Sunday, December 4

Tropical Storm Ian forecast to become major hurricane in Gulf of Mexico


Tropical Storm Ian formed Friday night over the central Caribbean north of Venezuela and is expected to become a hurricane by Sunday night. Meteorologists predict Ian will rapidly intensify as it passes through the northwest Caribbean, potentially reaching major hurricane status as it collides with Cuba en route to the Florida Gulf Coast.

Uncertainty remains over where Ian will strike, with a range of possibilities still on the table. There are some indications that it could make a more dramatic north and northeast turn before a landfall somewhere near Tampa, while other computer models simulate a northward movement and eventual stall before a landfall near the Panhandle.

What to know about the next big storm approaching Florida

The resulting forecast challenges mean it’s impossible to say how strong Ian will be when it makes landfall stateside, but the above-average warmth of ocean waters in the eastern Gulf of Mexico is a definite red flag for forecasters worried about a serious impact.

“Ian is forecast to move near or over western Cuba as a strengthening hurricane and then approach the Florida peninsula at or near major hurricane strength,” wrote the National Hurricane Center, “with the potential for significant impacts from storm surge, hurricane-force winds and heavy rainfall.”

According to the National Hurricane Center, the “earliest reasonable arrival time” of tropical storm-force winds would be sometime around midday Tuesday in Cape Canaveral.

On Saturday morning, Tropical Storm Ian was 300 miles south-southeast of Kingstown, Jamaica, or about 570 miles southeast of Grand Cayman. It was moving west-southwest at 15 mph. The Cayman Islands are under a hurricane watch, while a tropical storm watch covers Jamaica.

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Maximum sustained winds in the core of the storm were listed at 45 mph. On infrared satellite, Ian can be seen roiling with one solid mass of convection, or downpour and thunderstorm activity, showing that it is beginning to consolidate and organize more.

On Friday, the surface circulation of Ian was visibly exposed east of the system’s clustering of thunderstorms, a sign of disruptive wind shear, or a change in wind speed and/or direction with height, knocking the entire system off-kilter. Since then, shear has begun to relax acutely, meaning thunderstorms aren’t blown as far downwind of the surface vortex. Visible satellite imagery suggests that the surface whirl will soon pass under thunderstorm updrafts, and if that’s the case, it could become vertically stretched. That would then serve as the anchoring column of vorticity, or spin, around which the storm would develop and intensify.

Rapid intensification en route to Cuba

Once shear relaxes, there is nothing holding Ian back. He it’s moving over water with temperatures approaching 90 degrees, meaning the seas are replete with thermal energy to fuel the storm.

There will also be an upper-level high-pressure system slipping overhead, which will help “vent” Ian and fan exhaust air away from the storm’s center. That will induce a vacuum-like effect in the upper atmosphere to enhance upward motion, intensifying the storm as more warm, moist air in contact with the ocean rushes into the circulation from below.

The National Hurricane Center is explicitly forecasting rapid intensification as Ian closes in on Cuba by Monday night, at which point the storm may be approaching major hurricane strength.

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Major wild cards with forecast

It’s around Monday when weather models begin to markedly diverge from one another and the forecast becomes much more hazy. Part of that stems from where Ian is now. We can spot the system well on satellite, but we can’t “see” where its axis of rotation is from above, since it’s buried beneath thunderstorm anvils. A Hurricane Hunter reconnaissance aircraft was flying through the storm Saturday morning to pinpoint the storm’s center.

If preliminary data show the center is a bit farther southwest than originally thought, the storm may trend farther west toward the western tip of Cuba. That, in turn, might mean a northern gulf track rather than a western Florida landfall.

It all comes down to whether Ian is “captured” or scooped northeast by an approaching trough, or a dip in the jet stream that carries high-altitude cold air, low pressure and spin.

The European model is more optimistic on this northeastward tug toward Florida. In this model, Ian does link up with the trough, curving eastward more quickly and sweeping ashore toward the midcoast of Florida as a significant hurricane. The National Hurricane Center estimates that landfall could occur with a storm near or at Category 3 strength and winds gusting well over 100 mph.

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The Hurricane Center warns that Florida could see “the potential for significant impacts from storm surge, hurricane-force winds, and heavy rainfall.”

On the American GFS model, Ian “misses his ride” and is left to saunter north. By the time Ian actually makes it to the northern gulf on the GFS model, a wedge of dry air banked up works to cap its strength from him and even weaken it.

The forecast continues to quickly evolve, so it’s wise to check back for frequent updates on Ian’s progress.




www.washingtonpost.com

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