an open question until the crossing actually happens. Forecasters will fine-tune the track and strength forecasts over the next couple of days as they continue to monitor the storm’s structure and environment.
The odds for hazardous conditions in Florida will drop if Tropical Storm Dorian gets ripped apart by the mountains of Puerto Rico or Hispaniola. Folks in Florida will need to take steps to get ready for the storm if Dorian makes it across the island in one piece and manages to recover over open waters. We’ll know more on Thursday once the system crosses from the Caribbean to the Atlantic.
4) Pay Attention To The Cone Of Uncertainty
The cone of uncertainty is the most important part of a tropical cyclone forecast. As I wrote over the weekend, the cone of uncertainty is the margin of error in a track forecast. This margin of error is calculated using the agency’s previous track errors, giving us a reasonable idea of where the center of the storm could wind up based on past storms.
Forecasts are more accurate in the short term and grow less accurate out to five days. The average track error at five days is about 228 miles, and it can wind up much larger than that with a complex storm such as Dorian.
It’s tempting to focus on the center line and specific points on the forecast maps, especially when the forecast calls for a landfall, but it’s important to remember that the cone shows us a range of possibilities and it only applies to the center of the storm. The wind, rain, and storm surge of a storm can extend hundreds of miles away from the center of circulation, and these hazards are not represented by the cone of uncertainty.
5) It’s Not All About The Wind
Wind gets top billing in any tropical cyclone forecast. Strong winds are dangerous because they can snap trees, knock out power, and cause structural damage, but it’s not all about the wind. We’ve seen too many times in recent years that freshwater flooding from heavy rain is often a much greater threat than damaging winds. A landfalling storm can also bring the threat for storm surge flooding at the coast, rip currents, and even some tornadoes in the outer bands.
6) Even If It Loses Steam, Consider This Practice
It’s not uncommon to wonder why we bother going through the motions of preparing for a storm when there’s so much uncertainty in the forecast. It sounds simplistic, but it really is better to be prepared for nothing—putting you in a better position for the next storm—than not to have prepared at all and get stuck at home with no food or water and all the stores are closed because the power is out.
You don’t have to prepare for a storm like a doomsday prepper, but it is a good idea to keep some simple supplies on hand just in case the power goes out for an extended period of time or the roads are impassable and you can’t get around town.
Non-perishable food, drinking water, batteries, and battery-powered flashlights are good to have on hand no matter the season. Fresh refills of prescription medicine, personal hygiene items like hand sanitizer, and some cash (if you can afford it) are also good items to have in case the power goes out.
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