Tropical Storm Fiona forms, soon to lash Lesser Antilles, Puerto Rico

Tropical Storm Fiona, which formed Wednesday evening several hundred miles east of the Lesser Antilles, is set to lash the Leeward Islands and Puerto Rico with heavy rain, rough surf, coastal rip currents and strong gusty winds. That’s just the first act in what looks to be a long-lived tour of the western Atlantic, with increasing signs that Fiona could become an eventual hurricane and may be one to watch for Bermuda or the U.S. East Coast.

Tropical storm warnings have been hoisted in the northern Leeward Islands — including Saba and St. Eustatius, St. Maarten, Antigua, Barbuda, St. Kitts, Nevis, Montserrat and Anguilla — and could be expanded into Puerto Rico and the U.S. Virgin Islands by Thursday afternoon or evening. Existing watches will probably be upgraded to warnings as the 50 mph storm churns due west at 14 mph.

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Winds up to tropical storm force will probably get there beginning Friday night and will accompany heavy rainfall on the order of 3 to 6 inches. After passing near or over Puerto Rico, Fiona looks to curve northward, at which point a jigsaw puzzle of uncertain atmospheric ingredients will play a west-vs.-east tug of war to determine where it ultimately goes.

Fiona is the sixth named storm of what, until now, has been a relatively quiet Atlantic hurricane season. The Atlantic basin is running at about 47.4 percent of average for ACE, or accumulated cyclone energy — a measure of overall storm activity.

According to hurricane researcher Philip Klotzbach, it’s the slowest start to a season since 2014, defying expert predictions of a particularly active 2022 season. By comparison, the hyperactive season of 2021 had already cranked out 20 named storms and was on the verge of dipping into the Greek alphabet.

As of 11 a.m. Eastern time Thursday, the center of Fiona was located about 495 miles east of the Leeward Islands and was moving west at a typical pace. That westward motion is expected to continue through Friday, when Fiona will deliver impacts to the islands and Puerto Rico.

Maximum sustained winds were estimated at 50 mph, and the National Hurricane Center anticipates subtle strengthening to a 55 mph storm. Thereafter, a plateau in intensity is expected as it continues west. The agency has asked ships within 300 miles of the storm’s position to record and submit weather observations every three hours, which will aid in forecasting and modeling efforts. An Air Force Hurricane Hunter plane will be dispatched to investigate the storm later Thursday.

On infrared satellite imagery, Fiona is replete with deep convection, or shower and thunderstorm activity. That’s evidenced by the darker reds and whites, indicative of high, cold cloud tops. But the majority of the storminess is displaced to the east of its low-level circulation — notice in white the low-level cloud field spiraling into the center, which is obscured by higher clouds to the east.

That lack of vertical alignment of the system is the result of westerly to northwesterly wind shear, or a change of wind speed and/or direction with altitude. It knocks the system off-kilter, and until it’s able to better stack itself vertically, Fiona will struggle to intensify. Strengthening isn’t really expected in the near-term, as shear doesn’t look to relax any time soon.

Eventually, the low-level center may become stretched if a thunderstorm and its associated updraft pass over said vortex, but whether that will happen before arrival in Puerto Rico remains to be seen.

Fiona is expected to bring impacts to the northern Leeward Islands beginning late Friday, and its core should cross the archipelago sometime early Saturday. A general 3 to 6 inches of rain, with a chance of locally higher amounts, is expected. Gusty squalls with winds approaching 50 to 60 mph are likely as well, along with dangerous coastal rip currents.

From there, the American (GFS) model hints that Fiona could track north of Puerto Rico while still sideswiping the northeastern fringe of the U.S. territory.

Conversely, the European model simulates a track south of Puerto Rico and eventually into Hispaniola. That could shred the storm’s circulation before emergence over the waters of the southeastern Bahamas. The storm’s torrential downpours over the Dominican Republic and Haiti could well lead to flooding and landslides, especially in mountainous areas where there is potential for double-digit rainfall totals.

The Hurricane Center forecast for the track of Fiona splits the difference between the American and European models, calling for a path over Puerto Rico before Fiona navigates the Mona Passage west of the island and east of Hispaniola as it begins a northward curve. The ultimate wild card, and hence the different track scenarios, is when that right turn to the north will take place, which depends on the strength and position of high pressure to the northeast. That high acts as a guardrail.

Ultimately, Fiona will be steered to the north, where, if it evades land and its inner core remains intact, it could begin intensifying in the next five to seven days.

Some computer model simulations project it will pass ominously close to the Eastern Seaboard, shunted west by the Bermuda High and further lured toward the coast by approaching low pressure seeking to capture it. Other models allow it to escape out to sea, which would pose a greater risk to Bermuda. All told, it’s simply too early to tell — but this is one you’ll want to closely watch.

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