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Potential tropical storm Bonnie could hop from Atlantic to Pacific

Costa Rica and Nicaragua are in the crosshairs, while another system in the Gulf will bring heavy rain to Texas

Forecast track for potential tropical storm Bonnie. (National Hurricane Center)
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For more than a week, meteorologists have been forecasting the formation of a tropical storm in the Atlantic, first near the Lesser Antilles and then in the southern Caribbean. It hasn’t happened yet, but it could at any time — and soon-to-be Bonnie might have something unusual in store.

The nascent tropical disturbance is moving west at a breakneck pace, skimming along the northern edge of South America at about 30 mph. By the weekend, it could make landfall in Nicaragua or Costa Rica, bringing heavy rain and mudslides before slipping into the Pacific.

That’s where it could live a second life and develop into a hurricane, becoming a rare crossover storm that makes the trek across the Continental Divide from one ocean basin to another.

Tropical storm warnings and hurricane watches are up for the east coasts of much of Nicaragua and Costa Rica, where the storm should arrive Friday night into Saturday.

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There are also two other systems to watch in the Atlantic — including one in the wake of the first. That one could bring heavy rain to the Leeward Islands over the weekend. Another disturbance in the Gulf of Mexico is slated to deliver torrential rains and flooding to coastal Texas, with up to 8 inches expected.

Where potential Bonnie is now

As of 11 a.m. Thursday, “Potential Tropical Cyclone 2” — centered just north of Colombia in the southern Caribbean — still didn’t have a name. Its winds are sufficient to earn tropical storm status, but it lacks the requisite organization. That’s because there’s no obvious center of circulation that spans multiple layers of the atmosphere.

Until a cohesive central vortex forms, it won’t be classified as a tropical depression or storm. Despite this, 40 mph tropical-storm-force winds extend outward up to 80 miles from the de facto center, which itself is a mere swirl in surface cloud cover. Mid-level and high-altitude winds haven’t responded yet.

If a thunderstorm is able to develop overhead of, or move above, that low-level area of twist, it could entrain that spin and produce a central vortex. Then the system would probably come together rather quickly, since it already contains sufficient winds and copious thunderstorm activity.

An impediment to that could be its brush against the beaches of South America. That’s where friction from the adjacent land mass is slowing down warm, moist “inflow” into the system, preventing a swifter strengthening.

At the upper levels, outflow is evident on satellite imagery. That’s visible in the wispy, banded clouds wafting away from the center, marking where “spent” air caught up in the storm’s exhaust is fanning out of its circulation. That allows for more moisture-rich air to enter the storm from below, bolstering its opportunity to mature if land interaction doesn’t interfere.

Where potential Bonnie is going

The National Hurricane Center expects the disturbance to be named Bonnie by tonight. Until then, it’s continuing to chug westward at an unusually low latitude. It recently passed close to Aruba, marking only the fifth tropical system since 2000 to pass within 60 miles of the island. On Grenada, the Maurice Bishop International Airport registered gusts to 52 mph early Wednesday.

By late Thursday, it will be a couple hundred miles north of the Panama Canal, and should be a tropical storm by then. Thereafter, it’ll make landfall near the Nicaragua-Costa Rica border Friday night into Saturday morning. Winds gusting over 60 mph are possible along the immediate coastline, but the bigger story will be rainfall. A broad 4 to 8 inches, with localized one-foot totals, can be expected.

After that, it will emerge into the Pacific, where it will join an elite club of only a few tropical systems to make it from one ocean basin to another. By the time it begins to weaken over land, it will already be “feeling” the warm waters of the eastern tropical Pacific. There’s a chance it could even make a run at hurricane strength, and some models suggest it could spin around aimlessly until the second week of July.

The most recent crossover storm from the Atlantic was Otto, which made landfall in Costa Rica as a Category 3 in November 2016 before heading into the Pacific. A total of 18 tropical storms or hurricanes have crossed from the Atlantic to the Pacific and maintained their status since reliable bookkeeping began in 1851. Before 2000, crossover storms received a new name when they arrived in the Pacific.

This month, a quasi-crossover from the Pacific to the Atlantic occurred. After Hurricane Agatha slammed into Mexico’s west coast, its remnants crossed the country’s interior before emerging in the Gulf of Mexico. They eventually brought flooding rain to Miami. Those remnants then became part of the Tropical Storm Alex, the first named storm of the Atlantic hurricane season.

A rainstorm in the Gulf

There’s also another system to watch in the Gulf. That one has a 40 percent chance of development, according to the National Hurricane Center. Its window of opportunity is limited, as it’ll move ashore some time in the next 24 hours or so.

The heaviest rainfall will probably be displaced north of the center of the system, soaking areas along the immediate coastline from Matagorda Bay to Houston-Galveston and in the direction of the Golden Triangle and southwest Louisiana. A general 3 to 5 inches is expected, and a localized 7-inch total or two can’t be ruled out.

“The onset of the potential heavy rain tonight may start between midnight and 3am across the coast, and then up towards the US-59 corridor between 3am and sunrise on Friday,” the National Weather Service in Houston wrote in an online forecast discussion. “Deep moisture profiles … support rainfall rates of 2-3” per hour for stronger embedded storms within the more widespread rainfall.”

Fortunately, recent drought should mitigate flooding potential, although any neighborhoods that experience repetitive downpours may deal with some urban and small-stream flooding problems.