Tropical Storm Franklin became the second named storm to form on Sunday in the Atlantic, just hours after Tropical Storm Emily developed.
The National Hurricane Center estimated that Franklin had sustained winds of 45 miles per hour. Tropical disturbances that have sustained winds of 39 m.p.h. earn a name. Once winds reach 74 m.p.h., a storm becomes a hurricane, and at 111 m.p.h. it becomes a major hurricane.
The storm formed 270 miles south-southwest from San Juan, P.R., and was moving west-northwest at 14 m.p.h. Tropical Storm Emily, which also formed on Sunday, was not expected to pose any hazards to land.
Franklin is forecast to approach Hispaniola late on Tuesday and move across the island on Wednesday as a tropical storm, bringing a risk of life-threatening flooding, heavy rainfall, strong winds and dangerous waves along the coast, the Hurricane Center said.
The storm is also expected to bring heavy rainfall across parts of Puerto Rico through the middle of the week, raising the risk of flooding and mudslides, the center said.
Franklin is the seventh tropical cyclone to reach tropical storm strength this year.
The Hurricane Center announced in May that it had reassessed a storm that had formed off the northeastern United States in mid-January, determining that it was a subtropical storm and thus making it the Atlantic’s first cyclone of the year.
However, the storm was not retroactively given a name, making Arlene, which formed in the Gulf of Mexico on June 2, the first named Atlantic storm this year.
Bret and Cindy soon followed, the first time since 1968 that there were two named storms in the Atlantic in June at the same time, according to Philip Klotzbach, a researcher at Colorado State University who studies hurricanes.
Last month, Don became the season’s first hurricane before quickly losing strength.
The Atlantic hurricane season started on June 1 and runs through Nov. 30.
This year features an El Niño pattern, which started in June. The intermittent climate phenomenon can have wide-ranging effects on weather around the world, and it typically impedes the number of Atlantic hurricanes.
In the Atlantic, El Niño increases the amount of wind shear, or the change in wind speed and direction from the ocean or land surface into the atmosphere. Hurricanes need a calm environment to form, and the instability caused by increased wind shear makes those conditions less likely.
(El Niño has the opposite effect in the Pacific, reducing the amount of wind shear.)
At the same time, this year’s heightened sea surface temperatures pose a number of threats, including the ability to supercharge storms.
That unusual confluence of factors has made making storm predictions more difficult.
“Stuff just doesn’t feel right,” Mr. Klotzbach said after NOAA released its updated forecast in August. “There’s just a lot of kind of screwy things that we haven’t seen before.”
There is consensus among scientists that hurricanes are becoming more powerful because of climate change. Although there might not be more named storms overall, the likelihood of major hurricanes is increasing.
Climate change is also affecting the amount of rain that storms can produce.
In a warming world, the air can hold more moisture, which means that a named storm can hold and produce more rainfall, as Hurricane Harvey did in Texas in 2017, when some areas received more than 40 inches of rain in less than 48 hours.
Researchers have also found that over the past few decades storms have slowed, sitting over areas for longer.
When a storm slows over water, the amount of moisture the storm can absorb increases.
When a storm slows over land, the amount of rain that falls over a single location increases; in 2019, for example, Hurricane Dorian slowed to a crawl over the northwestern Bahamas, resulting in a total rainfall of 22.84 inches in Hope Town during the storm.
Other potential effects of climate change include greater storm surges, rapid intensification and a broader reach of tropical systems.
Eduardo Medina contributed reporting.