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Tropical storm warning issued for Puerto Rico as Fiona approaches

Some models project the storm could pass ominously close to the U.S. East Coast next week.

Tropical Storm Fiona midday Friday. (NOAA)

Tropical Storm Fiona has thrown forecasters a few curveballs thus far, and it doesn’t look like that’s about to change any time soon. The storm is set to slam into Puerto Rico and the Leeward Islands with strong winds and heavy rain in the next 36 to 48 hours, but thereafter, uncertainty skyrockets. There are even some indications that an intensifying Fiona could whir ominously close to the U.S. East Coast.

Tropical storm warnings are in effect for the majority of the northern Lesser Antilles — Antigua, Barbuda, St. Kitts, Nevis, Montserrat, Anguilla, Saba, St. Eustatius, St. Maarten, Guadeloupe, St. Barthelemy and St. Martin. Warnings had also been hoisted for Puerto Rico, including Vieques and Culebra, and the U.S. Virgin Islands as of 11 a.m. The British Virgin Islands and Dominica are under a tropical storm watch.

While the winds will be modest, gusting up to 50 mph or so near the storm’s center as it traverses the islands, the National Hurricane Center warned of “considerable flood impacts” that could be highly problematic.

“Flash and urban flooding, along with mudslides in areas of higher terrain” can be expected, it wrote.

Weather models have been very inconsistent in their simulations of Fiona’s evolution after 72 hours; some contend that the storm will curve northward, slipping harmlessly out to sea. Others paint a scenario a bit more concerning for the Southeast United States. Others still cling to the remote possibility of a weak system entering the Gulf of Mexico, but those odds are minimal. No matter how you slice it, Fiona is one to watch.

Fiona now

On Friday morning, Tropical Storm Fiona was centered about 175 miles east of Guadeloupe, drifting due west at 15 mph. It’s important not to get caught up on where the center is, however. That’s because Fiona is a “naked” system. That means strong upper-level winds have blown all the thunderstorms associated with Fiona east of its low-level center of circulation, leaving the center exposed.

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Fiona will continue struggling against shear, or a disruptive change of wind speed and/or direction with height, though Saturday. There’s a chance that shear could relax a bit during the latter half of the weekend, potentially allowing Fiona to flirt with low-end hurricane strength.

Impact on the Leeward Islands and Puerto Rico

Fiona’s circulation will cross the Leeward Islands, probably near Guadeloupe, during the late afternoon hours on Friday. Remember though — since the storm is lopsided, the bulk of the wind and entirety of the rainfall will hold off until after the low-level center passes. That means it probably won’t start raining until overnight Friday into Saturday. The heaviest rains, which could total 3 to 6 inches, will last only 18 to 24 hours, but some showers should linger into early next week; the U.S. and British Virgin Islands should see a similar amount of rain.

For Puerto Rico proper, the rain arrives Saturday night but could stick around through Monday as the system slows down its westward progress and potentially turns north up the Mona Passage — the ocean pass between Hispaniola and western Puerto Rico. Rainfall totals of 4 to 8 inches with localized 12-inch amounts are possible, along with flooding, especially where the U.S. territory’s higher terrain becomes a factor. That could precipitate a few mudslides. Gusts of 35 to 50 mph are probable, too.

The heaviest rains are projected in the eastern Dominican Republic, where 6 to 10 inches are possible, and even up to 16 inches in the mountains, where flash flooding and mudslides will become a threat.

Fiona’s long-term prospects

Fiona’s long-range forecast is especially murky.

As Fiona scrapes along the south side of Puerto Rico, its strength will determine how quickly it turns. At the low and mid-levels of the atmosphere, winds are out of the east — pushing Fiona west. At the upper levels, winds are southerly. As such, a weaker Fiona could continue progressing westward, but if Fiona strengthens, and therefore becomes taller, it’ll begin to “feel” southerly winds and curve to the north.

Picture a sailboat. It might be drifting in one direction, but if it hoists its sails high enough and catches a new wind direction, it’ll start changing course.

The tricky thing about Fiona is that subtleties in the storm’s trajectory will have enormous bearings on next steps. If Fiona waits to begin a northward curve, which is likely, it could encounter Hispaniola, a jagged land mass which would probably shred the storm’s inner circulation. That could prompt a messy reorganization and an almost impossible forecast.

Alternatively, Fiona could escape northward sooner into the southeast Bahamas. From there, its path would hinge on the strength of an Atlantic ridge of high pressure, which will act as a force field to suppress Fiona westward. A stronger ridge would shunt it closer to the East Coast, but a weaker high would allow it to meander out to sea.

At present, there are too many overlapping uncertainties to hatch a forecast with adequate confidence. As such, it’s temporarily a game of wait and see — but it’s too early to let one’s guard down.

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